\if, \elseif, \else, \endif (was Re: PSQL commands: \quit_if, \quit_unless)

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index 9915731..20091e5 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2007,6 +2007,78 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer) as has_customers,
+    EXISTS(SELECT 1 FROM employee) as has_employees
+\gset
+\if :has_users
+    SELECT * FROM customer ORDER BY creation_date LIMIT 5;
+\elif :has_employees
+    \echo 'no customers found'
+    SELECT * FROM employee ORDER BY creation_date LIMIT 5;
+\else
+    \if yes
+        \echo 'No customers or employees'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all 
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which is evaluated like other options booleans, so the valid values
+        are any unabiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Queries within a false branch of a conditional block will not be
+        sent to the server.
+        </para>
+        <para>
+        Non-conditional <command>\</command>-commands within a false branch
+        of a conditional block will not be evaluated for correctness. The
+        command will be ignored along with all remaining input to the end
+        of the line.
+        </para>
+        <para>
+        Expressions on <command>\if</command> and <command>\elif</command>
+        commands within a false branch of a conditional block will not be
+        evaluated.
+        </para>
+        <para>
+        A conditional block can at most one <command>\else</command> command.
+        </para>
+        <para>
+        The <command>\elif</command> command cannot follow the
+        <command>\else</command> command.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index 4139b77..feb9ddc 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && psqlscan_branch_active(scan_state))
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,68 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	bool	success = false;
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	/*
+	 * placeholder code until ParseVariableBool() ads error detection
+	 * once that patch is in place, use that instead
+	 */
+	if (value)
+	{
+		size_t		len;
+
+		if (value == NULL)
+			return false;			/* not set -> assume "off" */
+
+		len = strlen(value);
+
+		if ((pg_strncasecmp(value, "true", len) == 0) ||
+			(pg_strncasecmp(value, "yes", len) == 0) ||
+			(pg_strncasecmp(value, "on", (len > 2 ? len : 2)) == 0) ||
+			(pg_strcasecmp(value, "1") == 0))
+		{
+			success = true;
+			*result = true;
+		}
+		else if ((pg_strncasecmp(value, "false", len) == 0) ||
+				 (pg_strncasecmp(value, "no", len) == 0) ||
+				 (pg_strncasecmp(value, "off", (len > 2 ? len : 2)) == 0) ||
+				 (pg_strcasecmp(value, "0") == 0))
+		{
+			success = true;
+			*result = false;
+		}
+		else
+		{
+			psql_error("\\%s: invalid boolean expression: %s\n",
+						action, value);
+		}
+		free(value);
+	}
+	else
+	{
+		psql_error("\\%s: no expression given\n",action);
+	}
+	return success;
+}
+
+static bool
+is_branching_command(const char *cmd)
+{
+	return ((strcmp(cmd, "if") == 0 || \
+			strcmp(cmd, "elif") == 0 || \
+			strcmp(cmd, "else") == 0 || \
+			strcmp(cmd, "endif") == 0));
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +270,14 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!psqlscan_branch_active(scan_state) && !is_branching_command(cmd) )
+	{
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -984,6 +1055,114 @@ exec_command(const char *cmd,
 		}
 	}
 
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		if (psqlscan_branch_active(scan_state))
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "if", &if_true);
+			if (success)
+			{
+				if (if_true)
+					new_if_state = IFSTATE_TRUE;
+				else
+					new_if_state = IFSTATE_FALSE;
+			}
+		}
+		if (success)
+			psqlscan_branch_push(scan_state,new_if_state);
+		psql_scan_reset(scan_state);
+	}
+
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\elif\n");
+			success = false;
+		}
+		else
+		{
+			bool elif_true = false;
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_IGNORED:
+					/* inactive branch, do nothing */
+					break;
+				case IFSTATE_TRUE:
+					/* just finished true section of active branch */
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					break;
+				case IFSTATE_FALSE:
+					/* determine if this section is true or not */
+					success = read_boolean_expression(scan_state, "elif",
+														&elif_true);
+					if (success)
+					{
+						if (elif_true)
+							psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+					}
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\elif after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	else if (strcmp(cmd, "else") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\else\n");
+			success = false;
+		}
+		else
+		{
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_TRUE:
+					/* just finished true section of active branch */
+				case IFSTATE_IGNORED:
+					/* whole branch was inactive */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+					break;
+				case IFSTATE_FALSE:
+					/* just finished true section of active branch */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\else after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+		else
+		{
+			psqlscan_branch_end_state(scan_state);
+		}
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index bb306a4..7252824 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,7 +23,6 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
-
 /*
  * Main processing loop for reading lines of input
  *	and sending them to the backend.
@@ -51,6 +50,9 @@ MainLoop(FILE *source)
 	volatile int count_eof = 0;
 	volatile bool die_on_error = false;
 
+	/* only needed at the end to detect unbalanced ifs in scan_state */
+	bool if_endifs_balanced = true;
+
 	/* Save the prior command source */
 	FILE	   *prev_cmd_source;
 	bool		prev_cmd_interactive;
@@ -285,21 +287,28 @@ MainLoop(FILE *source)
 			if (scan_result == PSCAN_SEMICOLON ||
 				(scan_result == PSCAN_EOL && pset.singleline))
 			{
-				/*
-				 * Save query in history.  We use history_buf to accumulate
-				 * multi-line queries into a single history entry.
-				 */
-				if (pset.cur_cmd_interactive && !line_saved_in_history)
+				if (psqlscan_branch_active(scan_state))
 				{
-					pg_append_history(line, history_buf);
-					pg_send_history(history_buf);
-					line_saved_in_history = true;
+					/*
+					 * Save query in history.  We use history_buf to accumulate
+					 * multi-line queries into a single history entry.
+					 */
+					if (pset.cur_cmd_interactive && !line_saved_in_history)
+					{
+						pg_append_history(line, history_buf);
+						pg_send_history(history_buf);
+						line_saved_in_history = true;
+					}
+
+					/* execute query */
+					success = SendQuery(query_buf->data);
 				}
+				else
+					success = true;
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
+				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 
 				/* transfer query to previous_buf by pointer-swapping */
 				{
@@ -358,15 +367,21 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
-
-					/* transfer query to previous_buf by pointer-swapping */
+					if (psqlscan_branch_active(scan_state))
 					{
-						PQExpBuffer swap_buf = previous_buf;
+						success = SendQuery(query_buf->data);
 
-						previous_buf = query_buf;
-						query_buf = swap_buf;
+						/* transfer query to previous_buf by pointer-swapping */
+						{
+							PQExpBuffer swap_buf = previous_buf;
+
+							previous_buf = query_buf;
+							query_buf = swap_buf;
+						}
 					}
+					else
+						success = true;
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -425,12 +440,17 @@ MainLoop(FILE *source)
 	if (query_buf->len > 0 && !pset.cur_cmd_interactive &&
 		successResult == EXIT_SUCCESS)
 	{
-		/* save query in history */
-		if (pset.cur_cmd_interactive)
-			pg_send_history(history_buf);
+		if (psqlscan_branch_active(scan_state))
+		{
+			/* save query in history */
+			if (pset.cur_cmd_interactive)
+				pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+			/* execute query */
+			success = SendQuery(query_buf->data);
+		}
+		else
+			success = true;
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,11 +471,17 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	if (slashCmdStatus != PSQL_CMD_TERMINATE)
+		if_endifs_balanced = psqlscan_branch_empty(scan_state);
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
 	pset.cur_cmd_interactive = prev_cmd_interactive;
 	pset.lineno = prev_lineno;
 
+	if (! if_endifs_balanced )
+		psql_error("found EOF before closing \\endif(s)\n");
+
 	return successResult;
 }	/* MainLoop() */
diff --git a/src/bin/psql/mainloop.h b/src/bin/psql/mainloop.h
index 228a5e0..47f4c32 100644
--- a/src/bin/psql/mainloop.h
+++ b/src/bin/psql/mainloop.h
@@ -14,4 +14,5 @@ extern const PsqlScanCallbacks psqlscan_callbacks;
 
 extern int	MainLoop(FILE *source);
 
+
 #endif   /* MAINLOOP_H */
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..f70841c 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,9 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+	state->branch_block_active = true;
+
 	return state;
 }
 
@@ -919,6 +922,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1436,103 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_empty(PsqlScanState state)
+{
+	return (state->branch_stack == NULL);
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	return state->branch_block_active;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_update_active
+ *
+ * Scan the branch_stack to determine whether the next statements
+ * can execute or should be skipped. Cache this result in
+ * branch_block_active.
+ */
+static void
+psqlscan_branch_update_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	state->branch_block_active  = ( (s == IFSTATE_NONE) ||
+									(s == IFSTATE_TRUE) ||
+									(s == IFSTATE_ELSE_TRUE));
+}
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	psqlscan_branch_update_active(state);
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	psqlscan_branch_update_active(state);
+	return true;
+}
+
+/*
+ * psqlscan_branch_end_state
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_end_state(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	psqlscan_branch_update_active(state);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..734e719 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,12 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
+	bool branch_block_active;
 } PsqlScanStateData;
 
 
@@ -141,4 +171,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_end_state(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 464436a..1dcaa46 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2686,6 +2686,66 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else 
+	\echo 'should not print #1-4'
+\endif
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+\endif
+encountered un-matched \endif
+\else
+encountered un-matched \else
+\elif
+encountered un-matched \elif
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index 900aa7e..4f41894 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -357,6 +357,66 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+\endif
+
+\else
+
+\elif
+
+\if true
+\else
+\else
+\endif
+
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index 640fe12..fcf265b 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,78 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer) as has_customers,
+    EXISTS(SELECT 1 FROM employee) as has_employees
+\gset
+\if :has_users
+    SELECT * FROM customer ORDER BY creation_date LIMIT 5;
+\elif :has_employees
+    \echo 'no customers found'
+    SELECT * FROM employee ORDER BY creation_date LIMIT 5;
+\else
+    \if yes
+        \echo 'No customers or employees'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all 
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which is evaluated like other options booleans, so the valid values
+        are any unabiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Queries within a false branch of a conditional block will not be
+        sent to the server.
+        </para>
+        <para>
+        Non-conditional <command>\</command>-commands within a false branch
+        of a conditional block will not be evaluated for correctness. The
+        command will be ignored along with all remaining input to the end
+        of the line.
+        </para>
+        <para>
+        Expressions on <command>\if</command> and <command>\elif</command>
+        commands within a false branch of a conditional block will not be
+        evaluated.
+        </para>
+        <para>
+        A conditional block can at most one <command>\else</command> command.
+        </para>
+        <para>
+        The <command>\elif</command> command cannot follow the
+        <command>\else</command> command.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index 0c164a3..8235015 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,68 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	bool	success = false;
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	/*
+	 * placeholder code until ParseVariableBool() ads error detection
+	 * once that patch is in place, use that instead
+	 */
+	if (value)
+	{
+		size_t		len;
+
+		if (value == NULL)
+			return false;			/* not set -> assume "off" */
+
+		len = strlen(value);
+
+		if ((pg_strncasecmp(value, "true", len) == 0) ||
+			(pg_strncasecmp(value, "yes", len) == 0) ||
+			(pg_strncasecmp(value, "on", (len > 2 ? len : 2)) == 0) ||
+			(pg_strcasecmp(value, "1") == 0))
+		{
+			success = true;
+			*result = true;
+		}
+		else if ((pg_strncasecmp(value, "false", len) == 0) ||
+				 (pg_strncasecmp(value, "no", len) == 0) ||
+				 (pg_strncasecmp(value, "off", (len > 2 ? len : 2)) == 0) ||
+				 (pg_strcasecmp(value, "0") == 0))
+		{
+			success = true;
+			*result = false;
+		}
+		else
+		{
+			psql_error("\\%s: invalid boolean expression: %s\n",
+						action, value);
+		}
+		free(value);
+	}
+	else
+	{
+		psql_error("\\%s: no expression given\n",action);
+	}
+	return success;
+}
+
+static bool
+is_branching_command(const char *cmd)
+{
+	return ((strcmp(cmd, "if") == 0 || \
+			strcmp(cmd, "elif") == 0 || \
+			strcmp(cmd, "else") == 0 || \
+			strcmp(cmd, "endif") == 0));
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +270,14 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd) )
+	{
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1000,6 +1071,141 @@ exec_command(const char *cmd,
 		}
 	}
 
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "if", &if_true);
+			if (success)
+			{
+				if (if_true)
+					new_if_state = IFSTATE_TRUE;
+				else
+					new_if_state = IFSTATE_FALSE;
+			}
+		}
+		if (success)
+		{
+			psqlscan_branch_push(scan_state,new_if_state);
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\elif\n");
+			success = false;
+		}
+		else
+		{
+			bool elif_true = false;
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_IGNORED:
+					/*
+					 * inactive branch, do nothing:
+					 * either if-endif already had a true block,
+					 * or whole parent block is false.
+					 */
+					break;
+				case IFSTATE_TRUE:
+					/*
+					 * just finished true section of active branch
+					 * do not evaluate expression, just skip
+					 */
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+					break;
+				case IFSTATE_FALSE:
+					/*
+					 * have not yet found a true block in this if-endif,
+					 * determine if this section is true or not.
+					 */
+					/* switch variable expansion back on just for this line */
+					pset.active_branch = true;
+					success = read_boolean_expression(scan_state, "elif",
+														&elif_true);
+					pset.active_branch = false;
+					if (success)
+					{
+						if (elif_true)
+						{
+							psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+							pset.active_branch = true;
+						}
+					}
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\elif after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	else if (strcmp(cmd, "else") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\else\n");
+			success = false;
+		}
+		else
+		{
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_TRUE:
+					/* just finished true section of active branch */
+				case IFSTATE_IGNORED:
+					/* whole branch was inactive */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+					pset.active_branch = false;
+					break;
+				case IFSTATE_FALSE:
+					/* just finished true section of active branch */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+					pset.active_branch = true;
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\else after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+		else
+		{
+			psqlscan_branch_end_state(scan_state);
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		}
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index e1b04de..a526008 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -126,6 +126,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index bb306a4..b8d4887 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,7 +23,6 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
-
 /*
  * Main processing loop for reading lines of input
  *	and sending them to the backend.
@@ -51,6 +50,9 @@ MainLoop(FILE *source)
 	volatile int count_eof = 0;
 	volatile bool die_on_error = false;
 
+	/* only needed at the end to detect unbalanced ifs in scan_state */
+	bool if_endifs_balanced = true;
+
 	/* Save the prior command source */
 	FILE	   *prev_cmd_source;
 	bool		prev_cmd_interactive;
@@ -285,21 +287,28 @@ MainLoop(FILE *source)
 			if (scan_result == PSCAN_SEMICOLON ||
 				(scan_result == PSCAN_EOL && pset.singleline))
 			{
-				/*
-				 * Save query in history.  We use history_buf to accumulate
-				 * multi-line queries into a single history entry.
-				 */
-				if (pset.cur_cmd_interactive && !line_saved_in_history)
+				if (pset.active_branch)
 				{
-					pg_append_history(line, history_buf);
-					pg_send_history(history_buf);
-					line_saved_in_history = true;
+					/*
+					 * Save query in history.  We use history_buf to accumulate
+					 * multi-line queries into a single history entry.
+					 */
+					if (pset.cur_cmd_interactive && !line_saved_in_history)
+					{
+						pg_append_history(line, history_buf);
+						pg_send_history(history_buf);
+						line_saved_in_history = true;
+					}
+
+					/* execute query */
+					success = SendQuery(query_buf->data);
 				}
+				else
+					success = true;
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
+				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 
 				/* transfer query to previous_buf by pointer-swapping */
 				{
@@ -358,15 +367,21 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
-
-					/* transfer query to previous_buf by pointer-swapping */
+					if (pset.active_branch)
 					{
-						PQExpBuffer swap_buf = previous_buf;
+						success = SendQuery(query_buf->data);
 
-						previous_buf = query_buf;
-						query_buf = swap_buf;
+						/* transfer query to previous_buf by pointer-swapping */
+						{
+							PQExpBuffer swap_buf = previous_buf;
+
+							previous_buf = query_buf;
+							query_buf = swap_buf;
+						}
 					}
+					else
+						success = true;
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -425,12 +440,17 @@ MainLoop(FILE *source)
 	if (query_buf->len > 0 && !pset.cur_cmd_interactive &&
 		successResult == EXIT_SUCCESS)
 	{
-		/* save query in history */
-		if (pset.cur_cmd_interactive)
-			pg_send_history(history_buf);
+		if (pset.active_branch)
+		{
+			/* save query in history */
+			if (pset.cur_cmd_interactive)
+				pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+			/* execute query */
+			success = SendQuery(query_buf->data);
+		}
+		else
+			success = true;
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,11 +471,17 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	if (slashCmdStatus != PSQL_CMD_TERMINATE)
+		if_endifs_balanced = psqlscan_branch_empty(scan_state);
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
 	pset.cur_cmd_interactive = prev_cmd_interactive;
 	pset.lineno = prev_lineno;
 
+	if (! if_endifs_balanced )
+		psql_error("found EOF before closing \\endif(s)\n");
+
 	return successResult;
 }	/* MainLoop() */
diff --git a/src/bin/psql/mainloop.h b/src/bin/psql/mainloop.h
index 228a5e0..47f4c32 100644
--- a/src/bin/psql/mainloop.h
+++ b/src/bin/psql/mainloop.h
@@ -14,4 +14,5 @@ extern const PsqlScanCallbacks psqlscan_callbacks;
 
 extern int	MainLoop(FILE *source);
 
+
 #endif   /* MAINLOOP_H */
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 4c7c3b1..15d5599 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 85aac4a..b03ded4 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..a09d404 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,88 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_empty(PsqlScanState state)
+{
+	return (state->branch_stack == NULL);
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return ((s == IFSTATE_NONE) ||
+			(s == IFSTATE_TRUE) ||
+			(s == IFSTATE_ELSE_TRUE));
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_end_state
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_end_state(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..a24a5ac 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_end_state(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 464436a..5cc1130 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2686,6 +2686,78 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+\endif
+encountered un-matched \endif
+\else
+encountered un-matched \else
+\elif
+encountered un-matched \elif
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index 900aa7e..8793aa2 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -357,6 +357,74 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+\endif
+
+\else
+
+\elif
+
+\if true
+\else
+\else
+\endif
+
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index 4e51e90..aad62ac 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..a916671 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,29 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+static bool
+is_branching_command(const char *cmd)
+{
+	return ((strcmp(cmd, "if") == 0 || \
+			strcmp(cmd, "elif") == 0 || \
+			strcmp(cmd, "else") == 0 || \
+			strcmp(cmd, "endif") == 0));
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +231,14 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd) )
+	{
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1038,160 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+				new_if_state = (if_true) ? IFSTATE_TRUE : IFSTATE_FALSE;
+		}
+		psqlscan_branch_push(scan_state,new_if_state);
+		pset.active_branch = psqlscan_branch_active(scan_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\elif\n");
+			success = false;
+		}
+		else
+		{
+			bool elif_true = false;
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_IGNORED:
+					/*
+					 * inactive branch, do nothing:
+					 * either if-endif already had a true block,
+					 * or whole parent block is false.
+					 */
+					break;
+				case IFSTATE_TRUE:
+					/*
+					 * just finished true section of active branch
+					 * do not evaluate expression, just skip
+					 */
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+					break;
+				case IFSTATE_FALSE:
+					/*
+					 * have not yet found a true block in this if-endif,
+					 * determine if this section is true or not.
+					 * variable expansion must be temporarily turned back
+					 * on to read the boolean expression.
+					 */
+					pset.active_branch = true;
+					success = read_boolean_expression(scan_state, "\\elif <expr>",
+														&elif_true);
+					pset.active_branch = false;
+					if (!success)
+						psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					else if (elif_true)
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\elif after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\else\n");
+			success = false;
+		}
+		else
+		{
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_TRUE:
+				case IFSTATE_IGNORED:
+					/*
+					 * either just finished true section of an active branch,
+					 * or whole branch was inactive. either way, be on the
+					 * lookout for any invalid \endif or \else commands
+					 */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+					break;
+				case IFSTATE_FALSE:
+					/* just finished false section of an active branch */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+					pset.active_branch = true;
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\else after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		pset.active_branch = psqlscan_branch_active(scan_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		if (psqlscan_branch_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+		else
+		{
+			psqlscan_branch_end_state(scan_state);
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		}
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 6e3acdc..9f1a072 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index dc25b4b..9c609b4 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,7 +23,6 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
-
 /*
  * Main processing loop for reading lines of input
  *	and sending them to the backend.
@@ -296,10 +295,15 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				/* execute query if branch is active */
+				if (pset.active_branch)
+					success = SendQuery(query_buf->data);
+				else
+					success = true;
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
+				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 
 				/* transfer query to previous_buf by pointer-swapping */
 				{
@@ -358,7 +362,11 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					/* ignore queries on inactive branches */
+					if (pset.active_branch)
+						success = SendQuery(query_buf->data);
+					else
+						success = true;
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +375,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +438,11 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		/* execute query if branch is active*/
+		if (pset.active_branch)
+			success = SendQuery(query_buf->data);
+		else
+			success = true;
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +463,14 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/* check for unbalanced \if-\endifs unless user explicitly quit */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE && !psqlscan_branch_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/mainloop.h b/src/bin/psql/mainloop.h
index 228a5e0..47f4c32 100644
--- a/src/bin/psql/mainloop.h
+++ b/src/bin/psql/mainloop.h
@@ -14,4 +14,5 @@ extern const PsqlScanCallbacks psqlscan_callbacks;
 
 extern int	MainLoop(FILE *source);
 
+
 #endif   /* MAINLOOP_H */
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 4c7c3b1..15d5599 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 0574b5b..df25864 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..e8cc897
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,25 @@
+use strict;
+use warnings;
+
+use PostgresNode;
+use TestLib;
+use Test::More tests => 1;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $stdout;
+my $stderr;
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $retcode = $node->psql('postgres', '\if invalid_expression',
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+is($retcode,'0','Invalid \if respects ON_ERROR_STOP');
+
+$node->teardown_node;
+
+
+
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..a09d404 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,88 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_empty(PsqlScanState state)
+{
+	return (state->branch_stack == NULL);
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return ((s == IFSTATE_NONE) ||
+			(s == IFSTATE_TRUE) ||
+			(s == IFSTATE_ELSE_TRUE));
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_end_state
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_end_state(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..a24a5ac 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_end_state(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 420825a..ade98c6 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2695,6 +2695,84 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+\endif
+encountered un-matched \endif
+\else
+encountered un-matched \else
+\elif
+encountered un-matched \elif
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index 79624b9..482bab8 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -365,6 +365,80 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+\endif
+
+\else
+
+\elif
+
+\if true
+\else
+\else
+\endif
+
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..dcf567e 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,17 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd) )
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("inside inactive branch, command ignored.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1042,176 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+				new_if_state = (if_true) ? IFSTATE_TRUE : IFSTATE_FALSE;
+		}
+		psqlscan_branch_push(scan_state,new_if_state);
+		pset.active_branch = psqlscan_branch_active(scan_state);
+		psql_scan_reset(scan_state);
+		if (pset.cur_cmd_interactive && success)
+			psql_error("entered if: %s, %s commands\n",
+						(pset.active_branch) ? "active" : "inactive",
+						(pset.active_branch) ? "executing" : "ignoring");
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		if (psqlscan_branch_stack_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\elif\n");
+			success = false;
+		}
+		else
+		{
+			bool elif_true = false;
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_IGNORED:
+					/*
+					 * inactive branch, do nothing:
+					 * either if-endif already had a true block,
+					 * or whole parent block is false.
+					 */
+					break;
+				case IFSTATE_TRUE:
+					/*
+					 * just finished true section of active branch
+					 * do not evaluate expression, just skip
+					 */
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+					break;
+				case IFSTATE_FALSE:
+					/*
+					 * have not yet found a true block in this if-endif,
+					 * determine if this section is true or not.
+					 * variable expansion must be temporarily turned back
+					 * on to read the boolean expression.
+					 */
+					pset.active_branch = true;
+					success = read_boolean_expression(scan_state, "\\elif <expr>",
+														&elif_true);
+					pset.active_branch = false;
+					if (!success)
+						psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					else if (elif_true)
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\elif after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		psql_scan_reset(scan_state);
+		if (pset.cur_cmd_interactive && success)
+			psql_error("entered elif: %s, %s commands\n",
+						(pset.active_branch) ? "active" : "inactive",
+						(pset.active_branch) ? "executing" : "ignoring");
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		if (psqlscan_branch_stack_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\else\n");
+			success = false;
+		}
+		else
+		{
+			switch (psqlscan_branch_get_state(scan_state))
+			{
+				case IFSTATE_TRUE:
+				case IFSTATE_IGNORED:
+					/*
+					 * either just finished true section of an active branch,
+					 * or whole branch was inactive. either way, be on the
+					 * lookout for any invalid \endif or \else commands
+					 */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+					break;
+				case IFSTATE_FALSE:
+					/* just finished false section of an active branch */
+					psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+					pset.active_branch = true;
+					break;
+				case IFSTATE_ELSE_TRUE:
+				case IFSTATE_ELSE_FALSE:
+					psql_error("encountered \\else after \\else\n");
+					success = false;
+					break;
+				default:
+					break;
+			}
+		}
+		pset.active_branch = psqlscan_branch_active(scan_state);
+		psql_scan_reset(scan_state);
+		if (pset.cur_cmd_interactive && success)
+			psql_error("entered else: %s, %s commands\n",
+						(pset.active_branch) ? "active" : "inactive",
+						(pset.active_branch) ? "executing" : "ignoring");
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		if (psqlscan_branch_stack_empty(scan_state))
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+		else
+		{
+			psqlscan_branch_pop(scan_state);
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		}
+		psql_scan_reset(scan_state);
+		if (pset.cur_cmd_interactive && success)
+			psql_error("exited if: %s, %s commands\n",
+						(pset.active_branch) ? "active" : "inactive",
+						(pset.active_branch) ? "executing" : "ignoring");
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 6e3acdc..9f1a072 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..543b0d5 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -307,6 +307,21 @@ slashUsage(unsigned short int pager)
 					  "  \\lo_list\n"
 					  "  \\lo_unlink LOBOID      large object operations\n"));
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block, and execute subsequent\n"
+					  "                          commands/queires if <expr> is true, otherwise\n"
+					  "                          skip them without evaluation\n"));
+	fprintf(output, _("  \\elif <expr>           within a conditional block, exectue subsequennt\n"
+					  "                          commands/queires if <expr> is true and all\n"
+					  "                          previous \\if and \\elif expressions were false\n"));
+	fprintf(output, _("  \\elif <expr>           within a conditional block, exectue subsequennt\n"
+					  "                          commands/queires only if all previous \\if and\n"
+					  "                          \\elif expressions were false\n"));
+	fprintf(output, _("  \\endif                 end a conditional block, restoring\n"
+					  "                          executing/skipping state to what it was before\n"
+					  "                          the matching \\if command.\n"));
+	fprintf(output, "\n");
+
 	ClosePager(output);
 }
 
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..ed0330d 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,22 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("inside inactive branch, query ignored.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -296,8 +312,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +374,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +383,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +446,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +467,15 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/* check for unbalanced \if-\endifs unless user explicitly quit */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..2ed01c5
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,32 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 3;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $stdout;
+my $stderr;
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $script_1 = q(\if invalid_expression
+);
+
+my $stderr_1  = q(psql:<stdin>:1: unrecognized value "invalid_expression" for "\if <expr>": boolean expected
+psql:<stdin>:1: found EOF before closing \endif(s));
+
+my $retcode = $node->psql('postgres', $script_1,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+is($retcode,'3','Invalid \if respects ON_ERROR_STOP');
+is($stdout,'','STDOUT mismatch');
+is($stderr, $stderr_1, 'STDERR mismatch');
+
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..d615eb8 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,95 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+encountered un-matched \endif
+-- test un-matched else
+\else
+encountered un-matched \else
+-- test un-matched elif
+\elif
+encountered un-matched \elif
+-- test double-else error
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..ba41d9e 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..9058897 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,17 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("inside inactive branch, command ignored.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1042,251 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				if (if_true)
+				{
+					new_if_state = IFSTATE_TRUE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is true, executing commands\n");
+					pset.active_branch = true;
+				}
+				else
+				{
+					new_if_state = IFSTATE_FALSE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+					pset.active_branch = false;
+				}
+			}
+			else
+			{
+				/*
+				 * show this error in both interactive and script, because the
+				 * session is now in a state where all blocks will be ignored
+				 * regardless of expression truth
+				 */
+				psql_error("new \\if is invalid, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+			}
+		}
+		else
+		{
+			if (pset.cur_cmd_interactive)
+				psql_error("new \\if is inside ignored block, "
+						"ignoring commands until next \\endif\n");
+			pset.active_branch = false;
+		}
+
+		psqlscan_branch_push(scan_state,new_if_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is inside ignored block, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is after true block, "
+							"ignoring commands until next \\endif\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				if (success)
+				{
+					if (elif_true)
+					{
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is true, executing commands\n");
+						pset.active_branch = false;
+					}
+					else
+					{
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+						pset.active_branch = false;
+					}
+				}
+				else
+				{
+					/*
+					 * show this error in both interactive and script, because the
+					 * session is now in a state where all blocks will be ignored
+					 * regardless of expression truth
+					 */
+					psql_error("\\elif is invalid, "
+								"ignoring commands until next \\endif\n");
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+				}
+				pset.active_branch = psqlscan_branch_active(scan_state);
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\elif after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("encountered un-matched \\elif\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after true condition or in ignored block, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after all previous conditions false, "
+								"executing commands\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("encountered un-matched \\else\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\else after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		if (psqlscan_branch_pop(scan_state))
+		{
+			bool was_active = pset.active_branch;
+			pset.active_branch = psqlscan_branch_active(scan_state);
+
+			if (pset.cur_cmd_interactive)
+			{
+				if (psqlscan_branch_stack_empty(scan_state))
+					/* no more branches */
+					psql_error("exited \\if, executing commands\n");
+				else if (!pset.active_branch)
+					/* was ignoring, still ignoring */
+					psql_error("exited \\if to false parent branch, \n"
+							"ignoring commands until next \\endif\n");
+				else if (was_active)
+					/* was true, still true */
+					psql_error("exited \\if to true parent branch, \n"
+							"continuing executing commands\n");
+				else
+					/* was false, is true again */
+					psql_error("exited \\if to true parent branch, \n"
+							"resuming executing commands\n");
+			}
+		}
+		else
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 6e3acdc..9f1a072 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..4b30675 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,23 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("inside inactive branch, query ignored. "
+					"use \\endif to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -296,8 +313,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +375,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +384,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +447,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +468,15 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/* check for unbalanced \if-\endifs unless user explicitly quit */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..1c8c3c4
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,38 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 15;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+	[ "\\if invalid_expression\n\\endif\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
+	[ "\\else\\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
+	[ "\\endif\\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test respects ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..d4f1849 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,96 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+new \if is invalid, ignoring commands until next \endif
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+encountered un-matched \endif
+-- test un-matched else
+\else
+encountered un-matched \else
+-- test un-matched elif
+\elif
+encountered un-matched \elif
+-- test double-else error
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..ba41d9e 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..4a3e471 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,17 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("inside inactive branch, command ignored.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1042,251 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				if (if_true)
+				{
+					new_if_state = IFSTATE_TRUE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is true, executing commands\n");
+					pset.active_branch = true;
+				}
+				else
+				{
+					new_if_state = IFSTATE_FALSE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+					pset.active_branch = false;
+				}
+			}
+			else
+			{
+				/*
+				 * show this error in both interactive and script, because the
+				 * session is now in a state where all blocks will be ignored
+				 * regardless of expression truth
+				 */
+				psql_error("new \\if is invalid, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+			}
+		}
+		else
+		{
+			if (pset.cur_cmd_interactive)
+				psql_error("new \\if is inside ignored block, "
+						"ignoring commands until next \\endif\n");
+			pset.active_branch = false;
+		}
+
+		psqlscan_branch_push(scan_state,new_if_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is inside ignored block, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is after true block, "
+							"ignoring commands until next \\endif\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				if (success)
+				{
+					if (elif_true)
+					{
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is true, executing commands\n");
+						pset.active_branch = false;
+					}
+					else
+					{
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+						pset.active_branch = false;
+					}
+				}
+				else
+				{
+					/*
+					 * show this error in both interactive and script, because the
+					 * session is now in a state where all blocks will be ignored
+					 * regardless of expression truth
+					 */
+					psql_error("\\elif is invalid, "
+								"ignoring commands until next \\endif\n");
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+				}
+				pset.active_branch = psqlscan_branch_active(scan_state);
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\elif after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("encountered un-matched \\elif\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after true condition or in ignored block, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after all previous conditions false, "
+								"executing commands\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("encountered un-matched \\else\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\else after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		if (psqlscan_branch_pop(scan_state))
+		{
+			bool was_active = pset.active_branch;
+			pset.active_branch = psqlscan_branch_active(scan_state);
+
+			if (pset.cur_cmd_interactive)
+			{
+				if (psqlscan_branch_stack_empty(scan_state))
+					/* no more branches */
+					psql_error("exited \\if, executing commands\n");
+				else if (!pset.active_branch)
+					/* was ignoring, still ignoring */
+					psql_error("exited \\if to false parent branch, "
+							"ignoring commands until next \\endif\n");
+				else if (was_active)
+					/* was true, still true */
+					psql_error("exited \\if to true parent branch, "
+							"continuing executing commands\n");
+				else
+					/* was false, is true again */
+					psql_error("exited \\if to true parent branch, "
+							"resuming executing commands\n");
+			}
+		}
+		else
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 6e3acdc..9f1a072 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..099cf8c 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,23 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("inside inactive branch, query ignored. "
+					"use \\endif to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +139,35 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					bool was_active = pset.active_branch;
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+
+					if (psqlscan_branch_stack_empty(scan_state))
+						/* no more branches */
+						psql_error("escaped \\if, executing commands\n");
+					else if (!pset.active_branch)
+						/* was ignoring, still ignoring */
+						psql_error("escaped \\if to false parent branch, "
+								"ignoring commands until next \\endif\n");
+					else if (was_active)
+						/* was true, still true */
+						psql_error("escaped \\if to true parent branch, "
+								"continuing executing commands\n");
+					else
+						/* was false, is true again */
+						psql_error("escaped \\if to true parent branch, "
+								"resuming executing commands\n");
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +341,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +403,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +412,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +475,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +496,15 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/* check for unbalanced \if-\endifs unless user explicitly quit */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..68c9b15
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,37 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 15;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+	[ "\\if invalid_expression\n\\endif\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
+	[ "\\else\\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
+	[ "\\endif\\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test respects ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..d4f1849 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,96 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+new \if is invalid, ignoring commands until next \endif
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+encountered un-matched \endif
+-- test un-matched else
+\else
+encountered un-matched \else
+-- test un-matched elif
+\elif
+encountered un-matched \elif
+-- test double-else error
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..ba41d9e 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
index 68c9b15..a703cab 100644
--- a/src/bin/psql/t/001_if.pl
+++ b/src/bin/psql/t/001_if.pl
@@ -4,7 +4,7 @@ use warnings;
 use Config;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 15;
+use Test::More tests => 18;
 
 #
 # test invalid \if respects ON_ERROR_STOP
@@ -14,12 +14,14 @@ $node->init;
 $node->start;
 
 my $tests = [
-	[ "\\if invalid_expression\n\\endif\n", '', 'boolean expected', 'syntax error' ],
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
 	# unmatched checks
-	[ "\\if true\\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
-	[ "\\elif true\\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
-	[ "\\else\\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
-	[ "\\endif\\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
 ];
 
 # 3 checks per tests
@@ -29,7 +31,7 @@ for my $test (@$tests) {
   my $retcode = $node->psql('postgres', $script,
 		stdout => \$stdout, stderr => \$stderr,
 		on_error_stop => 1);
-  is($retcode,'3',"$name test respects ON_ERROR_STOP");
+  is($retcode,'3',"$name test ON_ERROR_STOP");
   is($stdout, $stdout_expect, "$name test STDOUT");
   like($stderr, qr/$stderr_re/, "$name test STDERR");
 }

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..4a3e471 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,17 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("inside inactive branch, command ignored.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1042,251 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				if (if_true)
+				{
+					new_if_state = IFSTATE_TRUE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is true, executing commands\n");
+					pset.active_branch = true;
+				}
+				else
+				{
+					new_if_state = IFSTATE_FALSE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+					pset.active_branch = false;
+				}
+			}
+			else
+			{
+				/*
+				 * show this error in both interactive and script, because the
+				 * session is now in a state where all blocks will be ignored
+				 * regardless of expression truth
+				 */
+				psql_error("new \\if is invalid, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+			}
+		}
+		else
+		{
+			if (pset.cur_cmd_interactive)
+				psql_error("new \\if is inside ignored block, "
+						"ignoring commands until next \\endif\n");
+			pset.active_branch = false;
+		}
+
+		psqlscan_branch_push(scan_state,new_if_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is inside ignored block, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is after true block, "
+							"ignoring commands until next \\endif\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				if (success)
+				{
+					if (elif_true)
+					{
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is true, executing commands\n");
+						pset.active_branch = false;
+					}
+					else
+					{
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+						pset.active_branch = false;
+					}
+				}
+				else
+				{
+					/*
+					 * show this error in both interactive and script, because the
+					 * session is now in a state where all blocks will be ignored
+					 * regardless of expression truth
+					 */
+					psql_error("\\elif is invalid, "
+								"ignoring commands until next \\endif\n");
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+				}
+				pset.active_branch = psqlscan_branch_active(scan_state);
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\elif after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("encountered un-matched \\elif\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after true condition or in ignored block, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after all previous conditions false, "
+								"executing commands\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("encountered un-matched \\else\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\else after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		if (psqlscan_branch_pop(scan_state))
+		{
+			bool was_active = pset.active_branch;
+			pset.active_branch = psqlscan_branch_active(scan_state);
+
+			if (pset.cur_cmd_interactive)
+			{
+				if (psqlscan_branch_stack_empty(scan_state))
+					/* no more branches */
+					psql_error("exited \\if, executing commands\n");
+				else if (!pset.active_branch)
+					/* was ignoring, still ignoring */
+					psql_error("exited \\if to false parent branch, "
+							"ignoring commands until next \\endif\n");
+				else if (was_active)
+					/* was true, still true */
+					psql_error("exited \\if to true parent branch, "
+							"continuing executing commands\n");
+				else
+					/* was false, is true again */
+					psql_error("exited \\if to true parent branch, "
+							"resuming executing commands\n");
+			}
+		}
+		else
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 6e3acdc..9f1a072 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..099cf8c 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,23 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("inside inactive branch, query ignored. "
+					"use \\endif to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +139,35 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					bool was_active = pset.active_branch;
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+
+					if (psqlscan_branch_stack_empty(scan_state))
+						/* no more branches */
+						psql_error("escaped \\if, executing commands\n");
+					else if (!pset.active_branch)
+						/* was ignoring, still ignoring */
+						psql_error("escaped \\if to false parent branch, "
+								"ignoring commands until next \\endif\n");
+					else if (was_active)
+						/* was true, still true */
+						psql_error("escaped \\if to true parent branch, "
+								"continuing executing commands\n");
+					else
+						/* was false, is true again */
+						psql_error("escaped \\if to true parent branch, "
+								"resuming executing commands\n");
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +341,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +403,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +412,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +475,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +496,15 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/* check for unbalanced \if-\endifs unless user explicitly quit */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..a703cab
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,39 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 18;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..d4f1849 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,96 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+new \if is invalid, ignoring commands until next \endif
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+encountered un-matched \endif
+-- test un-matched else
+\else
+encountered un-matched \else
+-- test un-matched elif
+\elif
+encountered un-matched \elif
+-- test double-else error
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..ba41d9e 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..f10d7ac 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,17 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("inside inactive branch, command ignored.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1042,259 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				if (if_true)
+				{
+					new_if_state = IFSTATE_TRUE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is true, executing commands\n");
+					pset.active_branch = true;
+				}
+				else
+				{
+					new_if_state = IFSTATE_FALSE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+					pset.active_branch = false;
+				}
+			}
+			else
+			{
+				/*
+				 * show this error in both interactive and script, because the
+				 * session is now in a state where all blocks will be ignored
+				 * regardless of expression truth. suppress this error in
+				 * cases where the script is already going to terminate.
+				 */
+				if (pset.on_error_stop)
+					psql_error("new \\if is invalid.\n");
+				else
+					psql_error("new \\if is invalid, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+			}
+		}
+		else
+		{
+			if (pset.cur_cmd_interactive)
+				psql_error("new \\if is inside ignored block, "
+						"ignoring commands until next \\endif\n");
+			pset.active_branch = false;
+		}
+
+		psqlscan_branch_push(scan_state,new_if_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is inside ignored block, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is after true block, "
+							"ignoring commands until next \\endif\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				if (success)
+				{
+					if (elif_true)
+					{
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is true, executing commands\n");
+						pset.active_branch = false;
+					}
+					else
+					{
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+						pset.active_branch = false;
+					}
+				}
+				else
+				{
+					/*
+					 * show this error in both interactive and script, because the
+					 * session is now in a state where all blocks will be ignored
+					 * regardless of expression truth. suppress this error in
+					 * cases where the script is already going to terminate.
+					 */
+					if (pset.on_error_stop)
+						psql_error("\\elif is invalid.\n");
+					else
+						psql_error("\\elif is invalid, "
+									"ignoring commands until next \\endif\n");
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+				}
+				pset.active_branch = psqlscan_branch_active(scan_state);
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\elif after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("encountered un-matched \\elif\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after true condition or in ignored block, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after all previous conditions false, "
+								"executing commands\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("encountered un-matched \\else\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\else after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		if (psqlscan_branch_pop(scan_state))
+		{
+			bool was_active = pset.active_branch;
+			pset.active_branch = psqlscan_branch_active(scan_state);
+
+			if (pset.cur_cmd_interactive)
+			{
+				if (psqlscan_branch_stack_empty(scan_state))
+					/* no more branches */
+					psql_error("exited \\if, executing commands\n");
+				else if (!pset.active_branch)
+					/* was ignoring, still ignoring */
+					psql_error("exited \\if to false parent branch, "
+							"ignoring commands until next \\endif\n");
+				else if (was_active)
+					/* was true, still true */
+					psql_error("exited \\if to true parent branch, "
+							"continuing executing commands\n");
+				else
+					/* was false, is true again */
+					psql_error("exited \\if to true parent branch, "
+							"resuming executing commands\n");
+			}
+		}
+		else
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 6e3acdc..9f1a072 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..b599581 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,23 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("inside inactive branch, query ignored. "
+					"use \\endif to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +139,35 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					bool was_active = pset.active_branch;
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+
+					if (psqlscan_branch_stack_empty(scan_state))
+						/* no more branches */
+						psql_error("escaped \\if, executing commands\n");
+					else if (!pset.active_branch)
+						/* was ignoring, still ignoring */
+						psql_error("escaped \\if to false parent branch, "
+								"ignoring commands until next \\endif\n");
+					else if (was_active)
+						/* was true, still true */
+						psql_error("escaped \\if to true parent branch, "
+								"continuing executing commands\n");
+					else
+						/* was false, is true again */
+						psql_error("escaped \\if to true parent branch, "
+								"resuming executing commands\n");
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +341,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +403,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +412,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +475,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +496,19 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/*
+	 * check for unbalanced \if-\endifs unless user explicitly quit,
+	 * or the script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& successResult != EXIT_USER
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..88f3013
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,41 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 21;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
+	# error stop messages
+	[ "\\set ON_ERROR_STOP on\n\\if error\n\\echo NO\n\\endif\n\\echo NO\n", '', 'new.*if is invalid.', 'invalid if'],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..d4f1849 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,96 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+new \if is invalid, ignoring commands until next \endif
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+encountered un-matched \endif
+-- test un-matched else
+\else
+encountered un-matched \else
+-- test un-matched elif
+\elif
+encountered un-matched \elif
+-- test double-else error
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..ba41d9e 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..63ddf0a 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,17 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("inside inactive branch, command ignored.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1042,259 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block.
+	 * <expr> must be a valid boolean expression, or the whole block will be
+	 * ignored.
+	 * If this \if is itself a part of a branch that is false/ignored, it too
+	 * will automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		ifState new_if_state = IFSTATE_IGNORED;
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				if (if_true)
+				{
+					new_if_state = IFSTATE_TRUE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is true, executing commands\n");
+					pset.active_branch = true;
+				}
+				else
+				{
+					new_if_state = IFSTATE_FALSE;
+					if (pset.cur_cmd_interactive)
+						psql_error("new \\if is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+					pset.active_branch = false;
+				}
+			}
+			else
+			{
+				/*
+				 * show this error in both interactive and script, because the
+				 * session is now in a state where all blocks will be ignored
+				 * regardless of expression truth. suppress this error in
+				 * cases where the script is already going to terminate.
+				 */
+				if (pset.on_error_stop && !pset.cur_cmd_interactive)
+					psql_error("new \\if is invalid.\n");
+				else
+					psql_error("new \\if is invalid, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+			}
+		}
+		else
+		{
+			if (pset.cur_cmd_interactive)
+				psql_error("new \\if is inside ignored block, "
+						"ignoring commands until next \\endif\n");
+			pset.active_branch = false;
+		}
+
+		psqlscan_branch_push(scan_state,new_if_state);
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is inside ignored block, "
+							"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\elif is after true block, "
+							"ignoring commands until next \\endif\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				if (success)
+				{
+					if (elif_true)
+					{
+						psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is true, executing commands\n");
+						pset.active_branch = false;
+					}
+					else
+					{
+						if (pset.cur_cmd_interactive)
+							psql_error("\\elif is false, ignoring commands "
+									"until next \\elif, \\else, or \\endif\n");
+						pset.active_branch = false;
+					}
+				}
+				else
+				{
+					/*
+					 * show this error in both interactive and script, because the
+					 * session is now in a state where all blocks will be ignored
+					 * regardless of expression truth. suppress this error in
+					 * cases where the script is already going to terminate.
+					 */
+					if (pset.on_error_stop && !pset.cur_cmd_interactive)
+						psql_error("\\elif is invalid.\n");
+					else
+						psql_error("\\elif is invalid, "
+									"ignoring commands until next \\endif\n");
+					psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+					pset.active_branch = false;
+				}
+				pset.active_branch = psqlscan_branch_active(scan_state);
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\elif after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("encountered un-matched \\elif\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after true condition or in ignored block, "
+								"ignoring commands until next \\endif\n");
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				if (pset.cur_cmd_interactive)
+					psql_error("\\else after all previous conditions false, "
+								"executing commands\n");
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("encountered un-matched \\else\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("encountered \\else after \\else\n");
+				success = false;
+				pset.active_branch = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		if (psqlscan_branch_pop(scan_state))
+		{
+			bool was_active = pset.active_branch;
+			pset.active_branch = psqlscan_branch_active(scan_state);
+
+			if (pset.cur_cmd_interactive)
+			{
+				if (psqlscan_branch_stack_empty(scan_state))
+					/* no more branches */
+					psql_error("exited \\if, executing commands\n");
+				else if (!pset.active_branch)
+					/* was ignoring, still ignoring */
+					psql_error("exited \\if to false parent branch, "
+							"ignoring commands until next \\endif\n");
+				else if (was_active)
+					/* was true, still true */
+					psql_error("exited \\if to true parent branch, "
+							"continuing executing commands\n");
+				else
+					/* was false, is true again */
+					psql_error("exited \\if to true parent branch, "
+							"resuming executing commands\n");
+			}
+		}
+		else
+		{
+			psql_error("encountered un-matched \\endif\n");
+			success = false;
+		}
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 5349c39..698ca1f 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..de93b52 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,23 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("inside inactive branch, query ignored. "
+					"use \\endif to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +139,35 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					bool was_active = pset.active_branch;
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+
+					if (psqlscan_branch_stack_empty(scan_state))
+						/* no more branches */
+						psql_error("escaped \\if, executing commands\n");
+					else if (!pset.active_branch)
+						/* was ignoring, still ignoring */
+						psql_error("escaped \\if to false parent branch, "
+								"ignoring commands until next \\endif\n");
+					else if (was_active)
+						/* was true, still true */
+						psql_error("escaped \\if to true parent branch, "
+								"continuing executing commands\n");
+					else
+						/* was false, is true again */
+						psql_error("escaped \\if to true parent branch, "
+								"resuming executing commands\n");
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +341,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +403,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +412,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +475,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +496,19 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/*
+	 * check for unbalanced \if-\endifs unless user explicitly quit,
+	 * or the script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& successResult != EXIT_USER
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error && !pset.cur_cmd_interactive)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..88f3013
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,41 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 21;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', 'encountered un-matched.*elif', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', 'encountered un-matched.*else', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', 'encountered un-matched.*endif', 'unmatched \endif' ],
+	# error stop messages
+	[ "\\set ON_ERROR_STOP on\n\\if error\n\\echo NO\n\\endif\n\\echo NO\n", '', 'new.*if is invalid.', 'invalid if'],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..d4f1849 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,96 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+new \if is invalid, ignoring commands until next \endif
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+encountered un-matched \endif
+-- test un-matched else
+\else
+encountered un-matched \else
+-- test un-matched elif
+\elif
+encountered un-matched \elif
+-- test double-else error
+\if true
+\else
+\else
+encountered \else after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+encountered \elif after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..ba41d9e 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- test invalidation of whole if-endif when invalid boolean is given
+\if invalid_boolean_expression
+	\echo 'should not print #6-1'
+\else
+	\echo 'should not print because whole if-then is wrecked #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..c0ba4c4 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,67 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..390bccd 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,18 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("command ignored, use \\endif or Ctrl-C to exit "
+						"current branch.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1043,154 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block. <expr> must be a
+	 * valid boolean expression, or the command will be ignored. If this \if
+	 * is itself a part of a branch that is false/ignored, it too will
+	 * automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				pset.active_branch = if_true;
+				psqlscan_branch_push(scan_state,
+					(if_true) ? IFSTATE_TRUE : IFSTATE_FALSE);
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				pset.active_branch = false;
+				if (success && elif_true)
+				{
+					psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+					pset.active_branch = true;
+				}
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\elif: cannot occur after \\else\n");
+				success = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("\\elif: no matching \\if\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("\\else: no matching \\if\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\else: cannot occur after \\else\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		success = psqlscan_branch_pop(scan_state);
+		if (success)
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		else
+			psql_error("\\endif: no matching \\if\n");
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 5349c39..698ca1f 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..82fbf20 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,22 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("query ignored, use \\endif or Ctrl-C to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +138,19 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					psql_error("\\if: escaped\n");
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +324,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +386,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +395,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +458,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +479,19 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/*
+	 * check for unbalanced \if-\endifs unless user explicitly quit,
+	 * or the script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& successResult != EXIT_USER
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error && !pset.cur_cmd_interactive)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..aadd3f0
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,42 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 21;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', '.elif: no matching .if', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', '.else: no matching .if', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', '.endif: no matching .if', 'unmatched \endif' ],
+	# error stop messages
+	[ "\\set ON_ERROR_STOP on\n\\if error\n\\echo NO\n\\endif\n\\echo NO\n", '', 
+		'unrecognized value "error" for ".if <expr>": boolean expected', 'if error'],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..998630a 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -919,6 +921,13 @@ psql_scan_destroy(PsqlScanState state)
 
 	yylex_destroy(state->scanner);
 
+	while (state->branch_stack != NULL)
+	{
+		IfStackElem *p = state->branch_stack;
+		state->branch_stack = state->branch_stack->next;
+		free(p);
+	}
+
 	free(state);
 }
 
@@ -1426,3 +1435,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..22b8102 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,99 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+	\echo 'will print anyway #6-1'
+will print anyway #6-1
+\else
+\else: no matching \if
+	\echo 'will print anyway #6-2'
+will print anyway #6-2
+\endif
+\endif: no matching \if
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+\endif: no matching \if
+-- test un-matched else
+\else
+\else: no matching \if
+-- test un-matched elif
+\elif
+\elif: no matching \if
+-- test double-else error
+\if true
+\else
+\else
+\else: cannot occur after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+\elif: cannot occur after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..c37e744 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+	\echo 'will print anyway #6-1'
+\else
+	\echo 'will print anyway #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..bdef6e9 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,77 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        <para>
+        Expressions that do not properly evaluate to true or false will
+        generate an error and cause the <command>\if</command> or
+        <command>\elif</command> command to fail.  Because that behavior may
+        change branching context in undesirable ways (executing code which
+        was intended to be skipped, causing <command>\elif</command>,
+        <command>\else</command>, and <command>\endif</command> commands to
+        pair with the wrong <command>\if</command>, etc), it is
+        recommended that scripts which use conditionals also set
+        <varname>ON_ERROR_STOP</varname>.
+        </para>
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..1492b66 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -21,7 +21,7 @@ REFDOCDIR= $(top_srcdir)/doc/src/sgml/ref
 override CPPFLAGS := -I. -I$(srcdir) -I$(libpq_srcdir) $(CPPFLAGS)
 LDFLAGS += -L$(top_builddir)/src/fe_utils -lpgfeutils -lpq
 
-OBJS=	command.o common.o help.o input.o stringutils.o mainloop.o copy.o \
+OBJS=	command.o common.o conditional.o help.o input.o stringutils.o mainloop.o copy.o \
 	startup.o prompt.o variables.o large_obj.o describe.o \
 	crosstabview.o tab-complete.o \
 	sql_help.o psqlscanslash.o \
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..fded2bc 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -38,6 +38,7 @@
 #include "fe_utils/string_utils.h"
 
 #include "common.h"
+#include "conditional.h"
 #include "copy.h"
 #include "crosstabview.h"
 #include "describe.h"
@@ -62,6 +63,7 @@ typedef enum EditableObjectType
 /* functions for use in this file */
 static backslashResult exec_command(const char *cmd,
 			 PsqlScanState scan_state,
+			 ConditionalStack cstack,
 			 PQExpBuffer query_buf);
 static bool do_edit(const char *filename_arg, PQExpBuffer query_buf,
 		int lineno, bool *edited);
@@ -109,6 +111,7 @@ static void checkWin32Codepage(void);
 
 backslashResult
 HandleSlashCmds(PsqlScanState scan_state,
+				ConditionalStack cstack,
 				PQExpBuffer query_buf)
 {
 	backslashResult status = PSQL_CMD_SKIP_LINE;
@@ -121,7 +124,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 	cmd = psql_scan_slash_command(scan_state);
 
 	/* And try to execute it */
-	status = exec_command(cmd, scan_state, query_buf);
+	status = exec_command(cmd, scan_state, cstack, query_buf);
 
 	if (status == PSQL_CMD_UNKNOWN)
 	{
@@ -132,7 +135,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +197,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -201,12 +228,25 @@ read_connect_arg(PsqlScanState scan_state)
 static backslashResult
 exec_command(const char *cmd,
 			 PsqlScanState scan_state,
+			 ConditionalStack cstack,
 			 PQExpBuffer query_buf)
 {
 	bool		success = true; /* indicate here if the command ran ok or
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("command ignored, use \\endif or Ctrl-C to exit "
+						"current branch.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1046,154 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block. <expr> must be a
+	 * valid boolean expression, or the command will be ignored. If this \if
+	 * is itself a part of a branch that is false/ignored, it too will
+	 * automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				pset.active_branch = if_true;
+				conditional_stack_push(cstack,
+					(if_true) ? IFSTATE_TRUE : IFSTATE_FALSE);
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (conditional_stack_peek(cstack))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				conditional_stack_poke(cstack, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				pset.active_branch = false;
+				if (success && elif_true)
+				{
+					conditional_stack_poke(cstack, IFSTATE_TRUE);
+					pset.active_branch = true;
+				}
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\elif: cannot occur after \\else\n");
+				success = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("\\elif: no matching \\if\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (conditional_stack_peek(cstack))
+		{
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				conditional_stack_poke(cstack, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				pset.active_branch = false;
+				conditional_stack_poke(cstack, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("\\else: no matching \\if\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\else: cannot occur after \\else\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		success = conditional_stack_pop(cstack);
+		if (success)
+			pset.active_branch = conditional_active(cstack);
+		else
+			psql_error("\\endif: no matching \\if\n");
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/command.h b/src/bin/psql/command.h
index d0c3264..a396f29 100644
--- a/src/bin/psql/command.h
+++ b/src/bin/psql/command.h
@@ -10,6 +10,7 @@
 
 #include "fe_utils/print.h"
 #include "fe_utils/psqlscan.h"
+#include "conditional.h"
 
 
 typedef enum _backslashResult
@@ -25,6 +26,7 @@ typedef enum _backslashResult
 
 
 extern backslashResult HandleSlashCmds(PsqlScanState scan_state,
+				ConditionalStack cstack,
 				PQExpBuffer query_buf);
 
 extern int	process_file(char *filename, bool use_relative_path);
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 5349c39..698ca1f 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/conditional.c b/src/bin/psql/conditional.c
new file mode 100644
index 0000000..1875329
--- /dev/null
+++ b/src/bin/psql/conditional.c
@@ -0,0 +1,107 @@
+/*
+ * psql - the PostgreSQL interactive terminal
+ *
+ * Copyright (c) 2000-2017, PostgreSQL Global Development Group
+ *
+ * src/bin/psql/conditional.c
+ */
+#include "postgres_fe.h"
+#include "conditional.h"
+
+/*
+ * create stack
+ */
+ConditionalStack
+conditional_stack_create()
+{
+	ConditionalStack cstack = pg_malloc0(sizeof(ConditionalStackData));
+	cstack->head = NULL;
+	return cstack;
+}
+
+/*
+ * destroy stack
+ */
+void
+conditional_stack_destroy(const ConditionalStack cstack)
+{
+	while (cstack->head != NULL)
+	{
+		IfStackElem *p = cstack->head;
+		cstack->head = cstack->head->next;
+		free(p);
+	}
+	free(cstack);
+}
+
+/*
+ * Create a new conditional branch.
+ */
+bool
+conditional_stack_push(const ConditionalStack cstack,
+						ifState new_state)
+{
+	IfStackElem *p = (IfStackElem*) pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = cstack->head;
+	cstack->head = p;
+	return true;
+}
+
+/*
+ * Destroy the topmost conditional branch.
+ * Returns false if there was no branch to end.
+ */
+bool
+conditional_stack_pop(const ConditionalStack cstack)
+{
+	IfStackElem *p = cstack->head;
+	if (!p)
+		return false;
+	cstack->head = cstack->head->next;
+	free(p);
+	return true;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+conditional_stack_peek(const ConditionalStack cstack)
+{
+	if (conditional_stack_empty(cstack))
+		return IFSTATE_NONE;
+	return cstack->head->if_state;
+}
+
+/*
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+conditional_stack_poke(const ConditionalStack cstack, ifState new_state)
+{
+	if (conditional_stack_empty(cstack))
+		return false;
+	cstack->head->if_state = new_state;
+	return true;
+}
+
+/*
+ * True if there are no active \if-structures
+ */
+bool
+conditional_stack_empty(const ConditionalStack cstack)
+{
+	return cstack->head == NULL;
+}
+
+/*
+ * True if the current conditional block (if any) is true
+ */
+bool
+conditional_active(const ConditionalStack cstack)
+{
+	ifState s = conditional_stack_peek(cstack);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
diff --git a/src/bin/psql/conditional.h b/src/bin/psql/conditional.h
new file mode 100644
index 0000000..e947760
--- /dev/null
+++ b/src/bin/psql/conditional.h
@@ -0,0 +1,65 @@
+/*
+ * psql - the PostgreSQL interactive terminal
+ *
+ * Copyright (c) 2000-2017, PostgreSQL Global Development Group
+ *
+ * src/bin/psql/conditional.h
+ */
+#ifndef CONDITIONAL_H
+#define CONDITIONAL_H
+
+#include "postgres_fe.h"
+
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
+/*
+ * A mutable stack needs an immutable structure to be passed around
+ */
+typedef struct ConditionalStackData
+{
+	IfStackElem	*head;
+} ConditionalStackData;
+
+typedef struct ConditionalStackData *ConditionalStack;
+
+extern ConditionalStack conditional_stack_create(void);
+
+extern void conditional_stack_destroy(ConditionalStack cstack);
+
+extern bool conditional_stack_empty(ConditionalStack cstack);
+
+extern bool conditional_stack_push(ConditionalStack cstack,
+									ifState new_state);
+
+extern bool conditional_stack_pop(ConditionalStack cstack);
+
+extern ifState conditional_stack_peek(ConditionalStack cstack);
+
+extern bool conditional_stack_poke(ConditionalStack cstack,
+									ifState new_state);
+
+extern bool conditional_active(ConditionalStack cstack);
+
+#endif   /* CONDITIONAL_H */
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..240357c 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -10,12 +10,13 @@
 
 #include "command.h"
 #include "common.h"
+#include "conditional.h"
 #include "input.h"
 #include "prompt.h"
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +24,22 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("query ignored, use \\endif or Ctrl-C to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -50,6 +67,7 @@ MainLoop(FILE *source)
 	volatile promptStatus_t prompt_status = PROMPT_READY;
 	volatile int count_eof = 0;
 	volatile bool die_on_error = false;
+	ConditionalStack cond_stack;
 
 	/* Save the prior command source */
 	FILE	   *prev_cmd_source;
@@ -69,6 +87,7 @@ MainLoop(FILE *source)
 
 	/* Create working state */
 	scan_state = psql_scan_create(&psqlscan_callbacks);
+	cond_stack = conditional_stack_create();
 
 	query_buf = createPQExpBuffer();
 	previous_buf = createPQExpBuffer();
@@ -122,7 +141,20 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!conditional_stack_empty(cond_stack))
+				{
+					psql_error("\\if: escaped\n");
+					conditional_stack_pop(cond_stack);
+					pset.active_branch =
+						conditional_active(cond_stack);
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +328,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -343,6 +375,7 @@ MainLoop(FILE *source)
 
 				/* execute backslash command */
 				slashCmdStatus = HandleSlashCmds(scan_state,
+												 cond_stack,
 												 query_buf->len > 0 ?
 												 query_buf : previous_buf);
 
@@ -358,7 +391,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +400,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +463,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,7 +484,21 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/*
+	 * check for unbalanced \if-\endifs unless user explicitly quit,
+	 * or the script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& successResult != EXIT_USER
+		&& !conditional_stack_empty(cond_stack))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error && !pset.cur_cmd_interactive)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
+	conditional_stack_destroy(cond_stack);
 
 	pset.cur_cmd_source = prev_cmd_source;
 	pset.cur_cmd_interactive = prev_cmd_interactive;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..a67c89f 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
@@ -335,19 +337,24 @@ main(int argc, char *argv[])
 			else if (cell->action == ACT_SINGLE_SLASH)
 			{
 				PsqlScanState scan_state;
+				ConditionalStack cond_stack;
 
 				if (pset.echo == PSQL_ECHO_ALL)
 					puts(cell->val);
 
 				scan_state = psql_scan_create(&psqlscan_callbacks);
+				cond_stack = conditional_stack_create();
 				psql_scan_setup(scan_state,
 								cell->val, strlen(cell->val),
 								pset.encoding, standard_strings());
 
-				successResult = HandleSlashCmds(scan_state, NULL) != PSQL_CMD_ERROR
+				successResult = HandleSlashCmds(scan_state,
+												cond_stack,
+												NULL) != PSQL_CMD_ERROR
 					? EXIT_SUCCESS : EXIT_FAILURE;
 
 				psql_scan_destroy(scan_state);
+				conditional_stack_destroy(cond_stack);
 			}
 			else if (cell->action == ACT_FILE)
 			{
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..15dbca1
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,42 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 21;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', 'syntax error' ],
+	# unmatched checks
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', '.elif: no matching .if', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', '.else: no matching .if', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', '.endif: no matching .if', 'unmatched \endif' ],
+	# error stop messages
+	[ "\\set ON_ERROR_STOP on\n\\if error\n\\echo NO\n\\endif\n\\echo NO\n", '',
+		'unrecognized value "error" for ".if <expr>": boolean expected', 'if error'],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..22b8102 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,99 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+	\echo 'will print anyway #6-1'
+will print anyway #6-1
+\else
+\else: no matching \if
+	\echo 'will print anyway #6-2'
+will print anyway #6-2
+\endif
+\endif: no matching \if
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+\endif: no matching \if
+-- test un-matched else
+\else
+\else: no matching \if
+-- test un-matched elif
+\elif
+\elif: no matching \if
+-- test double-else error
+\if true
+\else
+\else
+\else: cannot occur after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+\elif: cannot occur after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..c37e744 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+	\echo 'will print anyway #6-1'
+\else
+	\echo 'will print anyway #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..e86b66b 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,79 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+-- set ON_ERROR_STOP in case the variables are not valid boolean expressions
+\set ON_ERROR_STOP on
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Expressions that do not properly evaluate to true or false will
+        generate an error and cause the <command>\if</command> or
+        <command>\elif</command> command to fail.  Because that behavior may
+        change branching context in undesirable ways (executing code which
+        was intended to be skipped, causing <command>\elif</command>,
+        <command>\else</command>, and <command>\endif</command> commands to
+        pair with the wrong <command>\if</command>, etc), it is
+        recommended that scripts which use conditionals also set
+        <varname>ON_ERROR_STOP</varname>.
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..390bccd 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,18 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("command ignored, use \\endif or Ctrl-C to exit "
+						"current branch.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1043,154 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block. <expr> must be a
+	 * valid boolean expression, or the command will be ignored. If this \if
+	 * is itself a part of a branch that is false/ignored, it too will
+	 * automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				pset.active_branch = if_true;
+				psqlscan_branch_push(scan_state,
+					(if_true) ? IFSTATE_TRUE : IFSTATE_FALSE);
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				pset.active_branch = false;
+				if (success && elif_true)
+				{
+					psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+					pset.active_branch = true;
+				}
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\elif: cannot occur after \\else\n");
+				success = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("\\elif: no matching \\if\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("\\else: no matching \\if\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\else: cannot occur after \\else\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		success = psqlscan_branch_pop(scan_state);
+		if (success)
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		else
+			psql_error("\\endif: no matching \\if\n");
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 5349c39..698ca1f 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..82fbf20 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,22 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("query ignored, use \\endif or Ctrl-C to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +138,19 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					psql_error("\\if: escaped\n");
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +324,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +386,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +395,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +458,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +479,19 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/*
+	 * check for unbalanced \if-\endifs unless user explicitly quit,
+	 * or the script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& successResult != EXIT_USER
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error && !pset.cur_cmd_interactive)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..53a0b29 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -913,6 +915,9 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 void
 psql_scan_destroy(PsqlScanState state)
 {
+	while (psqlscan_branch_pop(state))
+		continue;
+
 	psql_scan_finish(state);
 
 	psql_scan_reset(state);
@@ -1426,3 +1431,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..22b8102 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,99 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+	\echo 'will print anyway #6-1'
+will print anyway #6-1
+\else
+\else: no matching \if
+	\echo 'will print anyway #6-2'
+will print anyway #6-2
+\endif
+\endif: no matching \if
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+\endif: no matching \if
+-- test un-matched else
+\else
+\else: no matching \if
+-- test un-matched elif
+\elif
+\elif: no matching \if
+-- test double-else error
+\if true
+\else
+\else
+\else: cannot occur after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+\elif: cannot occur after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..c37e744 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+	\echo 'will print anyway #6-1'
+\else
+	\echo 'will print anyway #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index ae58708..7e59192 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -2035,6 +2035,81 @@ hello 10
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\if</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\elif</literal> <replaceable class="parameter">expr</replaceable></term>
+        <term><literal>\else</literal></term>
+        <term><literal>\endif</literal></term>
+        <listitem>
+        <para>
+        This group of commands implements nestable conditional blocks, like
+        this:
+        </para>
+<programlisting>
+-- stop the script if the condition variables are invalid boolean expressions
+\set ON_ERROR_STOP on
+-- check for the existence of two separate records in the database and store
+-- the results in separate psql variables
+SELECT
+    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
+    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
+\gset
+\if :is_customer
+    SELECT * FROM customer WHERE customer_id = 123;
+\elif :is_employee
+    \echo 'is not a customer but is an employee'
+    SELECT * FROM employee WHERE employee_id = 456;
+\else
+    \if yes
+        \echo 'not a customer or employee'
+    \else
+        \echo 'this should never print'
+    \endif
+\endif
+</programlisting>
+        <para>
+        Conditional blocks must begin with a <command>\if</command> and end
+        with an <command>\endif</command>, and the pairs must be found in
+        the same source file. If an EOF is reached on the main file or an
+        <command>\include</command>-ed file before all local
+        <command>\if</command>-<command>\endif</command> are matched, then
+        psql will raise an error.
+        </para>
+        <para>
+        A conditional block can have any number of 
+        <command>\elif</command> clauses, which may optionally be followed by a
+        single <command>\else</command> clause.
+        </para>
+        <para>
+        The <command>\if</command> and <command>\elif</command> commands
+        read the rest of the line and evaluate it as a boolean expression.
+        Currently, expressions are limited to a single unquoted string
+        which are evaluated like other on/off options, so the valid values
+        are any unambiguous case insensitive matches for one of:
+        <literal>true</literal>, <literal>false</literal>, <literal>1</literal>,
+        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
+        <literal>yes</literal>, <literal>no</literal>.  So 
+        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
+        will all match <literal>true</literal>.
+        </para>
+        <para>
+        Expressions that do not properly evaluate to true or false will
+        generate an error and cause the <command>\if</command> or
+        <command>\elif</command> command to fail.  Because that behavior may
+        change branching context in undesirable ways (executing code which
+        was intended to be skipped, causing <command>\elif</command>,
+        <command>\else</command>, and <command>\endif</command> commands to
+        pair with the wrong <command>\if</command>, etc), it is
+        recommended that scripts which use conditionals also set
+        <varname>ON_ERROR_STOP</varname>.
+        <para>
+        Lines within false branches are not evaluated in any way: queries are
+        not sent to the server, non-conditional commands are not evaluated but
+        bluntly ignored, nested if-expressions in such branches are also not
+        evaluated but are tallied to check for proper nesting.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
diff --git a/src/bin/psql/Makefile b/src/bin/psql/Makefile
index c53733f..7a418c6 100644
--- a/src/bin/psql/Makefile
+++ b/src/bin/psql/Makefile
@@ -61,8 +61,16 @@ uninstall:
 
 clean distclean:
 	rm -f psql$(X) $(OBJS) lex.backup
+	rm -rf tmp_check
 
 # files removed here are supposed to be in the distribution tarball,
 # so do not clean them in the clean/distclean rules
 maintainer-clean: distclean
 	rm -f sql_help.h sql_help.c psqlscanslash.c
+
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f17f610..390bccd 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -49,6 +49,7 @@
 #include "psqlscanslash.h"
 #include "settings.h"
 #include "variables.h"
+#include "fe_utils/psqlscan_int.h"
 
 /*
  * Editable database object types.
@@ -132,7 +133,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		status = PSQL_CMD_ERROR;
 	}
 
-	if (status != PSQL_CMD_ERROR)
+	if (status != PSQL_CMD_ERROR && pset.active_branch)
 	{
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
@@ -194,6 +195,30 @@ read_connect_arg(PsqlScanState scan_state)
 	return result;
 }
 
+/*
+ * Read and interpret argument as a boolean expression.
+ * Return true if a boolean value was successfully parsed.
+ */
+static bool
+read_boolean_expression(PsqlScanState scan_state, char *action,
+						bool *result)
+{
+	char	*value = psql_scan_slash_option(scan_state,
+											OT_NORMAL, NULL, false);
+	bool	success = ParseVariableBool(value, action, result);
+	free(value);
+	return success;
+}
+
+/*
+ * Return true if the command given is a branching command.
+ */
+static bool
+is_branching_command(const char *cmd)
+{
+	return (strcmp(cmd, "if") == 0 || strcmp(cmd, "elif") == 0
+			|| strcmp(cmd, "else") == 0 || strcmp(cmd, "endif") == 0);
+}
 
 /*
  * Subroutine to actually try to execute a backslash command.
@@ -207,6 +232,18 @@ exec_command(const char *cmd,
 								 * failed */
 	backslashResult status = PSQL_CMD_SKIP_LINE;
 
+	if (!pset.active_branch && !is_branching_command(cmd))
+	{
+		if (pset.cur_cmd_interactive)
+			psql_error("command ignored, use \\endif or Ctrl-C to exit "
+						"current branch.\n");
+
+		/* Continue with an empty buffer as if the command were never read */
+		resetPQExpBuffer(query_buf);
+		psql_scan_reset(scan_state);
+		return status;
+	}
+
 	/*
 	 * \a -- toggle field alignment This makes little sense but we keep it
 	 * around.
@@ -1006,6 +1043,154 @@ exec_command(const char *cmd,
 		}
 	}
 
+	/*
+	 * \if <expr> is the beginning of an \if..\endif block. <expr> must be a
+	 * valid boolean expression, or the command will be ignored. If this \if
+	 * is itself a part of a branch that is false/ignored, it too will
+	 * automatically be ignored.
+	 */
+	else if (strcmp(cmd, "if") == 0)
+	{
+		/*
+		 * only evaluate the expression for truth if the underlying
+		 * branch is active
+		 */
+		if (pset.active_branch)
+		{
+			bool if_true = false;
+			success = read_boolean_expression(scan_state, "\\if <expr>", &if_true);
+			if (success)
+			{
+				pset.active_branch = if_true;
+				psqlscan_branch_push(scan_state,
+					(if_true) ? IFSTATE_TRUE : IFSTATE_FALSE);
+			}
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \elif <expr> is part of an \if..\endif block
+	 * <expr> will only be evalated for boolean truth if no previous
+	 * \if or \endif in the block has evaluated to true and the \if..\endif
+	 * block is not itself being ignored.
+	 * in the event that <expr> does not conform to a proper boolean expression,
+	 * all following statements in the block will be ignored until \endif is
+	 * encountered.
+	 *
+	 */
+	else if (strcmp(cmd, "elif") == 0)
+	{
+		bool elif_true = false;
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_IGNORED:
+				/*
+				 * inactive branch, do nothing:
+				 * either if-endif already had a true block,
+				 * or whole parent block is false.
+				 */
+				break;
+			case IFSTATE_TRUE:
+				/*
+				 * just finished true section of active branch
+				 * do not evaluate expression, just skip
+				 */
+				psqlscan_branch_set_state(scan_state, IFSTATE_IGNORED);
+				pset.active_branch = false;
+				break;
+			case IFSTATE_FALSE:
+				/*
+				 * have not yet found a true block in this if-endif,
+				 * determine if this section is true or not.
+				 * variable expansion must be temporarily turned back
+				 * on to read the boolean expression.
+				 */
+				pset.active_branch = true;
+				success = read_boolean_expression(scan_state, "\\elif <expr>",
+													&elif_true);
+				pset.active_branch = false;
+				if (success && elif_true)
+				{
+					psqlscan_branch_set_state(scan_state, IFSTATE_TRUE);
+					pset.active_branch = true;
+				}
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\elif: cannot occur after \\else\n");
+				success = false;
+				break;
+			case IFSTATE_NONE:
+				/* no if to elif from */
+				psql_error("\\elif: no matching \\if\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \else is part of an \if..\endif block
+	 * the statements within an \else branch will only be executed if
+	 * all previous \if and \endif expressions evaluated to false
+	 * and the block was not itself being ignored.
+	 */
+	else if (strcmp(cmd, "else") == 0)
+	{
+		switch (psqlscan_branch_get_state(scan_state))
+		{
+			case IFSTATE_FALSE:
+				/* just finished false section of an active branch */
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_TRUE);
+				pset.active_branch = true;
+				break;
+			case IFSTATE_TRUE:
+			case IFSTATE_IGNORED:
+				/*
+				 * either just finished true section of an active branch,
+				 * or whole branch was inactive. either way, be on the
+				 * lookout for any invalid \endif or \else commands
+				 */
+				pset.active_branch = false;
+				psqlscan_branch_set_state(scan_state, IFSTATE_ELSE_FALSE);
+				break;
+			case IFSTATE_NONE:
+				/* no if to else from */
+				psql_error("\\else: no matching \\if\n");
+				success = false;
+				break;
+			case IFSTATE_ELSE_TRUE:
+			case IFSTATE_ELSE_FALSE:
+				psql_error("\\else: cannot occur after \\else\n");
+				success = false;
+				break;
+			default:
+				break;
+		}
+		psql_scan_reset(scan_state);
+	}
+
+	/*
+	 * \endif - closing statment of an \if...\endif block
+	 */
+	else if (strcmp(cmd, "endif") == 0)
+	{
+		/*
+		 * get rid of this ifstate element and look at the previous
+		 * one, if any
+		 */
+		success = psqlscan_branch_pop(scan_state);
+		if (success)
+			pset.active_branch = psqlscan_branch_active(scan_state);
+		else
+			psql_error("\\endif: no matching \\if\n");
+
+		psql_scan_reset(scan_state);
+	}
+
 	/* \l is list databases */
 	else if (strcmp(cmd, "l") == 0 || strcmp(cmd, "list") == 0 ||
 			 strcmp(cmd, "l+") == 0 || strcmp(cmd, "list+") == 0)
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 5349c39..698ca1f 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -119,6 +119,11 @@ setQFout(const char *fname)
  * If "escape" is true, return the value suitably quoted and escaped,
  * as an identifier or string literal depending on "as_ident".
  * (Failure in escaping should lead to returning NULL.)
+ *
+ * Variables are not expanded if the current branch is inactive
+ * (part of an \if..\endif section which is false). \elif branches
+ * will need temporarily mark the branch active in order to
+ * properly evaluate conditionals.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident)
@@ -126,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident)
 	char	   *result;
 	const char *value;
 
+	/* do not expand variables if the branch is inactive */
+	if (!pset.active_branch)
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 3e3cab4..9f9e1a6 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -210,6 +210,13 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\qecho [STRING]        write string to query output stream (see \\o)\n"));
 	fprintf(output, "\n");
 
+	fprintf(output, _("Conditionals\n"));
+	fprintf(output, _("  \\if <expr>             begin a conditional block\n"));
+	fprintf(output, _("  \\elif <expr>           else if in the current conditional block\n"));
+	fprintf(output, _("  \\else                  else in the current conditional block\n"));
+	fprintf(output, _("  \\endif                 end current conditional block\n"));
+	fprintf(output, "\n");
+
 	fprintf(output, _("Informational\n"));
 	fprintf(output, _("  (options: S = show system objects, + = additional detail)\n"));
 	fprintf(output, _("  \\d[S+]                 list tables, views, and sequences\n"));
diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2..82fbf20 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -15,7 +15,7 @@
 #include "settings.h"
 
 #include "mb/pg_wchar.h"
-
+#include "fe_utils/psqlscan_int.h"
 
 /* callback functions for our flex lexer */
 const PsqlScanCallbacks psqlscan_callbacks = {
@@ -23,6 +23,22 @@ const PsqlScanCallbacks psqlscan_callbacks = {
 	psql_error
 };
 
+/*
+ * execute query if branch is active.
+ * warn interactive users about ignored queries.
+ */
+static
+bool send_query(const char *query)
+{
+	/* execute query if branch is active */
+	if (pset.active_branch)
+		return SendQuery(query);
+
+	if (pset.cur_cmd_interactive)
+		psql_error("query ignored, use \\endif or Ctrl-C to exit current branch.\n");
+
+	return true;
+}
 
 /*
  * Main processing loop for reading lines of input
@@ -122,7 +138,19 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+				/*
+				 * if interactive user is in a branch, then Ctrl-C will exit
+				 * from the inner-most branch
+				 */
+				if (!psqlscan_branch_stack_empty(scan_state))
+				{
+					psql_error("\\if: escaped\n");
+					psqlscan_branch_pop(scan_state);
+					pset.active_branch = psqlscan_branch_active(scan_state);
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -296,8 +324,8 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
+				success = send_query(query_buf->data);
+
 				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
 				pset.stmt_lineno = 1;
 
@@ -358,7 +386,7 @@ MainLoop(FILE *source)
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
-					success = SendQuery(query_buf->data);
+					success = send_query(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
 					{
@@ -367,6 +395,7 @@ MainLoop(FILE *source)
 						previous_buf = query_buf;
 						query_buf = swap_buf;
 					}
+
 					resetPQExpBuffer(query_buf);
 
 					/* flush any paren nesting info after forced send */
@@ -429,8 +458,7 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		success = send_query(query_buf->data);
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -451,6 +479,19 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(previous_buf);
 	destroyPQExpBuffer(history_buf);
 
+	/*
+	 * check for unbalanced \if-\endifs unless user explicitly quit,
+	 * or the script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE
+		&& successResult != EXIT_USER
+		&& !psqlscan_branch_stack_empty(scan_state))
+	{
+		psql_error("found EOF before closing \\endif(s)\n");
+		if (die_on_error && !pset.cur_cmd_interactive)
+			successResult = EXIT_USER;
+	}
+
 	psql_scan_destroy(scan_state);
 
 	pset.cur_cmd_source = prev_cmd_source;
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 195f5a1..c8aeab6 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -114,6 +114,9 @@ typedef struct _psqlSettings
 
 	VariableSpace vars;			/* "shell variable" repository */
 
+	bool		active_branch;	/* true if session is not in an \if branch
+								 * or the current branch is true */
+
 	/*
 	 * The remaining fields are set by assign hooks associated with entries in
 	 * "vars".  They should not be set directly except by those hook
diff --git a/src/bin/psql/startup.c b/src/bin/psql/startup.c
index 88d686a..5c8760a 100644
--- a/src/bin/psql/startup.c
+++ b/src/bin/psql/startup.c
@@ -128,6 +128,8 @@ main(int argc, char *argv[])
 	setvbuf(stderr, NULL, _IONBF, 0);
 #endif
 
+	pset.active_branch = true;
+
 	pset.progname = get_progname(argv[0]);
 
 	pset.db = NULL;
diff --git a/src/bin/psql/t/001_if.pl b/src/bin/psql/t/001_if.pl
new file mode 100644
index 0000000..ced7e98
--- /dev/null
+++ b/src/bin/psql/t/001_if.pl
@@ -0,0 +1,42 @@
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 21;
+
+#
+# test invalid \if respects ON_ERROR_STOP
+#
+my $node = get_new_node('master');
+$node->init;
+$node->start;
+
+my $tests = [
+    # syntax errors
+	[ "\\if invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', '\if syntax error' ],
+	[ "\\if false\n\\elif invalid\n\\echo NO\n\\endif\n\\echo NO\n", '', 'boolean expected', '\elif syntax error' ],
+	# unmatched checks
+	[ "\\if true\n", '', 'found EOF before closing.*endif', 'unmatched \if' ],
+	[ "\\elif true\n\\echo NO\n", '', '.elif: no matching .if', 'unmatched \elif' ],
+	[ "\\else\n\\echo NO\n", '', '.else: no matching .if', 'unmatched \else' ],
+	[ "\\endif\n\\echo NO\n", '', '.endif: no matching .if', 'unmatched \endif' ],
+	# error stop messages
+	[ "\\set ON_ERROR_STOP on\n\\if error\n\\echo NO\n\\endif\n\\echo NO\n", '',
+		'unrecognized value "error" for ".if <expr>": boolean expected', 'if error'],
+];
+
+# 3 checks per tests
+for my $test (@$tests) {
+  my ($script, $stdout_expect, $stderr_re, $name) = @$test;
+  my ($stdout, $stderr);
+  my $retcode = $node->psql('postgres', $script,
+		stdout => \$stdout, stderr => \$stderr,
+		on_error_stop => 1);
+  is($retcode,'3',"$name test ON_ERROR_STOP");
+  is($stdout, $stdout_expect, "$name test STDOUT");
+  like($stderr, qr/$stderr_re/, "$name test STDERR");
+}
+
+$node->teardown_node;
diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 1b29341..53a0b29 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -904,6 +904,8 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 
 	psql_scan_reset(state);
 
+	state->branch_stack = NULL;
+
 	return state;
 }
 
@@ -913,6 +915,9 @@ psql_scan_create(const PsqlScanCallbacks *callbacks)
 void
 psql_scan_destroy(PsqlScanState state)
 {
+	while (psqlscan_branch_pop(state))
+		continue;
+
 	psql_scan_finish(state);
 
 	psql_scan_reset(state);
@@ -1426,3 +1431,86 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+/*
+ * psqlscan_branch_stack_empty
+ *
+ * True if there are no active \if-structures
+ */
+bool
+psqlscan_branch_stack_empty(PsqlScanState state)
+{
+	return state->branch_stack == NULL;
+}
+
+/*
+ * Fetch the current state of the top of the stack
+ */
+ifState
+psqlscan_branch_get_state(PsqlScanState state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return IFSTATE_NONE;
+	return state->branch_stack->if_state;
+}
+
+/*
+ * psqlscan_branch_active
+ *
+ * True if the current \if-block (if any) is true and queries/commands
+ * should be executed.
+ */
+bool
+psqlscan_branch_active(PsqlScanState state)
+{
+	ifState s = psqlscan_branch_get_state(state);
+	return s == IFSTATE_NONE || s == IFSTATE_TRUE || s == IFSTATE_ELSE_TRUE;
+}
+
+
+/*
+ * psqlscan_branch_push
+ *
+ * Create a new \if branch.
+ */
+bool
+psqlscan_branch_push(PsqlScanState state, ifState new_state)
+{
+	IfStackElem *p = pg_malloc0(sizeof(IfStackElem));
+	p->if_state = new_state;
+	p->next = state->branch_stack;
+	state->branch_stack = p;
+	return true;
+}
+
+/*
+ * psqlscan_branch_set_state
+ *
+ * Change the state of the topmost branch.
+ * Returns false if there was branch state to set.
+ */
+bool
+psqlscan_branch_set_state(PsqlScanState state, ifState new_state)
+{
+	if (psqlscan_branch_stack_empty(state))
+		return false;
+	state->branch_stack->if_state = new_state;
+	return true;
+}
+
+/*
+ * psqlscan_branch_pop
+ *
+ * Destroy the topmost branch because and \endif was encountered.
+ * Returns false if there was no branch to end.
+ */
+bool
+psqlscan_branch_pop(PsqlScanState state)
+{
+	IfStackElem *p = state->branch_stack;
+	if (!p)
+		return false;
+	state->branch_stack = state->branch_stack->next;
+	free(p);
+	return true;
+}
diff --git a/src/include/fe_utils/psqlscan_int.h b/src/include/fe_utils/psqlscan_int.h
index 0fddc7a..321d455 100644
--- a/src/include/fe_utils/psqlscan_int.h
+++ b/src/include/fe_utils/psqlscan_int.h
@@ -75,6 +75,30 @@ typedef struct StackElem
 	struct StackElem *next;
 } StackElem;
 
+typedef enum _ifState
+{
+	IFSTATE_NONE = 0,	/* Not currently in an \if block */
+	IFSTATE_TRUE,		/* currently in an \if or \elif which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,		/* currently in an \if or \elif which is false
+						 * but no true branch has yet been seen,
+						 * and all parent branches (if any) are true */
+	IFSTATE_IGNORED,	/* currently in an \elif which follows a true \if
+						 * or the whole \if is a child of a false parent */
+	IFSTATE_ELSE_TRUE,	/* currently in an \else which is true
+						 * and all parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE	/* currently in an \else which is false or ignored */
+} ifState;
+
+/*
+ * The state of nested ifs is stored in a stack.
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;
+	struct IfStackElem *next;
+} IfStackElem;
+
 /*
  * All working state of the lexer must be stored in PsqlScanStateData
  * between calls.  This allows us to have multiple open lexer operations,
@@ -118,6 +142,11 @@ typedef struct PsqlScanStateData
 	 * Callback functions provided by the program making use of the lexer.
 	 */
 	const PsqlScanCallbacks *callbacks;
+
+	/*
+	 * \if branch state variables
+	 */
+	IfStackElem *branch_stack;
 } PsqlScanStateData;
 
 
@@ -141,4 +170,21 @@ extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 bool as_ident);
 
+/*
+ * branching commands
+ */
+extern bool psqlscan_branch_stack_empty(PsqlScanState state);
+
+extern bool psqlscan_branch_active(PsqlScanState state);
+
+extern ifState psqlscan_branch_get_state(PsqlScanState state);
+
+extern bool psqlscan_branch_push(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_set_state(PsqlScanState state,
+									ifState new_state);
+
+extern bool psqlscan_branch_pop(PsqlScanState state);
+
 #endif   /* PSQLSCAN_INT_H */
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 026a4f0..22b8102 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2712,6 +2712,99 @@ deallocate q;
 \pset format aligned
 \pset expanded off
 \pset border 1
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+all true
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+all false
+\endif
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+first thing true
+\else
+	\echo 'should not print #3-1'
+\endif
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+second thing true
+\else
+	\echo 'should not print #5-1'
+\endif
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+unrecognized value "invalid_boolean_expression" for "\if <expr>": boolean expected
+	\echo 'will print anyway #6-1'
+will print anyway #6-1
+\else
+\else: no matching \if
+	\echo 'will print anyway #6-2'
+will print anyway #6-2
+\endif
+\endif: no matching \if
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+\endif
+-- test un-matched endif
+\endif
+\endif: no matching \if
+-- test un-matched else
+\else
+\else: no matching \if
+-- test un-matched elif
+\elif
+\elif: no matching \if
+-- test double-else error
+\if true
+\else
+\else
+\else: cannot occur after \else
+\endif
+-- test elif out-of-order
+\if false
+\else
+\elif
+\elif: cannot occur after \else
+\endif
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index d823d11..c37e744 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -375,6 +375,91 @@ deallocate q;
 \pset expanded off
 \pset border 1
 
+-- test a large nested if using a variety of true-equivalents
+\if true
+	\if 1
+		\if yes
+			\if on
+				\echo 'all true'
+			\else
+				\echo 'should not print #1-1'
+			\endif
+		\else
+			\echo 'should not print #1-2'
+		\endif
+	\else
+		\echo 'should not print #1-3'
+	\endif
+\else
+	\echo 'should not print #1-4'
+\endif
+
+-- test a variety of false-equivalents in an if/elif/else structure
+\if false
+	\echo 'should not print #2-1'
+\elif 0
+	\echo 'should not print #2-2'
+\elif no
+	\echo 'should not print #2-3'
+\elif off
+	\echo 'should not print #2-4'
+\else
+	\echo 'all false'
+\endif
+
+-- test simple true-then-else
+\if true
+	\echo 'first thing true'
+\else
+	\echo 'should not print #3-1'
+\endif
+
+-- test simple false-true-else
+\if false
+	\echo 'should not print #4-1'
+\elif true
+	\echo 'second thing true'
+\else
+	\echo 'should not print #5-1'
+\endif
+
+-- invalid boolean expressions mean the \if is ignored
+\if invalid_boolean_expression
+	\echo 'will print anyway #6-1'
+\else
+	\echo 'will print anyway #6-2'
+\endif
+
+-- test non-evaluation of variables in false blocks
+\set var 'ab''cd'
+-- select :var;
+\if false
+  select :var ;
+\else
+  select 1;
+\endif
+
+-- test un-matched endif
+\endif
+
+-- test un-matched else
+\else
+
+-- test un-matched elif
+\elif
+
+-- test double-else error
+\if true
+\else
+\else
+\endif
+
+-- test elif out-of-order
+\if false
+\else
+\elif
+\endif
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never