ALTER TABLE support for dropping generation expression

From 465312aafd88325a48d934a2c64ca6b9d12dea3f Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Sat, 28 Dec 2019 17:38:12 +0100
Subject: [PATCH v2] ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION

Add an ALTER TABLE subcommand for dropping the generated property from
a column, per SQL standard.

Discussion: https://www.postgresql.org/message-id/flat/2f7f1d9c-946e-0453-d841-4f38eb9d69b6%402ndquadrant.com
---
 doc/src/sgml/ref/alter_table.sgml       |  18 ++++
 src/backend/catalog/sql_features.txt    |   2 +-
 src/backend/commands/tablecmds.c        | 131 +++++++++++++++++++++++-
 src/backend/parser/gram.y               |  20 +++-
 src/bin/psql/tab-complete.c             |   2 +-
 src/include/nodes/parsenodes.h          |   1 +
 src/include/parser/kwlist.h             |   1 +
 src/test/regress/expected/generated.out |  80 +++++++++++++++
 src/test/regress/sql/generated.sql      |  32 ++++++
 9 files changed, 283 insertions(+), 4 deletions(-)

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 8403c797e2..4bf449587c 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -46,6 +46,7 @@
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET DEFAULT <replaceable class="parameter">expression</replaceable>
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP DEFAULT
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET | DROP } NOT NULL
+    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP EXPRESSION [ IF EXISTS ]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET GENERATED { ALWAYS | BY DEFAULT } | SET <replaceable>sequence_option</replaceable> | RESTART [ [ WITH ] <replaceable class="parameter">restart</replaceable> ] } [...]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP IDENTITY [ IF EXISTS ]
@@ -241,6 +242,23 @@ <title>Description</title>
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>DROP EXPRESSION [ IF EXISTS ]</literal></term>
+    <listitem>
+     <para>
+      This form turns a stored generated column into a normal base column.
+      Existing data in the columns is retained, but future changes will no
+      longer apply the generation expression.
+     </para>
+
+     <para>
+      If <literal>DROP EXPRESSION IF EXISTS</literal> is specified and the
+      column is not a stored generated column, no error is thrown.  In this
+      case a notice is issued instead.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY</literal></term>
     <term><literal>SET GENERATED { ALWAYS | BY DEFAULT }</literal></term>
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index ab3e381cff..9f840ddfd2 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -252,7 +252,7 @@ F381	Extended schema manipulation	03	ALTER TABLE statement: DROP CONSTRAINT clau
 F382	Alter column data type			YES	
 F383	Set column not null clause			YES	
 F384	Drop identity property clause			YES	
-F385	Drop column generation expression clause			NO	
+F385	Drop column generation expression clause			YES	
 F386	Set identity column generation clause			YES	
 F391	Long identifiers			YES	
 F392	Unicode escapes in identifiers			YES	
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 5b882f80bf..603779ae9d 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -386,6 +386,7 @@ static ObjectAddress ATExecAddIdentity(Relation rel, const char *colName,
 static ObjectAddress ATExecSetIdentity(Relation rel, const char *colName,
 									   Node *def, LOCKMODE lockmode);
 static ObjectAddress ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode);
+static ObjectAddress ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode);
 static ObjectAddress ATExecSetStatistics(Relation rel, const char *colName, int16 colNum,
 										 Node *newValue, LOCKMODE lockmode);
 static ObjectAddress ATExecSetOptions(Relation rel, const char *colName,
@@ -3669,6 +3670,7 @@ AlterTableGetLockLevel(List *cmds)
 			case AT_AddIdentity:
 			case AT_DropIdentity:
 			case AT_SetIdentity:
+			case AT_DropExpression:
 				cmd_lockmode = AccessExclusiveLock;
 				break;
 
@@ -3943,6 +3945,11 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			/* No command-specific prep needed */
 			pass = AT_PASS_COL_ATTRS;
 			break;
+		case AT_DropExpression:	/* ALTER COLUMN DROP EXPRESSION */
+			ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);
+			ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);
+			pass = AT_PASS_DROP;
+			break;
 		case AT_SetStatistics:	/* ALTER COLUMN SET STATISTICS */
 			ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW | ATT_INDEX | ATT_PARTITIONED_INDEX | ATT_FOREIGN_TABLE);
 			ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);
@@ -4262,6 +4269,9 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_CheckNotNull:	/* check column is already marked NOT NULL */
 			ATExecCheckNotNull(tab, rel, cmd->name, lockmode);
 			break;
+		case AT_DropExpression:
+			address = ATExecDropExpression(rel, cmd->name, cmd->missing_ok, lockmode);
+			break;
 		case AT_SetStatistics:	/* ALTER COLUMN SET STATISTICS */
 			address = ATExecSetStatistics(rel, cmd->name, cmd->num, cmd->def, lockmode);
 			break;
@@ -6428,7 +6438,9 @@ ATExecColumnDefault(Relation rel, const char *colName,
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("column \"%s\" of relation \"%s\" is a generated column",
-						colName, RelationGetRelationName(rel))));
+						colName, RelationGetRelationName(rel)),
+				 newDefault || TupleDescAttr(tupdesc, attnum - 1)->attgenerated != ATTRIBUTE_GENERATED_STORED ? 0 :
+				 errhint("Use ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION instead.")));
 
 	/*
 	 * Remove any old default for the column.  We use RESTRICT here for
@@ -6696,6 +6708,123 @@ ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE
 	return address;
 }
 
+/*
+ * ALTER TABLE ALTER COLUMN DROP EXPRESSION
+ *
+ * Return the address of the affected column.
+ */
+static ObjectAddress
+ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode)
+{
+	HeapTuple	tuple;
+	Form_pg_attribute attTup;
+	AttrNumber	attnum;
+	Relation	attrelation;
+	ObjectAddress address;
+
+	attrelation = table_open(AttributeRelationId, RowExclusiveLock);
+	tuple = SearchSysCacheCopyAttName(RelationGetRelid(rel), colName);
+	if (!HeapTupleIsValid(tuple))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_COLUMN),
+				 errmsg("column \"%s\" of relation \"%s\" does not exist",
+						colName, RelationGetRelationName(rel))));
+
+	attTup = (Form_pg_attribute) GETSTRUCT(tuple);
+	attnum = attTup->attnum;
+
+	if (attnum <= 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot alter system column \"%s\"",
+						colName)));
+
+	if (attTup->attgenerated != ATTRIBUTE_GENERATED_STORED)
+	{
+		if (!missing_ok)
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("column \"%s\" of relation \"%s\" is not a stored generated column",
+							colName, RelationGetRelationName(rel))));
+		else
+		{
+			ereport(NOTICE,
+					(errmsg("column \"%s\" of relation \"%s\" is not a stored generated column, skipping",
+							colName, RelationGetRelationName(rel))));
+			heap_freetuple(tuple);
+			table_close(attrelation, RowExclusiveLock);
+			return InvalidObjectAddress;
+		}
+	}
+
+	attTup->attgenerated = '\0';
+	CatalogTupleUpdate(attrelation, &tuple->t_self, tuple);
+
+	InvokeObjectPostAlterHook(RelationRelationId,
+							  RelationGetRelid(rel),
+							  attTup->attnum);
+	ObjectAddressSubSet(address, RelationRelationId,
+						RelationGetRelid(rel), attnum);
+	heap_freetuple(tuple);
+
+	table_close(attrelation, RowExclusiveLock);
+
+	CommandCounterIncrement();
+
+	RemoveAttrDefault(RelationGetRelid(rel), attnum, DROP_RESTRICT, false, false);
+
+	/*
+	 * Remove all dependencies of this (formerly generated) column on other
+	 * columns in the same table.  (See StoreAttrDefault() for which
+	 * dependencies are created.)  We don't expect there to be dependencies
+	 * between columns of the same table for other reasons, so it's okay to
+	 * remove all of them.
+	 */
+	{
+		Relation	depRel;
+		ScanKeyData key[3];
+		SysScanDesc scan;
+		HeapTuple	tup;
+
+		depRel = table_open(DependRelationId, RowExclusiveLock);
+
+		ScanKeyInit(&key[0],
+					Anum_pg_depend_classid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(RelationRelationId));
+		ScanKeyInit(&key[1],
+					Anum_pg_depend_objid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(RelationGetRelid(rel)));
+		ScanKeyInit(&key[2],
+					Anum_pg_depend_objsubid,
+					BTEqualStrategyNumber, F_INT4EQ,
+					Int32GetDatum(attnum));
+
+		scan = systable_beginscan(depRel, DependDependerIndexId, true,
+								  NULL, 3, key);
+
+		while (HeapTupleIsValid(tup = systable_getnext(scan)))
+		{
+			Form_pg_depend depform = (Form_pg_depend) GETSTRUCT(tup);
+
+			if (depform->refclassid == RelationRelationId &&
+				depform->refobjid == RelationGetRelid(rel) &&
+				depform->refobjsubid != 0 &&
+				depform->deptype == DEPENDENCY_AUTO)
+			{
+				CatalogTupleDelete(depRel, &tup->t_self);
+			}
+		}
+
+		systable_endscan(scan);
+
+		table_close(depRel, RowExclusiveLock);
+	}
+
+	return address;
+}
+
 /*
  * ALTER TABLE ALTER COLUMN SET STATISTICS
  *
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index c5086846de..25e1aa66d9 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -636,7 +636,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	DOUBLE_P DROP
 
 	EACH ELSE ENABLE_P ENCODING ENCRYPTED END_P ENUM_P ESCAPE EVENT EXCEPT
-	EXCLUDE EXCLUDING EXCLUSIVE EXECUTE EXISTS EXPLAIN
+	EXCLUDE EXCLUDING EXCLUSIVE EXECUTE EXISTS EXPLAIN EXPRESSION
 	EXTENSION EXTERNAL EXTRACT
 
 	FALSE_P FAMILY FETCH FILTER FIRST_P FLOAT_P FOLLOWING FOR
@@ -2126,6 +2126,23 @@ alter_table_cmd:
 					n->name = $3;
 					$$ = (Node *)n;
 				}
+			/* ALTER TABLE <name> ALTER [COLUMN] <colname> DROP EXPRESSION */
+			| ALTER opt_column ColId DROP EXPRESSION
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_DropExpression;
+					n->name = $3;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ALTER [COLUMN] <colname> DROP EXPRESSION IF EXISTS */
+			| ALTER opt_column ColId DROP EXPRESSION IF_P EXISTS
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_DropExpression;
+					n->name = $3;
+					n->missing_ok = true;
+					$$ = (Node *)n;
+				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> SET STATISTICS <SignedIconst> */
 			| ALTER opt_column ColId SET STATISTICS SignedIconst
 				{
@@ -15193,6 +15210,7 @@ unreserved_keyword:
 			| EXCLUSIVE
 			| EXECUTE
 			| EXPLAIN
+			| EXPRESSION
 			| EXTENSION
 			| EXTERNAL
 			| FAMILY
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index 5e0db3515d..7e7e6d5636 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -2009,7 +2009,7 @@ psql_completion(const char *text, int start, int end)
 	/* ALTER TABLE ALTER [COLUMN] <foo> DROP */
 	else if (Matches("ALTER", "TABLE", MatchAny, "ALTER", "COLUMN", MatchAny, "DROP") ||
 			 Matches("ALTER", "TABLE", MatchAny, "ALTER", MatchAny, "DROP"))
-		COMPLETE_WITH("DEFAULT", "IDENTITY", "NOT NULL");
+		COMPLETE_WITH("DEFAULT", "EXPRESSION", "IDENTITY", "NOT NULL");
 	else if (Matches("ALTER", "TABLE", MatchAny, "CLUSTER"))
 		COMPLETE_WITH("ON");
 	else if (Matches("ALTER", "TABLE", MatchAny, "CLUSTER", "ON"))
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index ff626cbe61..b7aaab69dc 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1764,6 +1764,7 @@ typedef enum AlterTableType
 	AT_ColumnDefault,			/* alter column default */
 	AT_DropNotNull,				/* alter column drop not null */
 	AT_SetNotNull,				/* alter column set not null */
+	AT_DropExpression,			/* alter column drop expression */
 	AT_CheckNotNull,			/* check column is already marked not null */
 	AT_SetStatistics,			/* alter column set statistics */
 	AT_SetOptions,				/* alter column set ( options ) */
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 00ace8425e..5bf0c13dca 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -155,6 +155,7 @@ PG_KEYWORD("exclusive", EXCLUSIVE, UNRESERVED_KEYWORD)
 PG_KEYWORD("execute", EXECUTE, UNRESERVED_KEYWORD)
 PG_KEYWORD("exists", EXISTS, COL_NAME_KEYWORD)
 PG_KEYWORD("explain", EXPLAIN, UNRESERVED_KEYWORD)
+PG_KEYWORD("expression", EXPRESSION, UNRESERVED_KEYWORD)
 PG_KEYWORD("extension", EXTENSION, UNRESERVED_KEYWORD)
 PG_KEYWORD("external", EXTERNAL, UNRESERVED_KEYWORD)
 PG_KEYWORD("extract", EXTRACT, COL_NAME_KEYWORD)
diff --git a/src/test/regress/expected/generated.out b/src/test/regress/expected/generated.out
index f62c93f468..e03112869b 100644
--- a/src/test/regress/expected/generated.out
+++ b/src/test/regress/expected/generated.out
@@ -621,6 +621,7 @@ ALTER TABLE gtest27 ALTER COLUMN b TYPE boolean USING b <> 0;  -- error
 ERROR:  generation expression for column "b" cannot be cast automatically to type boolean
 ALTER TABLE gtest27 ALTER COLUMN b DROP DEFAULT;  -- error
 ERROR:  column "b" of relation "gtest27" is a generated column
+HINT:  Use ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION instead.
 \d gtest27
                              Table "public.gtest27"
  Column |  Type   | Collation | Nullable |               Default                
@@ -628,6 +629,85 @@ ERROR:  column "b" of relation "gtest27" is a generated column
  a      | integer |           |          | 
  b      | numeric |           |          | generated always as ((a * 2)) stored
 
+-- ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION
+ALTER TABLE gtest27 ALTER COLUMN a DROP EXPRESSION;  -- error
+ERROR:  column "a" of relation "gtest27" is not a stored generated column
+ALTER TABLE gtest27 ALTER COLUMN a DROP EXPRESSION IF EXISTS;  -- notice
+NOTICE:  column "a" of relation "gtest27" is not a stored generated column, skipping
+ALTER TABLE gtest27 ALTER COLUMN b DROP EXPRESSION;
+INSERT INTO gtest27 (a) VALUES (5);
+INSERT INTO gtest27 (a, b) VALUES (6, 66);
+SELECT * FROM gtest27;
+ a | b  
+---+----
+ 3 |  6
+ 4 |  8
+ 5 |   
+ 6 | 66
+(4 rows)
+
+\d gtest27
+              Table "public.gtest27"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | numeric |           |          | 
+
+-- check that dependencies between columns have also been removed
+ALTER TABLE gtest27 DROP COLUMN a;  -- should not drop b
+\d gtest27
+              Table "public.gtest27"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ b      | numeric |           |          | 
+
+-- with inheritance
+CREATE TABLE gtest29 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest29_1 () INHERITS (gtest29);
+ALTER TABLE gtest29 ALTER COLUMN b DROP EXPRESSION;
+\d gtest29
+              Table "public.gtest29"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Number of child tables: 1 (Use \d+ to list them.)
+
+\d gtest29_1
+             Table "public.gtest29_1"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Inherits: gtest29
+
+DROP TABLE gtest29 CASCADE;
+NOTICE:  drop cascades to table gtest29_1
+CREATE TABLE gtest29 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest29_1 () INHERITS (gtest29);
+ALTER TABLE ONLY gtest29 ALTER COLUMN b DROP EXPRESSION;
+\d gtest29
+              Table "public.gtest29"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Number of child tables: 1 (Use \d+ to list them.)
+
+\d gtest29_1
+                           Table "public.gtest29_1"
+ Column |  Type   | Collation | Nullable |              Default               
+--------+---------+-----------+----------+------------------------------------
+ a      | integer |           |          | 
+ b      | integer |           |          | generated always as (a * 2) stored
+Inherits: gtest29
+
 -- triggers
 CREATE TABLE gtest26 (
     a int PRIMARY KEY,
diff --git a/src/test/regress/sql/generated.sql b/src/test/regress/sql/generated.sql
index 6a56ae260f..a981e616a1 100644
--- a/src/test/regress/sql/generated.sql
+++ b/src/test/regress/sql/generated.sql
@@ -332,6 +332,38 @@ CREATE TABLE gtest27 (
 ALTER TABLE gtest27 ALTER COLUMN b DROP DEFAULT;  -- error
 \d gtest27
 
+-- ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION
+ALTER TABLE gtest27 ALTER COLUMN a DROP EXPRESSION;  -- error
+ALTER TABLE gtest27 ALTER COLUMN a DROP EXPRESSION IF EXISTS;  -- notice
+ALTER TABLE gtest27 ALTER COLUMN b DROP EXPRESSION;
+INSERT INTO gtest27 (a) VALUES (5);
+INSERT INTO gtest27 (a, b) VALUES (6, 66);
+SELECT * FROM gtest27;
+\d gtest27
+
+-- check that dependencies between columns have also been removed
+ALTER TABLE gtest27 DROP COLUMN a;  -- should not drop b
+\d gtest27
+
+-- with inheritance
+CREATE TABLE gtest29 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest29_1 () INHERITS (gtest29);
+ALTER TABLE gtest29 ALTER COLUMN b DROP EXPRESSION;
+\d gtest29
+\d gtest29_1
+DROP TABLE gtest29 CASCADE;
+CREATE TABLE gtest29 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest29_1 () INHERITS (gtest29);
+ALTER TABLE ONLY gtest29 ALTER COLUMN b DROP EXPRESSION;
+\d gtest29
+\d gtest29_1
+
 -- triggers
 CREATE TABLE gtest26 (
     a int PRIMARY KEY,

base-commit: 27a3b2ad836c9e7dd243bfebc760a9df9d6fd5a3
-- 
2.24.1

From d59ecf25468bfd10cc7ddf941faf2fb1092cb01c Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Sat, 11 Jan 2020 07:33:49 +0100
Subject: [PATCH v3] ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION

Add an ALTER TABLE subcommand for dropping the generated property from
a column, per SQL standard.

Reviewed-by: Sergei Kornilov <sk@zsrv.org>
Discussion: https://www.postgresql.org/message-id/flat/2f7f1d9c-946e-0453-d841-4f38eb9d69b6%402ndquadrant.com
---
 doc/src/sgml/ref/alter_table.sgml       |  18 +++
 src/backend/catalog/sql_features.txt    |   2 +-
 src/backend/commands/tablecmds.c        | 161 +++++++++++++++++++++++-
 src/backend/parser/gram.y               |  20 ++-
 src/bin/psql/tab-complete.c             |   2 +-
 src/include/nodes/parsenodes.h          |   1 +
 src/include/parser/kwlist.h             |   1 +
 src/test/regress/expected/generated.out |  87 +++++++++++++
 src/test/regress/sql/generated.sql      |  38 ++++++
 9 files changed, 326 insertions(+), 4 deletions(-)

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 8403c797e2..4bf449587c 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -46,6 +46,7 @@
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET DEFAULT <replaceable class="parameter">expression</replaceable>
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP DEFAULT
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET | DROP } NOT NULL
+    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP EXPRESSION [ IF EXISTS ]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET GENERATED { ALWAYS | BY DEFAULT } | SET <replaceable>sequence_option</replaceable> | RESTART [ [ WITH ] <replaceable class="parameter">restart</replaceable> ] } [...]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP IDENTITY [ IF EXISTS ]
@@ -241,6 +242,23 @@ <title>Description</title>
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>DROP EXPRESSION [ IF EXISTS ]</literal></term>
+    <listitem>
+     <para>
+      This form turns a stored generated column into a normal base column.
+      Existing data in the columns is retained, but future changes will no
+      longer apply the generation expression.
+     </para>
+
+     <para>
+      If <literal>DROP EXPRESSION IF EXISTS</literal> is specified and the
+      column is not a stored generated column, no error is thrown.  In this
+      case a notice is issued instead.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY</literal></term>
     <term><literal>SET GENERATED { ALWAYS | BY DEFAULT }</literal></term>
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index ab3e381cff..9f840ddfd2 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -252,7 +252,7 @@ F381	Extended schema manipulation	03	ALTER TABLE statement: DROP CONSTRAINT clau
 F382	Alter column data type			YES	
 F383	Set column not null clause			YES	
 F384	Drop identity property clause			YES	
-F385	Drop column generation expression clause			NO	
+F385	Drop column generation expression clause			YES	
 F386	Set identity column generation clause			YES	
 F391	Long identifiers			YES	
 F392	Unicode escapes in identifiers			YES	
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 421bc28727..2ec3fc5014 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -388,6 +388,8 @@ static ObjectAddress ATExecAddIdentity(Relation rel, const char *colName,
 static ObjectAddress ATExecSetIdentity(Relation rel, const char *colName,
 									   Node *def, LOCKMODE lockmode);
 static ObjectAddress ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode);
+static void ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recursing);
+static ObjectAddress ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode);
 static ObjectAddress ATExecSetStatistics(Relation rel, const char *colName, int16 colNum,
 										 Node *newValue, LOCKMODE lockmode);
 static ObjectAddress ATExecSetOptions(Relation rel, const char *colName,
@@ -3672,6 +3674,7 @@ AlterTableGetLockLevel(List *cmds)
 			case AT_AddIdentity:
 			case AT_DropIdentity:
 			case AT_SetIdentity:
+			case AT_DropExpression:
 				cmd_lockmode = AccessExclusiveLock;
 				break;
 
@@ -3946,6 +3949,12 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			/* No command-specific prep needed */
 			pass = AT_PASS_COL_ATTRS;
 			break;
+		case AT_DropExpression:	/* ALTER COLUMN DROP EXPRESSION */
+			ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);
+			ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);
+			ATPrepDropExpression(rel, cmd, recursing);
+			pass = AT_PASS_DROP;
+			break;
 		case AT_SetStatistics:	/* ALTER COLUMN SET STATISTICS */
 			ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW | ATT_INDEX | ATT_PARTITIONED_INDEX | ATT_FOREIGN_TABLE);
 			ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);
@@ -4265,6 +4274,9 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_CheckNotNull:	/* check column is already marked NOT NULL */
 			ATExecCheckNotNull(tab, rel, cmd->name, lockmode);
 			break;
+		case AT_DropExpression:
+			address = ATExecDropExpression(rel, cmd->name, cmd->missing_ok, lockmode);
+			break;
 		case AT_SetStatistics:	/* ALTER COLUMN SET STATISTICS */
 			address = ATExecSetStatistics(rel, cmd->name, cmd->num, cmd->def, lockmode);
 			break;
@@ -6457,7 +6469,9 @@ ATExecColumnDefault(Relation rel, const char *colName,
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("column \"%s\" of relation \"%s\" is a generated column",
-						colName, RelationGetRelationName(rel))));
+						colName, RelationGetRelationName(rel)),
+				 newDefault || TupleDescAttr(tupdesc, attnum - 1)->attgenerated != ATTRIBUTE_GENERATED_STORED ? 0 :
+				 errhint("Use ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION instead.")));
 
 	/*
 	 * Remove any old default for the column.  We use RESTRICT here for
@@ -6725,6 +6739,151 @@ ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE
 	return address;
 }
 
+/*
+ * ALTER TABLE ALTER COLUMN DROP EXPRESSION
+ */
+static void
+ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recursing)
+{
+	/*
+	 * Cannot drop generation expression from inherited columns.
+	 */
+	if (!recursing)
+	{
+		HeapTuple	tuple;
+		Form_pg_attribute attTup;
+
+		tuple = SearchSysCacheCopyAttName(RelationGetRelid(rel), cmd->name);
+		if (!HeapTupleIsValid(tuple))
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_COLUMN),
+					 errmsg("column \"%s\" of relation \"%s\" does not exist",
+							cmd->name, RelationGetRelationName(rel))));
+
+		attTup = (Form_pg_attribute) GETSTRUCT(tuple);
+
+		if (attTup->attinhcount > 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
+					 errmsg("cannot drop generation expression from inherited column")));
+	}
+}
+
+/*
+ * Return the address of the affected column.
+ */
+static ObjectAddress
+ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode)
+{
+	HeapTuple	tuple;
+	Form_pg_attribute attTup;
+	AttrNumber	attnum;
+	Relation	attrelation;
+	ObjectAddress address;
+
+	attrelation = table_open(AttributeRelationId, RowExclusiveLock);
+	tuple = SearchSysCacheCopyAttName(RelationGetRelid(rel), colName);
+	if (!HeapTupleIsValid(tuple))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_COLUMN),
+				 errmsg("column \"%s\" of relation \"%s\" does not exist",
+						colName, RelationGetRelationName(rel))));
+
+	attTup = (Form_pg_attribute) GETSTRUCT(tuple);
+	attnum = attTup->attnum;
+
+	if (attnum <= 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot alter system column \"%s\"",
+						colName)));
+
+	if (attTup->attgenerated != ATTRIBUTE_GENERATED_STORED)
+	{
+		if (!missing_ok)
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("column \"%s\" of relation \"%s\" is not a stored generated column",
+							colName, RelationGetRelationName(rel))));
+		else
+		{
+			ereport(NOTICE,
+					(errmsg("column \"%s\" of relation \"%s\" is not a stored generated column, skipping",
+							colName, RelationGetRelationName(rel))));
+			heap_freetuple(tuple);
+			table_close(attrelation, RowExclusiveLock);
+			return InvalidObjectAddress;
+		}
+	}
+
+	attTup->attgenerated = '\0';
+	CatalogTupleUpdate(attrelation, &tuple->t_self, tuple);
+
+	InvokeObjectPostAlterHook(RelationRelationId,
+							  RelationGetRelid(rel),
+							  attTup->attnum);
+	ObjectAddressSubSet(address, RelationRelationId,
+						RelationGetRelid(rel), attnum);
+	heap_freetuple(tuple);
+
+	table_close(attrelation, RowExclusiveLock);
+
+	CommandCounterIncrement();
+
+	RemoveAttrDefault(RelationGetRelid(rel), attnum, DROP_RESTRICT, false, false);
+
+	/*
+	 * Remove all dependencies of this (formerly generated) column on other
+	 * columns in the same table.  (See StoreAttrDefault() for which
+	 * dependencies are created.)  We don't expect there to be dependencies
+	 * between columns of the same table for other reasons, so it's okay to
+	 * remove all of them.
+	 */
+	{
+		Relation	depRel;
+		ScanKeyData key[3];
+		SysScanDesc scan;
+		HeapTuple	tup;
+
+		depRel = table_open(DependRelationId, RowExclusiveLock);
+
+		ScanKeyInit(&key[0],
+					Anum_pg_depend_classid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(RelationRelationId));
+		ScanKeyInit(&key[1],
+					Anum_pg_depend_objid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(RelationGetRelid(rel)));
+		ScanKeyInit(&key[2],
+					Anum_pg_depend_objsubid,
+					BTEqualStrategyNumber, F_INT4EQ,
+					Int32GetDatum(attnum));
+
+		scan = systable_beginscan(depRel, DependDependerIndexId, true,
+								  NULL, 3, key);
+
+		while (HeapTupleIsValid(tup = systable_getnext(scan)))
+		{
+			Form_pg_depend depform = (Form_pg_depend) GETSTRUCT(tup);
+
+			if (depform->refclassid == RelationRelationId &&
+				depform->refobjid == RelationGetRelid(rel) &&
+				depform->refobjsubid != 0 &&
+				depform->deptype == DEPENDENCY_AUTO)
+			{
+				CatalogTupleDelete(depRel, &tup->t_self);
+			}
+		}
+
+		systable_endscan(scan);
+
+		table_close(depRel, RowExclusiveLock);
+	}
+
+	return address;
+}
+
 /*
  * ALTER TABLE ALTER COLUMN SET STATISTICS
  *
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index ad5be902b0..bb1e499d52 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -636,7 +636,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	DOUBLE_P DROP
 
 	EACH ELSE ENABLE_P ENCODING ENCRYPTED END_P ENUM_P ESCAPE EVENT EXCEPT
-	EXCLUDE EXCLUDING EXCLUSIVE EXECUTE EXISTS EXPLAIN
+	EXCLUDE EXCLUDING EXCLUSIVE EXECUTE EXISTS EXPLAIN EXPRESSION
 	EXTENSION EXTERNAL EXTRACT
 
 	FALSE_P FAMILY FETCH FILTER FIRST_P FLOAT_P FOLLOWING FOR
@@ -2126,6 +2126,23 @@ alter_table_cmd:
 					n->name = $3;
 					$$ = (Node *)n;
 				}
+			/* ALTER TABLE <name> ALTER [COLUMN] <colname> DROP EXPRESSION */
+			| ALTER opt_column ColId DROP EXPRESSION
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_DropExpression;
+					n->name = $3;
+					$$ = (Node *)n;
+				}
+			/* ALTER TABLE <name> ALTER [COLUMN] <colname> DROP EXPRESSION IF EXISTS */
+			| ALTER opt_column ColId DROP EXPRESSION IF_P EXISTS
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_DropExpression;
+					n->name = $3;
+					n->missing_ok = true;
+					$$ = (Node *)n;
+				}
 			/* ALTER TABLE <name> ALTER [COLUMN] <colname> SET STATISTICS <SignedIconst> */
 			| ALTER opt_column ColId SET STATISTICS SignedIconst
 				{
@@ -15193,6 +15210,7 @@ unreserved_keyword:
 			| EXCLUSIVE
 			| EXECUTE
 			| EXPLAIN
+			| EXPRESSION
 			| EXTENSION
 			| EXTERNAL
 			| FAMILY
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index 2fd88866c9..b52396c17a 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -2015,7 +2015,7 @@ psql_completion(const char *text, int start, int end)
 	/* ALTER TABLE ALTER [COLUMN] <foo> DROP */
 	else if (Matches("ALTER", "TABLE", MatchAny, "ALTER", "COLUMN", MatchAny, "DROP") ||
 			 Matches("ALTER", "TABLE", MatchAny, "ALTER", MatchAny, "DROP"))
-		COMPLETE_WITH("DEFAULT", "IDENTITY", "NOT NULL");
+		COMPLETE_WITH("DEFAULT", "EXPRESSION", "IDENTITY", "NOT NULL");
 	else if (Matches("ALTER", "TABLE", MatchAny, "CLUSTER"))
 		COMPLETE_WITH("ON");
 	else if (Matches("ALTER", "TABLE", MatchAny, "CLUSTER", "ON"))
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index cdfa0568f7..28d837b8fa 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1785,6 +1785,7 @@ typedef enum AlterTableType
 	AT_ColumnDefault,			/* alter column default */
 	AT_DropNotNull,				/* alter column drop not null */
 	AT_SetNotNull,				/* alter column set not null */
+	AT_DropExpression,			/* alter column drop expression */
 	AT_CheckNotNull,			/* check column is already marked not null */
 	AT_SetStatistics,			/* alter column set statistics */
 	AT_SetOptions,				/* alter column set ( options ) */
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 0fe4e6cb20..3c1c232d37 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -155,6 +155,7 @@ PG_KEYWORD("exclusive", EXCLUSIVE, UNRESERVED_KEYWORD)
 PG_KEYWORD("execute", EXECUTE, UNRESERVED_KEYWORD)
 PG_KEYWORD("exists", EXISTS, COL_NAME_KEYWORD)
 PG_KEYWORD("explain", EXPLAIN, UNRESERVED_KEYWORD)
+PG_KEYWORD("expression", EXPRESSION, UNRESERVED_KEYWORD)
 PG_KEYWORD("extension", EXTENSION, UNRESERVED_KEYWORD)
 PG_KEYWORD("external", EXTERNAL, UNRESERVED_KEYWORD)
 PG_KEYWORD("extract", EXTRACT, COL_NAME_KEYWORD)
diff --git a/src/test/regress/expected/generated.out b/src/test/regress/expected/generated.out
index 8cffef0477..a6d3670af4 100644
--- a/src/test/regress/expected/generated.out
+++ b/src/test/regress/expected/generated.out
@@ -648,6 +648,7 @@ ALTER TABLE gtest27 ALTER COLUMN x TYPE boolean USING x <> 0;  -- error
 ERROR:  generation expression for column "x" cannot be cast automatically to type boolean
 ALTER TABLE gtest27 ALTER COLUMN x DROP DEFAULT;  -- error
 ERROR:  column "x" of relation "gtest27" is a generated column
+HINT:  Use ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION instead.
 -- It's possible to alter the column types this way:
 ALTER TABLE gtest27
   DROP COLUMN x,
@@ -683,6 +684,92 @@ SELECT * FROM gtest27;
  4 | 11 | 30
 (2 rows)
 
+-- ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION
+CREATE TABLE gtest29 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+INSERT INTO gtest29 (a) VALUES (3), (4);
+ALTER TABLE gtest29 ALTER COLUMN a DROP EXPRESSION;  -- error
+ERROR:  column "a" of relation "gtest29" is not a stored generated column
+ALTER TABLE gtest29 ALTER COLUMN a DROP EXPRESSION IF EXISTS;  -- notice
+NOTICE:  column "a" of relation "gtest29" is not a stored generated column, skipping
+ALTER TABLE gtest29 ALTER COLUMN b DROP EXPRESSION;
+INSERT INTO gtest29 (a) VALUES (5);
+INSERT INTO gtest29 (a, b) VALUES (6, 66);
+SELECT * FROM gtest29;
+ a | b  
+---+----
+ 3 |  6
+ 4 |  8
+ 5 |   
+ 6 | 66
+(4 rows)
+
+\d gtest29
+              Table "public.gtest29"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+
+-- check that dependencies between columns have also been removed
+ALTER TABLE gtest29 DROP COLUMN a;  -- should not drop b
+\d gtest29
+              Table "public.gtest29"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ b      | integer |           |          | 
+
+-- with inheritance
+CREATE TABLE gtest30 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest30_1 () INHERITS (gtest30);
+ALTER TABLE gtest30 ALTER COLUMN b DROP EXPRESSION;
+\d gtest30
+              Table "public.gtest30"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Number of child tables: 1 (Use \d+ to list them.)
+
+\d gtest30_1
+             Table "public.gtest30_1"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Inherits: gtest30
+
+DROP TABLE gtest30 CASCADE;
+NOTICE:  drop cascades to table gtest30_1
+CREATE TABLE gtest30 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest30_1 () INHERITS (gtest30);
+ALTER TABLE ONLY gtest30 ALTER COLUMN b DROP EXPRESSION;
+\d gtest30
+              Table "public.gtest30"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Number of child tables: 1 (Use \d+ to list them.)
+
+\d gtest30_1
+                           Table "public.gtest30_1"
+ Column |  Type   | Collation | Nullable |              Default               
+--------+---------+-----------+----------+------------------------------------
+ a      | integer |           |          | 
+ b      | integer |           |          | generated always as (a * 2) stored
+Inherits: gtest30
+
+ALTER TABLE gtest30_1 ALTER COLUMN b DROP EXPRESSION;  -- error
+ERROR:  cannot drop generation expression from inherited column
 -- triggers
 CREATE TABLE gtest26 (
     a int PRIMARY KEY,
diff --git a/src/test/regress/sql/generated.sql b/src/test/regress/sql/generated.sql
index ff5c8607de..f0e6a22dac 100644
--- a/src/test/regress/sql/generated.sql
+++ b/src/test/regress/sql/generated.sql
@@ -351,6 +351,44 @@ CREATE TABLE gtest27 (
 \d gtest27
 SELECT * FROM gtest27;
 
+-- ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION
+CREATE TABLE gtest29 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+INSERT INTO gtest29 (a) VALUES (3), (4);
+ALTER TABLE gtest29 ALTER COLUMN a DROP EXPRESSION;  -- error
+ALTER TABLE gtest29 ALTER COLUMN a DROP EXPRESSION IF EXISTS;  -- notice
+ALTER TABLE gtest29 ALTER COLUMN b DROP EXPRESSION;
+INSERT INTO gtest29 (a) VALUES (5);
+INSERT INTO gtest29 (a, b) VALUES (6, 66);
+SELECT * FROM gtest29;
+\d gtest29
+
+-- check that dependencies between columns have also been removed
+ALTER TABLE gtest29 DROP COLUMN a;  -- should not drop b
+\d gtest29
+
+-- with inheritance
+CREATE TABLE gtest30 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest30_1 () INHERITS (gtest30);
+ALTER TABLE gtest30 ALTER COLUMN b DROP EXPRESSION;
+\d gtest30
+\d gtest30_1
+DROP TABLE gtest30 CASCADE;
+CREATE TABLE gtest30 (
+    a int,
+    b int GENERATED ALWAYS AS (a * 2) STORED
+);
+CREATE TABLE gtest30_1 () INHERITS (gtest30);
+ALTER TABLE ONLY gtest30 ALTER COLUMN b DROP EXPRESSION;
+\d gtest30
+\d gtest30_1
+ALTER TABLE gtest30_1 ALTER COLUMN b DROP EXPRESSION;  -- error
+
 -- triggers
 CREATE TABLE gtest26 (
     a int PRIMARY KEY,
-- 
2.24.1