Add CINE for ALTER TABLE ... ADD COLUMN

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 6a82730..3041b09 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -36,7 +36,7 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+    ADD [ COLUMN ] [ IF NOT EXISTS ]<replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> [ RESTRICT | CASCADE ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ USING <replaceable class="PARAMETER">expression</replaceable> ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
@@ -96,11 +96,12 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
   <variablelist>
    <varlistentry>
-    <term><literal>ADD COLUMN</literal></term>
+    <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term>
     <listitem>
      <para>
       This form adds a new column to the table, using the same syntax as
-      <xref linkend="SQL-CREATETABLE">.
+      <xref linkend="SQL-CREATETABLE">. If <literal>IF NOT EXISTS</literal>
+      is specified and the column already exists, no error is thrown.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 06e4332..8cd436d 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -328,8 +328,8 @@ static void ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recu
 				bool is_view, AlterTableCmd *cmd, LOCKMODE lockmode);
 static ObjectAddress ATExecAddColumn(List **wqueue, AlteredTableInfo *tab,
 				Relation rel, ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode);
-static void check_for_column_name_collision(Relation rel, const char *colname);
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode);
+static bool check_for_column_name_collision(Relation rel, const char *colname, bool if_not_exists);
 static void add_column_datatype_dependency(Oid relid, int32 attnum, Oid typid);
 static void add_column_collation_dependency(Oid relid, int32 attnum, Oid collid);
 static void ATPrepAddOids(List **wqueue, Relation rel, bool recurse,
@@ -2294,7 +2294,7 @@ renameatt_internal(Oid myrelid,
 						oldattname)));
 
 	/* new name should not already exist */
-	check_for_column_name_collision(targetrelation, newattname);
+	check_for_column_name_collision(targetrelation, newattname, false);
 
 	/* apply the update */
 	namestrcpy(&(attform->attname), newattname);
@@ -3443,11 +3443,11 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddColumnToView:		/* add column via CREATE OR REPLACE
 										 * VIEW */
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, false, false, lockmode);
+							false, false, false, false, lockmode);
 			break;
 		case AT_AddColumnRecurse:
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, true, false, lockmode);
+									  false, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_ColumnDefault:	/* ALTER COLUMN DEFAULT */
 			address = ATExecColumnDefault(rel, cmd->name, cmd->def, lockmode);
@@ -3490,19 +3490,19 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			break;
 		case AT_AddConstraint:	/* ADD CONSTRAINT */
 			address =
-				ATExecAddConstraint(wqueue, tab, rel, (Constraint *) cmd->def,
-									false, false, lockmode);
+			ATExecAddConstraint(wqueue, tab, rel, (Constraint *) cmd->def,
+								false, false, lockmode);
 			break;
 		case AT_AddConstraintRecurse:	/* ADD CONSTRAINT with recursion */
 			address =
-				ATExecAddConstraint(wqueue, tab, rel, (Constraint *) cmd->def,
-									true, false, lockmode);
+			ATExecAddConstraint(wqueue, tab, rel, (Constraint *) cmd->def,
+								true, false, lockmode);
 			break;
 		case AT_ReAddConstraint:		/* Re-add pre-existing check
 										 * constraint */
 			address =
-				ATExecAddConstraint(wqueue, tab, rel, (Constraint *) cmd->def,
-									false, true, lockmode);
+			ATExecAddConstraint(wqueue, tab, rel, (Constraint *) cmd->def,
+								false, true, lockmode);
 			break;
 		case AT_AddIndexConstraint:		/* ADD CONSTRAINT USING INDEX */
 			address = ATExecAddIndexConstraint(tab, rel, (IndexStmt *) cmd->def,
@@ -3557,14 +3557,14 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, false, false, lockmode);
+									true, false, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_AddOidsRecurse:	/* SET WITH OIDS */
 			/* Use the ADD COLUMN code, unless prep decided to do nothing */
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, true, false, lockmode);
+									true, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_DropOids:		/* SET WITHOUT OIDS */
 
@@ -4658,7 +4658,7 @@ ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 static ObjectAddress
 ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 				ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode)
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode)
 {
 	Oid			myrelid = RelationGetRelid(rel);
 	Relation	pgclass,
@@ -4753,7 +4753,13 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	relkind = ((Form_pg_class) GETSTRUCT(reltup))->relkind;
 
 	/* new name should not already exist */
-	check_for_column_name_collision(rel, colDef->colname);
+	if (!check_for_column_name_collision(rel, colDef->colname, if_not_exists))
+	{
+		heap_close(attrdesc, RowExclusiveLock);
+		heap_freetuple(reltup);
+		heap_close(pgclass, RowExclusiveLock);
+		return InvalidObjectAddress;
+	}
 
 	/* Determine the new attribute's number */
 	if (isOid)
@@ -4981,9 +4987,10 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		/* Find or create work queue entry for this table */
 		childtab = ATGetQueueEntry(wqueue, childrel);
 
-		/* Recurse to child; return value is ignored */
+		/* Recurse to child */
 		ATExecAddColumn(wqueue, childtab, childrel,
-						colDef, isOid, recurse, true, lockmode);
+						colDef, isOid, recurse, true,
+						if_not_exists, lockmode);
 
 		heap_close(childrel, NoLock);
 	}
@@ -4996,8 +5003,8 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
  * If a new or renamed column will collide with the name of an existing
  * column, error out.
  */
-static void
-check_for_column_name_collision(Relation rel, const char *colname)
+static bool
+check_for_column_name_collision(Relation rel, const char *colname, bool if_not_exists)
 {
 	HeapTuple	attTuple;
 	int			attnum;
@@ -5010,7 +5017,7 @@ check_for_column_name_collision(Relation rel, const char *colname)
 							   ObjectIdGetDatum(RelationGetRelid(rel)),
 							   PointerGetDatum(colname));
 	if (!HeapTupleIsValid(attTuple))
-		return;
+		return true;
 
 	attnum = ((Form_pg_attribute) GETSTRUCT(attTuple))->attnum;
 	ReleaseSysCache(attTuple);
@@ -5026,10 +5033,23 @@ check_for_column_name_collision(Relation rel, const char *colname)
 			 errmsg("column name \"%s\" conflicts with a system column name",
 					colname)));
 	else
+	{
+		if (if_not_exists)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("column \"%s\" of relation \"%s\" already exists, skipping",
+							colname, RelationGetRelationName(rel))));
+			return false;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_COLUMN),
 				 errmsg("column \"%s\" of relation \"%s\" already exists",
 						colname, RelationGetRelationName(rel))));
+	}
+
+	return true;
 }
 
 /*
@@ -5936,17 +5956,17 @@ ATExecAddIndexConstraint(AlteredTableInfo *tab, Relation rel,
 
 	/* Create the catalog entries for the constraint */
 	address = index_constraint_create(rel,
-									  index_oid,
-									  indexInfo,
-									  constraintName,
-									  constraintType,
-									  stmt->deferrable,
-									  stmt->initdeferred,
-									  stmt->primary,
-									  true,		/* update pg_index */
-									  true,		/* remove old dependencies */
-									  allowSystemTableMods,
-									  false);		/* is_internal */
+							index_oid,
+							indexInfo,
+							constraintName,
+							constraintType,
+							stmt->deferrable,
+							stmt->initdeferred,
+							stmt->primary,
+							true,		/* update pg_index */
+							true,		/* remove old dependencies */
+							allowSystemTableMods,
+							false);		/* is_internal */
 
 	index_close(indexRel, NoLock);
 
@@ -5977,9 +5997,9 @@ ATExecAddConstraint(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	{
 		case CONSTR_CHECK:
 			address =
-				ATAddCheckConstraint(wqueue, tab, rel,
-									 newConstraint, recurse, false, is_readd,
-									 lockmode);
+			ATAddCheckConstraint(wqueue, tab, rel,
+								 newConstraint, recurse, false, is_readd,
+								 lockmode);
 			break;
 
 		case CONSTR_FOREIGN:
@@ -6075,7 +6095,7 @@ ATAddCheckConstraint(List **wqueue, AlteredTableInfo *tab, Relation rel,
 										!recursing,		/* is_local */
 										is_readd);		/* is_internal */
 
-	/* we don't expect more than one constraint here */
+	/* Add each to-be-validated constraint to Phase 3's queue */
 	Assert(list_length(newcons) <= 1);
 
 	/* Add each to-be-validated constraint to Phase 3's queue */
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 5818858..79f6ae7 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1919,6 +1919,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $2;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD IF NOT EXISTS <coldef> */
+			| ADD_P IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $5;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ADD COLUMN <coldef> */
@@ -1927,6 +1937,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $3;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD COLUMN IF NOT EXISTS <coldef> */
+			| ADD_P COLUMN IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $6;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> {SET DEFAULT <expr>|DROP DEFAULT} */
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 65274bc..a4960e3 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2539,3 +2539,92 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+ c4     | integer | 
+
+DROP TABLE test_add_column;
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index b5ee7b0..0d88e8a 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1687,3 +1687,34 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+\d test_add_column
+DROP TABLE test_add_column;

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 207fec1..339320e 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -36,7 +36,7 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+    ADD [ COLUMN ] [ IF NOT EXISTS ]<replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> [ RESTRICT | CASCADE ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ USING <replaceable class="PARAMETER">expression</replaceable> ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
@@ -96,11 +96,12 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
   <variablelist>
    <varlistentry>
-    <term><literal>ADD COLUMN</literal></term>
+    <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term>
     <listitem>
      <para>
       This form adds a new column to the table, using the same syntax as
-      <xref linkend="SQL-CREATETABLE">.
+      <xref linkend="SQL-CREATETABLE">. If <literal>IF NOT EXISTS</literal>
+      is specified and the column already exists, no error is thrown.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index d394713..2257ca2 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -306,14 +306,14 @@ static void createForeignKeyTriggers(Relation rel, Oid refRelOid,
 						 Constraint *fkconstraint,
 						 Oid constraintOid, Oid indexOid);
 static void ATController(AlterTableStmt *parsetree,
-			 Relation rel, List *cmds, bool recurse, LOCKMODE lockmode);
+						 Relation rel, List *cmds, bool recurse, LOCKMODE lockmode);
 static void ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 		  bool recurse, bool recursing, LOCKMODE lockmode);
 static void ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode);
 static void ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		  AlterTableCmd *cmd, LOCKMODE lockmode);
 static void ATRewriteTables(AlterTableStmt *parsetree,
-				List **wqueue, LOCKMODE lockmode);
+							List **wqueue, LOCKMODE lockmode);
 static void ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode);
 static AlteredTableInfo *ATGetQueueEntry(List **wqueue, Relation rel);
 static void ATSimplePermissions(Relation rel, int allowed_targets);
@@ -328,8 +328,8 @@ static void ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recu
 				bool is_view, AlterTableCmd *cmd, LOCKMODE lockmode);
 static ObjectAddress ATExecAddColumn(List **wqueue, AlteredTableInfo *tab,
 				Relation rel, ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode);
-static void check_for_column_name_collision(Relation rel, const char *colname);
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode);
+static bool check_for_column_name_collision(Relation rel, const char *colname, bool if_not_exists);
 static void add_column_datatype_dependency(Oid relid, int32 attnum, Oid typid);
 static void add_column_collation_dependency(Oid relid, int32 attnum, Oid collid);
 static void ATPrepAddOids(List **wqueue, Relation rel, bool recurse,
@@ -631,7 +631,7 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 
 			cooked = (CookedConstraint *) palloc(sizeof(CookedConstraint));
 			cooked->contype = CONSTR_DEFAULT;
-			cooked->conoid = InvalidOid;		/* until created */
+			cooked->conoid = InvalidOid;	/* until created */
 			cooked->name = NULL;
 			cooked->attnum = attnum;
 			cooked->expr = colDef->cooked_default;
@@ -1751,7 +1751,7 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 
 					cooked = (CookedConstraint *) palloc(sizeof(CookedConstraint));
 					cooked->contype = CONSTR_CHECK;
-					cooked->conoid = InvalidOid;		/* until created */
+					cooked->conoid = InvalidOid;	/* until created */
 					cooked->name = pstrdup(name);
 					cooked->attnum = 0; /* not used for constraints */
 					cooked->expr = expr;
@@ -1781,7 +1781,7 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 	 */
 	if (inhSchema != NIL)
 	{
-		int			schema_attno = 0;
+		int		schema_attno = 0;
 
 		foreach(entry, schema)
 		{
@@ -1809,14 +1809,14 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 				 * Yes, try to merge the two column definitions. They must
 				 * have the same type, typmod, and collation.
 				 */
-				if (exist_attno == schema_attno)
+				 if (exist_attno == schema_attno)
 					ereport(NOTICE,
-					(errmsg("merging column \"%s\" with inherited definition",
-							attributeName)));
+					   (errmsg("merging column \"%s\" with inherited definition",
+							   attributeName)));
 				else
 					ereport(NOTICE,
-							(errmsg("moving and merging column \"%s\" with inherited definition", attributeName),
-							 errdetail("User-specified column moved to the position of the inherited column.")));
+					   (errmsg("moving and merging column \"%s\" with inherited definition", attributeName),
+						errdetail("User-specified column moved to the position of the inherited column.")));
 				def = (ColumnDef *) list_nth(inhSchema, exist_attno - 1);
 				typenameTypeIdAndMod(NULL, def->typeName, &defTypeId, &deftypmod);
 				typenameTypeIdAndMod(NULL, newdef->typeName, &newTypeId, &newtypmod);
@@ -2302,7 +2302,7 @@ renameatt_internal(Oid myrelid,
 						oldattname)));
 
 	/* new name should not already exist */
-	check_for_column_name_collision(targetrelation, newattname);
+	check_for_column_name_collision(targetrelation, newattname, false);
 
 	/* apply the update */
 	namestrcpy(&(attform->attname), newattname);
@@ -3453,11 +3453,11 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddColumnToView:		/* add column via CREATE OR REPLACE
 										 * VIEW */
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, false, false, lockmode);
+									  false, false, false, false, lockmode);
 			break;
 		case AT_AddColumnRecurse:
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, true, false, lockmode);
+									  false, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_ColumnDefault:	/* ALTER COLUMN DEFAULT */
 			address = ATExecColumnDefault(rel, cmd->name, cmd->def, lockmode);
@@ -3496,7 +3496,7 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			break;
 		case AT_ReAddIndex:		/* ADD INDEX */
 			address = ATExecAddIndex(tab, rel, (IndexStmt *) cmd->def, true,
-									 lockmode);
+									lockmode);
 			break;
 		case AT_AddConstraint:	/* ADD CONSTRAINT */
 			address =
@@ -3567,14 +3567,14 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, false, false, lockmode);
+									true, false, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_AddOidsRecurse:	/* SET WITH OIDS */
 			/* Use the ADD COLUMN code, unless prep decided to do nothing */
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, true, false, lockmode);
+									true, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_DropOids:		/* SET WITHOUT OIDS */
 
@@ -3803,7 +3803,7 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode)
 			 * And fire it only once.
 			 */
 			if (parsetree)
-				EventTriggerTableRewrite((Node *) parsetree,
+				EventTriggerTableRewrite((Node *)parsetree,
 										 tab->relid,
 										 tab->rewrite);
 
@@ -4672,7 +4672,7 @@ ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 static ObjectAddress
 ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 				ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode)
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode)
 {
 	Oid			myrelid = RelationGetRelid(rel);
 	Relation	pgclass,
@@ -4767,7 +4767,13 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	relkind = ((Form_pg_class) GETSTRUCT(reltup))->relkind;
 
 	/* new name should not already exist */
-	check_for_column_name_collision(rel, colDef->colname);
+	if (!check_for_column_name_collision(rel, colDef->colname, if_not_exists))
+	{
+		heap_close(attrdesc, RowExclusiveLock);
+		heap_freetuple(reltup);
+		heap_close(pgclass, RowExclusiveLock);
+		return InvalidObjectAddress;
+	}
 
 	/* Determine the new attribute's number */
 	if (isOid)
@@ -4997,7 +5003,8 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* Recurse to child; return value is ignored */
 		ATExecAddColumn(wqueue, childtab, childrel,
-						colDef, isOid, recurse, true, lockmode);
+						colDef, isOid, recurse, true,
+						if_not_exists, lockmode);
 
 		heap_close(childrel, NoLock);
 	}
@@ -5010,8 +5017,8 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
  * If a new or renamed column will collide with the name of an existing
  * column, error out.
  */
-static void
-check_for_column_name_collision(Relation rel, const char *colname)
+static bool
+check_for_column_name_collision(Relation rel, const char *colname, bool if_not_exists)
 {
 	HeapTuple	attTuple;
 	int			attnum;
@@ -5024,7 +5031,7 @@ check_for_column_name_collision(Relation rel, const char *colname)
 							   ObjectIdGetDatum(RelationGetRelid(rel)),
 							   PointerGetDatum(colname));
 	if (!HeapTupleIsValid(attTuple))
-		return;
+		return true;
 
 	attnum = ((Form_pg_attribute) GETSTRUCT(attTuple))->attnum;
 	ReleaseSysCache(attTuple);
@@ -5040,10 +5047,23 @@ check_for_column_name_collision(Relation rel, const char *colname)
 			 errmsg("column name \"%s\" conflicts with a system column name",
 					colname)));
 	else
+	{
+		if (if_not_exists)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("column \"%s\" of relation \"%s\" already exists, skipping",
+							colname, RelationGetRelationName(rel))));
+			return false;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_COLUMN),
 				 errmsg("column \"%s\" of relation \"%s\" already exists",
 						colname, RelationGetRelationName(rel))));
+	}
+
+	return true;
 }
 
 /*
@@ -5960,7 +5980,7 @@ ATExecAddIndexConstraint(AlteredTableInfo *tab, Relation rel,
 									  true,		/* update pg_index */
 									  true,		/* remove old dependencies */
 									  allowSystemTableMods,
-									  false);	/* is_internal */
+									  false);		/* is_internal */
 
 	index_close(indexRel, NoLock);
 
@@ -6906,7 +6926,7 @@ ATExecValidateConstraint(Relation rel, char *constrName, bool recurse,
 						 HeapTupleGetOid(tuple));
 	}
 	else
-		address = InvalidObjectAddress; /* already validated */
+		address = InvalidObjectAddress;		/* already validated */
 
 	systable_endscan(scan);
 
@@ -7900,8 +7920,8 @@ ATPrepAlterColumnType(List **wqueue,
 			else
 				ereport(ERROR,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),
-						 errmsg("column \"%s\" cannot be cast automatically to type %s",
-								colName, format_type_be(targettype)),
+				  errmsg("column \"%s\" cannot be cast automatically to type %s",
+						 colName, format_type_be(targettype)),
 				/* translator: USING is SQL, don't translate it */
 					   errhint("You might need to specify \"USING %s::%s\".",
 							   quote_identifier(colName),
@@ -9717,9 +9737,9 @@ AlterTableMoveAll(AlterTableMoveAllStmt *stmt)
 			!ConditionalLockRelationOid(relOid, AccessExclusiveLock))
 			ereport(ERROR,
 					(errcode(ERRCODE_OBJECT_IN_USE),
-					 errmsg("aborting because lock on relation \"%s\".\"%s\" is not available",
-							get_namespace_name(relForm->relnamespace),
-							NameStr(relForm->relname))));
+			   errmsg("aborting because lock on relation \"%s\".\"%s\" is not available",
+					  get_namespace_name(relForm->relnamespace),
+					  NameStr(relForm->relname))));
 		else
 			LockRelationOid(relOid, AccessExclusiveLock);
 
@@ -10939,9 +10959,9 @@ ATExecReplicaIdentity(Relation rel, ReplicaIdentityStmt *stmt, LOCKMODE lockmode
 static void
 ATExecEnableRowSecurity(Relation rel)
 {
-	Relation	pg_class;
-	Oid			relid;
-	HeapTuple	tuple;
+	Relation		pg_class;
+	Oid				relid;
+	HeapTuple		tuple;
 
 	relid = RelationGetRelid(rel);
 
@@ -10965,9 +10985,9 @@ ATExecEnableRowSecurity(Relation rel)
 static void
 ATExecDisableRowSecurity(Relation rel)
 {
-	Relation	pg_class;
-	Oid			relid;
-	HeapTuple	tuple;
+	Relation		pg_class;
+	Oid				relid;
+	HeapTuple		tuple;
 
 	relid = RelationGetRelid(rel);
 
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e0ff6f1..a7b09e4 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1941,6 +1941,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $2;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD IF NOT EXISTS <coldef> */
+			| ADD_P IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $5;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ADD COLUMN <coldef> */
@@ -1949,6 +1959,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $3;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD COLUMN IF NOT EXISTS <coldef> */
+			| ADD_P COLUMN IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $6;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> {SET DEFAULT <expr>|DROP DEFAULT} */
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 3ad2c55..ae81ff6 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2542,3 +2542,92 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+ c4     | integer | 
+
+DROP TABLE test_add_column;
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index 29c1875..bf7654c 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1688,3 +1688,34 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+\d test_add_column
+DROP TABLE test_add_column;

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 207fec1..339320e 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -36,7 +36,7 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+    ADD [ COLUMN ] [ IF NOT EXISTS ]<replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> [ RESTRICT | CASCADE ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ USING <replaceable class="PARAMETER">expression</replaceable> ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
@@ -96,11 +96,12 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
   <variablelist>
    <varlistentry>
-    <term><literal>ADD COLUMN</literal></term>
+    <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term>
     <listitem>
      <para>
       This form adds a new column to the table, using the same syntax as
-      <xref linkend="SQL-CREATETABLE">.
+      <xref linkend="SQL-CREATETABLE">. If <literal>IF NOT EXISTS</literal>
+      is specified and the column already exists, no error is thrown.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index d394713..94791e6 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -328,8 +328,8 @@ static void ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recu
 				bool is_view, AlterTableCmd *cmd, LOCKMODE lockmode);
 static ObjectAddress ATExecAddColumn(List **wqueue, AlteredTableInfo *tab,
 				Relation rel, ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode);
-static void check_for_column_name_collision(Relation rel, const char *colname);
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode);
+static bool check_for_column_name_collision(Relation rel, const char *colname, bool if_not_exists);
 static void add_column_datatype_dependency(Oid relid, int32 attnum, Oid typid);
 static void add_column_collation_dependency(Oid relid, int32 attnum, Oid collid);
 static void ATPrepAddOids(List **wqueue, Relation rel, bool recurse,
@@ -2302,7 +2302,7 @@ renameatt_internal(Oid myrelid,
 						oldattname)));
 
 	/* new name should not already exist */
-	check_for_column_name_collision(targetrelation, newattname);
+	check_for_column_name_collision(targetrelation, newattname, false);
 
 	/* apply the update */
 	namestrcpy(&(attform->attname), newattname);
@@ -3453,11 +3453,11 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddColumnToView:		/* add column via CREATE OR REPLACE
 										 * VIEW */
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, false, false, lockmode);
+									  false, false, false, false, lockmode);
 			break;
 		case AT_AddColumnRecurse:
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, true, false, lockmode);
+									  false, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_ColumnDefault:	/* ALTER COLUMN DEFAULT */
 			address = ATExecColumnDefault(rel, cmd->name, cmd->def, lockmode);
@@ -3567,14 +3567,14 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, false, false, lockmode);
+									true, false, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_AddOidsRecurse:	/* SET WITH OIDS */
 			/* Use the ADD COLUMN code, unless prep decided to do nothing */
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, true, false, lockmode);
+									true, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_DropOids:		/* SET WITHOUT OIDS */
 
@@ -4672,7 +4672,7 @@ ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 static ObjectAddress
 ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 				ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode)
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode)
 {
 	Oid			myrelid = RelationGetRelid(rel);
 	Relation	pgclass,
@@ -4767,7 +4767,13 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	relkind = ((Form_pg_class) GETSTRUCT(reltup))->relkind;
 
 	/* new name should not already exist */
-	check_for_column_name_collision(rel, colDef->colname);
+	if (!check_for_column_name_collision(rel, colDef->colname, if_not_exists))
+	{
+		heap_close(attrdesc, RowExclusiveLock);
+		heap_freetuple(reltup);
+		heap_close(pgclass, RowExclusiveLock);
+		return InvalidObjectAddress;
+	}
 
 	/* Determine the new attribute's number */
 	if (isOid)
@@ -4997,7 +5003,8 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* Recurse to child; return value is ignored */
 		ATExecAddColumn(wqueue, childtab, childrel,
-						colDef, isOid, recurse, true, lockmode);
+						colDef, isOid, recurse, true,
+						if_not_exists, lockmode);
 
 		heap_close(childrel, NoLock);
 	}
@@ -5008,10 +5015,10 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 /*
  * If a new or renamed column will collide with the name of an existing
- * column, error out.
+ * column and if_not_exists is false then error out, else do nothing.
  */
-static void
-check_for_column_name_collision(Relation rel, const char *colname)
+static bool
+check_for_column_name_collision(Relation rel, const char *colname, bool if_not_exists)
 {
 	HeapTuple	attTuple;
 	int			attnum;
@@ -5024,7 +5031,7 @@ check_for_column_name_collision(Relation rel, const char *colname)
 							   ObjectIdGetDatum(RelationGetRelid(rel)),
 							   PointerGetDatum(colname));
 	if (!HeapTupleIsValid(attTuple))
-		return;
+		return true;
 
 	attnum = ((Form_pg_attribute) GETSTRUCT(attTuple))->attnum;
 	ReleaseSysCache(attTuple);
@@ -5040,10 +5047,23 @@ check_for_column_name_collision(Relation rel, const char *colname)
 			 errmsg("column name \"%s\" conflicts with a system column name",
 					colname)));
 	else
+	{
+		if (if_not_exists)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("column \"%s\" of relation \"%s\" already exists, skipping",
+							colname, RelationGetRelationName(rel))));
+			return false;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_COLUMN),
 				 errmsg("column \"%s\" of relation \"%s\" already exists",
 						colname, RelationGetRelationName(rel))));
+	}
+
+	return true;
 }
 
 /*
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e0ff6f1..a7b09e4 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1941,6 +1941,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $2;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD IF NOT EXISTS <coldef> */
+			| ADD_P IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $5;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ADD COLUMN <coldef> */
@@ -1949,6 +1959,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $3;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD COLUMN IF NOT EXISTS <coldef> */
+			| ADD_P COLUMN IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $6;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> {SET DEFAULT <expr>|DROP DEFAULT} */
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 3ad2c55..ae81ff6 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2542,3 +2542,92 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+ c4     | integer | 
+
+DROP TABLE test_add_column;
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index 29c1875..bf7654c 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1688,3 +1688,34 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+\d test_add_column
+DROP TABLE test_add_column;

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 207fec1..5a71c3c 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -36,7 +36,7 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+    ADD [ COLUMN ] [ IF NOT EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> [ RESTRICT | CASCADE ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ USING <replaceable class="PARAMETER">expression</replaceable> ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
@@ -96,11 +96,12 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
   <variablelist>
    <varlistentry>
-    <term><literal>ADD COLUMN</literal></term>
+    <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term>
     <listitem>
      <para>
       This form adds a new column to the table, using the same syntax as
-      <xref linkend="SQL-CREATETABLE">.
+      <xref linkend="SQL-CREATETABLE">. If <literal>IF NOT EXISTS</literal>
+      is specified and the column already exists, no error is thrown.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 1c7eded..05fbe51 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -328,8 +328,9 @@ static void ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recu
 				bool is_view, AlterTableCmd *cmd, LOCKMODE lockmode);
 static ObjectAddress ATExecAddColumn(List **wqueue, AlteredTableInfo *tab,
 				Relation rel, ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode);
-static void check_for_column_name_collision(Relation rel, const char *colname);
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode);
+static bool check_for_column_name_collision(Relation rel, const char *colname,
+				bool if_not_exists);
 static void add_column_datatype_dependency(Oid relid, int32 attnum, Oid typid);
 static void add_column_collation_dependency(Oid relid, int32 attnum, Oid collid);
 static void ATPrepAddOids(List **wqueue, Relation rel, bool recurse,
@@ -2304,7 +2305,7 @@ renameatt_internal(Oid myrelid,
 						oldattname)));
 
 	/* new name should not already exist */
-	check_for_column_name_collision(targetrelation, newattname);
+	(void) check_for_column_name_collision(targetrelation, newattname, false);
 
 	/* apply the update */
 	namestrcpy(&(attform->attname), newattname);
@@ -3455,11 +3456,11 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddColumnToView:		/* add column via CREATE OR REPLACE
 										 * VIEW */
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, false, false, lockmode);
+									  false, false, false, false, lockmode);
 			break;
 		case AT_AddColumnRecurse:
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, true, false, lockmode);
+									  false, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_ColumnDefault:	/* ALTER COLUMN DEFAULT */
 			address = ATExecColumnDefault(rel, cmd->name, cmd->def, lockmode);
@@ -3572,14 +3573,14 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, false, false, lockmode);
+									true, false, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_AddOidsRecurse:	/* SET WITH OIDS */
 			/* Use the ADD COLUMN code, unless prep decided to do nothing */
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, true, false, lockmode);
+									true, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_DropOids:		/* SET WITHOUT OIDS */
 
@@ -4677,7 +4678,7 @@ ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 static ObjectAddress
 ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 				ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode)
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode)
 {
 	Oid			myrelid = RelationGetRelid(rel);
 	Relation	pgclass,
@@ -4771,8 +4772,14 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		elog(ERROR, "cache lookup failed for relation %u", myrelid);
 	relkind = ((Form_pg_class) GETSTRUCT(reltup))->relkind;
 
-	/* new name should not already exist */
-	check_for_column_name_collision(rel, colDef->colname);
+	/* skipp if the name already exists and if_not_exists is true */
+	if (!check_for_column_name_collision(rel, colDef->colname, if_not_exists))
+	{
+		heap_close(attrdesc, RowExclusiveLock);
+		heap_freetuple(reltup);
+		heap_close(pgclass, RowExclusiveLock);
+		return InvalidObjectAddress;
+	}
 
 	/* Determine the new attribute's number */
 	if (isOid)
@@ -5002,7 +5009,8 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* Recurse to child; return value is ignored */
 		ATExecAddColumn(wqueue, childtab, childrel,
-						colDef, isOid, recurse, true, lockmode);
+						colDef, isOid, recurse, true,
+						if_not_exists, lockmode);
 
 		heap_close(childrel, NoLock);
 	}
@@ -5013,10 +5021,11 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 /*
  * If a new or renamed column will collide with the name of an existing
- * column, error out.
+ * column and if_not_exists is false then error out, else do nothing.
  */
-static void
-check_for_column_name_collision(Relation rel, const char *colname)
+static bool
+check_for_column_name_collision(Relation rel, const char *colname,
+								bool if_not_exists)
 {
 	HeapTuple	attTuple;
 	int			attnum;
@@ -5029,7 +5038,7 @@ check_for_column_name_collision(Relation rel, const char *colname)
 							   ObjectIdGetDatum(RelationGetRelid(rel)),
 							   PointerGetDatum(colname));
 	if (!HeapTupleIsValid(attTuple))
-		return;
+		return true;
 
 	attnum = ((Form_pg_attribute) GETSTRUCT(attTuple))->attnum;
 	ReleaseSysCache(attTuple);
@@ -5045,10 +5054,23 @@ check_for_column_name_collision(Relation rel, const char *colname)
 			 errmsg("column name \"%s\" conflicts with a system column name",
 					colname)));
 	else
+	{
+		if (if_not_exists)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("column \"%s\" of relation \"%s\" already exists, skipping",
+							colname, RelationGetRelationName(rel))));
+			return false;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_COLUMN),
 				 errmsg("column \"%s\" of relation \"%s\" already exists",
 						colname, RelationGetRelationName(rel))));
+	}
+
+	return true;
 }
 
 /*
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 2b02a2e..ea36cdf 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1942,6 +1942,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $2;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD IF NOT EXISTS <coldef> */
+			| ADD_P IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $5;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ADD COLUMN <coldef> */
@@ -1950,6 +1960,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $3;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD COLUMN IF NOT EXISTS <coldef> */
+			| ADD_P COLUMN IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $6;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> {SET DEFAULT <expr>|DROP DEFAULT} */
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 8d12496..028d6ed 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2605,3 +2605,92 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+ c4     | integer | 
+
+DROP TABLE test_add_column;
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index 3feed7c..aefb5c9 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1724,3 +1724,34 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+\d test_add_column
+DROP TABLE test_add_column;

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 207fec1..5a71c3c 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -36,7 +36,7 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+    ADD [ COLUMN ] [ IF NOT EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> [ RESTRICT | CASCADE ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ USING <replaceable class="PARAMETER">expression</replaceable> ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
@@ -96,11 +96,12 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
 
   <variablelist>
    <varlistentry>
-    <term><literal>ADD COLUMN</literal></term>
+    <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term>
     <listitem>
      <para>
       This form adds a new column to the table, using the same syntax as
-      <xref linkend="SQL-CREATETABLE">.
+      <xref linkend="SQL-CREATETABLE">. If <literal>IF NOT EXISTS</literal>
+      is specified and the column already exists, no error is thrown.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 1c7eded..92b2662 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -328,8 +328,9 @@ static void ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recu
 				bool is_view, AlterTableCmd *cmd, LOCKMODE lockmode);
 static ObjectAddress ATExecAddColumn(List **wqueue, AlteredTableInfo *tab,
 				Relation rel, ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode);
-static void check_for_column_name_collision(Relation rel, const char *colname);
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode);
+static bool check_for_column_name_collision(Relation rel, const char *colname,
+				bool if_not_exists);
 static void add_column_datatype_dependency(Oid relid, int32 attnum, Oid typid);
 static void add_column_collation_dependency(Oid relid, int32 attnum, Oid collid);
 static void ATPrepAddOids(List **wqueue, Relation rel, bool recurse,
@@ -2304,7 +2305,7 @@ renameatt_internal(Oid myrelid,
 						oldattname)));
 
 	/* new name should not already exist */
-	check_for_column_name_collision(targetrelation, newattname);
+	(void) check_for_column_name_collision(targetrelation, newattname, false);
 
 	/* apply the update */
 	namestrcpy(&(attform->attname), newattname);
@@ -3455,11 +3456,11 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddColumnToView:		/* add column via CREATE OR REPLACE
 										 * VIEW */
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, false, false, lockmode);
+									  false, false, false, false, lockmode);
 			break;
 		case AT_AddColumnRecurse:
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									  false, true, false, lockmode);
+									  false, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_ColumnDefault:	/* ALTER COLUMN DEFAULT */
 			address = ATExecColumnDefault(rel, cmd->name, cmd->def, lockmode);
@@ -3572,14 +3573,14 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, false, false, lockmode);
+									true, false, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_AddOidsRecurse:	/* SET WITH OIDS */
 			/* Use the ADD COLUMN code, unless prep decided to do nothing */
 			if (cmd->def != NULL)
 				address =
 					ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
-									true, true, false, lockmode);
+									true, true, false, cmd->missing_ok, lockmode);
 			break;
 		case AT_DropOids:		/* SET WITHOUT OIDS */
 
@@ -4677,7 +4678,7 @@ ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 static ObjectAddress
 ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 				ColumnDef *colDef, bool isOid,
-				bool recurse, bool recursing, LOCKMODE lockmode)
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode)
 {
 	Oid			myrelid = RelationGetRelid(rel);
 	Relation	pgclass,
@@ -4771,8 +4772,14 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		elog(ERROR, "cache lookup failed for relation %u", myrelid);
 	relkind = ((Form_pg_class) GETSTRUCT(reltup))->relkind;
 
-	/* new name should not already exist */
-	check_for_column_name_collision(rel, colDef->colname);
+	/* skip if the name already exists and if_not_exists is true */
+	if (!check_for_column_name_collision(rel, colDef->colname, if_not_exists))
+	{
+		heap_close(attrdesc, RowExclusiveLock);
+		heap_freetuple(reltup);
+		heap_close(pgclass, RowExclusiveLock);
+		return InvalidObjectAddress;
+	}
 
 	/* Determine the new attribute's number */
 	if (isOid)
@@ -5002,7 +5009,8 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* Recurse to child; return value is ignored */
 		ATExecAddColumn(wqueue, childtab, childrel,
-						colDef, isOid, recurse, true, lockmode);
+						colDef, isOid, recurse, true,
+						if_not_exists, lockmode);
 
 		heap_close(childrel, NoLock);
 	}
@@ -5013,10 +5021,11 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 /*
  * If a new or renamed column will collide with the name of an existing
- * column, error out.
+ * column and if_not_exists is false then error out, else do nothing.
  */
-static void
-check_for_column_name_collision(Relation rel, const char *colname)
+static bool
+check_for_column_name_collision(Relation rel, const char *colname,
+								bool if_not_exists)
 {
 	HeapTuple	attTuple;
 	int			attnum;
@@ -5029,7 +5038,7 @@ check_for_column_name_collision(Relation rel, const char *colname)
 							   ObjectIdGetDatum(RelationGetRelid(rel)),
 							   PointerGetDatum(colname));
 	if (!HeapTupleIsValid(attTuple))
-		return;
+		return true;
 
 	attnum = ((Form_pg_attribute) GETSTRUCT(attTuple))->attnum;
 	ReleaseSysCache(attTuple);
@@ -5045,10 +5054,23 @@ check_for_column_name_collision(Relation rel, const char *colname)
 			 errmsg("column name \"%s\" conflicts with a system column name",
 					colname)));
 	else
+	{
+		if (if_not_exists)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("column \"%s\" of relation \"%s\" already exists, skipping",
+							colname, RelationGetRelationName(rel))));
+			return false;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_COLUMN),
 				 errmsg("column \"%s\" of relation \"%s\" already exists",
 						colname, RelationGetRelationName(rel))));
+	}
+
+	return true;
 }
 
 /*
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 2b02a2e..ea36cdf 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1942,6 +1942,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $2;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD IF NOT EXISTS <coldef> */
+			| ADD_P IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $5;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ADD COLUMN <coldef> */
@@ -1950,6 +1960,16 @@ alter_table_cmd:
 					AlterTableCmd *n = makeNode(AlterTableCmd);
 					n->subtype = AT_AddColumn;
 					n->def = $3;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ADD COLUMN IF NOT EXISTS <coldef> */
+			| ADD_P COLUMN IF_P NOT EXISTS columnDef
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_AddColumn;
+					n->def = $6;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> {SET DEFAULT <expr>|DROP DEFAULT} */
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 8d12496..028d6ed 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2605,3 +2605,92 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+ERROR:  column "c2" of relation "test_add_column" already exists
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+NOTICE:  column "c3" of relation "test_add_column" already exists, skipping
+\d test_add_column
+Table "public.test_add_column"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ c1     | integer | 
+ c2     | integer | 
+ c3     | integer | 
+ c4     | integer | 
+
+DROP TABLE test_add_column;
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index 3feed7c..aefb5c9 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1724,3 +1724,34 @@ ALTER TABLE logged1 SET UNLOGGED; -- silently do nothing
 DROP TABLE logged3;
 DROP TABLE logged2;
 DROP TABLE logged1;
+
+-- test ADD COLUMN IF NOT EXISTS
+CREATE TABLE test_add_column(c1 integer);
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN c2 integer, -- fail because c2 already exists
+	ADD COLUMN c3 integer;
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN c3 integer; -- fail because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer; -- skipping because c3 already exists
+\d test_add_column
+ALTER TABLE test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer, -- skipping because c2 already exists
+	ADD COLUMN IF NOT EXISTS c3 integer, -- skipping because c3 already exists
+	ADD COLUMN c4 integer;
+\d test_add_column
+DROP TABLE test_add_column;