Add IF NOT EXISTS processing to ALTER TABLE ADD COLUMN

Fabrízio de Royes Mello, reviewed by Payal Singh, Alvaro Herrera and
Michael Paquier.

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 +96,13 @@ 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 a column already exists with this name,
+      no error is thrown.
      </para>
     </listitem>
    </varlistentry>

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);
+				bool recurse, bool recursing, bool if_not_exists, LOCKMODE lockmode);
-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);
 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 */
+	/* skip if the name already exists and if_not_exists is true */
-	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)
@@ -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
+static bool
-check_for_column_name_collision(Relation rel, const char *colname)
+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;
 }
 
 /*

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} */

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;

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;