Fix usage of "tableoid" in GENERATED expressions.

We consider this supported (though I've got my doubts that it's a
good idea, because tableoid is not immutable).  However, several
code paths failed to fill the field in soon enough, causing such
a GENERATED expression to see zero or the wrong value.  This
occurred when ALTER TABLE adds a new GENERATED column to a table
with existing rows, and during regular INSERT or UPDATE on a
foreign table with GENERATED columns.

Noted during investigation of a report from Vitaly Ustinov.
Back-patch to v12 where GENERATED came in.

Discussion: https://postgr.es/m/CAM_DEiWR2DPT6U4xb-Ehigozzd3n3G37ZB1+867zbsEVtYoJww@mail.gmail.com

src/backend/commands/tablecmds.c

@@ -5761,6 +5761,14 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 				foreach(lc, dropped_attrs)
 					newslot->tts_isnull[lfirst_int(lc)] = true;
 
+				/*
+				 * Constraints and GENERATED expressions might reference the
+				 * tableoid column, so fill tts_tableOid with the desired
+				 * value.  (We must do this each time, because it gets
+				 * overwritten with newrel's OID during storing.)
+				 */
+				newslot->tts_tableOid = RelationGetRelid(oldrel);
+
 				/*
 				 * Process supplied expressions to replace selected columns.
 				 *
@@ -5804,11 +5812,6 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 									   &newslot->tts_isnull[ex->attnum - 1]);
 				}
 
-				/*
-				 * Constraints might reference the tableoid column, so
-				 * initialize t_tableOid before evaluating them.
-				 */
-				newslot->tts_tableOid = RelationGetRelid(oldrel);
 				insertslot = newslot;
 			}
 			else

src/backend/executor/nodeModifyTable.c

@@ -658,6 +658,12 @@ ExecInsert(ModifyTableState *mtstate,
 	}
 	else if (resultRelInfo->ri_FdwRoutine)
 	{
+		/*
+		 * GENERATED expressions might reference the tableoid column, so
+		 * (re-)initialize tts_tableOid before evaluating them.
+		 */
+		slot->tts_tableOid = RelationGetRelid(resultRelInfo->ri_RelationDesc);
+
 		/*
 		 * Compute stored generated columns
 		 */
@@ -729,7 +735,7 @@ ExecInsert(ModifyTableState *mtstate,
 		/*
 		 * AFTER ROW Triggers or RETURNING expressions might reference the
 		 * tableoid column, so (re-)initialize tts_tableOid before evaluating
-		 * them.
+		 * them.  (This covers the case where the FDW replaced the slot.)
 		 */
 		slot->tts_tableOid = RelationGetRelid(resultRelInfo->ri_RelationDesc);
 	}
@@ -738,8 +744,8 @@ ExecInsert(ModifyTableState *mtstate,
 		WCOKind		wco_kind;
 
 		/*
-		 * Constraints might reference the tableoid column, so (re-)initialize
+		 * Constraints and GENERATED expressions might reference the tableoid
-		 * tts_tableOid before evaluating them.
+		 * column, so (re-)initialize tts_tableOid before evaluating them.
 		 */
 		slot->tts_tableOid = RelationGetRelid(resultRelationDesc);
 
@@ -1629,6 +1635,12 @@ ExecUpdate(ModifyTableState *mtstate,
 	}
 	else if (resultRelInfo->ri_FdwRoutine)
 	{
+		/*
+		 * GENERATED expressions might reference the tableoid column, so
+		 * (re-)initialize tts_tableOid before evaluating them.
+		 */
+		slot->tts_tableOid = RelationGetRelid(resultRelInfo->ri_RelationDesc);
+
 		/*
 		 * Compute stored generated columns
 		 */
@@ -1651,7 +1663,7 @@ ExecUpdate(ModifyTableState *mtstate,
 		/*
 		 * AFTER ROW Triggers or RETURNING expressions might reference the
 		 * tableoid column, so (re-)initialize tts_tableOid before evaluating
-		 * them.
+		 * them.  (This covers the case where the FDW replaced the slot.)
 		 */
 		slot->tts_tableOid = RelationGetRelid(resultRelationDesc);
 	}
@@ -1662,8 +1674,8 @@ ExecUpdate(ModifyTableState *mtstate,
 		bool		update_indexes;
 
 		/*
-		 * Constraints might reference the tableoid column, so (re-)initialize
+		 * Constraints and GENERATED expressions might reference the tableoid
-		 * tts_tableOid before evaluating them.
+		 * column, so (re-)initialize tts_tableOid before evaluating them.
 		 */
 		slot->tts_tableOid = RelationGetRelid(resultRelationDesc);
 

src/test/regress/expected/generated.out

@@ -64,6 +64,7 @@ ERROR:  both identity and generation expression specified for column "b" of tabl
 LINE 1: ...t PRIMARY KEY, b int GENERATED ALWAYS AS identity GENERATED ...
                                                              ^
 -- reference to system column not allowed in generated column
+-- (except tableoid, which we test below)
 CREATE TABLE gtest_err_6a (a int PRIMARY KEY, b bool GENERATED ALWAYS AS (xmin <> 37) STORED);
 ERROR:  cannot use system column "xmin" in column generation expression
 LINE 1: ...a (a int PRIMARY KEY, b bool GENERATED ALWAYS AS (xmin <> 37...
@@ -455,14 +456,16 @@ DROP TYPE double_int;
 -- using tableoid is allowed
 CREATE TABLE gtest_tableoid (
   a int PRIMARY KEY,
-  b bool GENERATED ALWAYS AS (tableoid <> 0) STORED
+  b bool GENERATED ALWAYS AS (tableoid = 'gtest_tableoid'::regclass) STORED
 );
 INSERT INTO gtest_tableoid VALUES (1), (2);
+ALTER TABLE gtest_tableoid ADD COLUMN
+  c regclass GENERATED ALWAYS AS (tableoid) STORED;
 SELECT * FROM gtest_tableoid;
- a | b 
+ a | b |       c        
----+---
+---+---+----------------
- 1 | t
+ 1 | t | gtest_tableoid
- 2 | t
+ 2 | t | gtest_tableoid
 (2 rows)
 
 -- drop column behavior

src/test/regress/sql/generated.sql

@@ -29,6 +29,7 @@ CREATE TABLE gtest_err_5a (a int PRIMARY KEY, b int DEFAULT 5 GENERATED ALWAYS A
 CREATE TABLE gtest_err_5b (a int PRIMARY KEY, b int GENERATED ALWAYS AS identity GENERATED ALWAYS AS (a * 2) STORED);
 
 -- reference to system column not allowed in generated column
+-- (except tableoid, which we test below)
 CREATE TABLE gtest_err_6a (a int PRIMARY KEY, b bool GENERATED ALWAYS AS (xmin <> 37) STORED);
 
 -- various prohibited constructs
@@ -218,9 +219,11 @@ DROP TYPE double_int;
 -- using tableoid is allowed
 CREATE TABLE gtest_tableoid (
   a int PRIMARY KEY,
-  b bool GENERATED ALWAYS AS (tableoid <> 0) STORED
+  b bool GENERATED ALWAYS AS (tableoid = 'gtest_tableoid'::regclass) STORED
 );
 INSERT INTO gtest_tableoid VALUES (1), (2);
+ALTER TABLE gtest_tableoid ADD COLUMN
+  c regclass GENERATED ALWAYS AS (tableoid) STORED;
 SELECT * FROM gtest_tableoid;
 
 -- drop column behavior