Fix handling of generated columns in ALTER TABLE.

ALTER TABLE failed if a column referenced in a GENERATED expression
had been added or changed in type earlier in the ALTER command.
That's because the GENERATED expression needs to be evaluated
against the table's updated tuples, but it was being evaluated
against the original tuples.  (Fortunately the executor has adequate
cross-checks to notice the mismatch, so we just got an obscure error
message and not anything more dangerous.)

Per report from Andreas Joseph Krogh.  Back-patch to v12 where
GENERATED was added.

Discussion: https://postgr.es/m/VisenaEmail.200.231b0a41523275d0.16ea7f800c7@tc7-visena

src/backend/commands/tablecmds.c

@@ -192,13 +192,15 @@ typedef struct NewConstraint
  * Phase 3 copy (this could be either a new column with a non-null default, or
  * a column that we're changing the type of).  Columns without such an entry
  * are just copied from the old table during ATRewriteTable.  Note that the
- * expr is an expression over *old* table values.
+ * expr is an expression over *old* table values, except when is_generated
+ * is true; then it is an expression over columns of the *new* tuple.
  */
 typedef struct NewColumnValue
 {
 	AttrNumber	attnum;			/* which column */
 	Expr	   *expr;			/* expression to compute */
 	ExprState  *exprstate;		/* execution state */
+	bool		is_generated;	/* is it a GENERATED expression? */
 } NewColumnValue;
 
 /*
@@ -4961,7 +4963,9 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 
 				/*
 				 * Process supplied expressions to replace selected columns.
-				 * Expression inputs come from the old tuple.
+				 *
+				 * First, evaluate expressions whose inputs come from the old
+				 * tuple.
 				 */
 				econtext->ecxt_scantuple = oldslot;
 
@@ -4969,6 +4973,9 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 				{
 					NewColumnValue *ex = lfirst(l);
 
+					if (ex->is_generated)
+						continue;
+
 					newslot->tts_values[ex->attnum - 1]
 						= ExecEvalExpr(ex->exprstate,
 									   econtext,
@@ -4977,6 +4984,26 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 
 				ExecStoreVirtualTuple(newslot);
 
+				/*
+				 * Now, evaluate any expressions whose inputs come from the
+				 * new tuple.  We assume these columns won't reference each
+				 * other, so that there's no ordering dependency.
+				 */
+				econtext->ecxt_scantuple = newslot;
+
+				foreach(l, tab->newvals)
+				{
+					NewColumnValue *ex = lfirst(l);
+
+					if (!ex->is_generated)
+						continue;
+
+					newslot->tts_values[ex->attnum - 1]
+						= ExecEvalExpr(ex->exprstate,
+									   econtext,
+									   &newslot->tts_isnull[ex->attnum - 1]);
+				}
+
 				/*
 				 * Constraints might reference the tableoid column, so
 				 * initialize t_tableOid before evaluating them.
@@ -5892,6 +5919,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			newval = (NewColumnValue *) palloc0(sizeof(NewColumnValue));
 			newval->attnum = attribute.attnum;
 			newval->expr = expression_planner(defval);
+			newval->is_generated = (colDef->generated != '\0');
 
 			tab->newvals = lappend(tab->newvals, newval);
 		}
@@ -10379,6 +10407,7 @@ ATPrepAlterColumnType(List **wqueue,
 		newval = (NewColumnValue *) palloc0(sizeof(NewColumnValue));
 		newval->attnum = attnum;
 		newval->expr = (Expr *) transform;
+		newval->is_generated = false;
 
 		tab->newvals = lappend(tab->newvals, newval);
 		if (ATColumnChangeRequiresRewrite(transform, attnum))

src/test/regress/expected/generated.out

@@ -593,40 +593,95 @@ ERROR:  cannot use generated column "b" in column generation expression
 DETAIL:  A generated column cannot reference another generated column.
 ALTER TABLE gtest25 ADD COLUMN x int GENERATED ALWAYS AS (z * 4) STORED;  -- error
 ERROR:  column "z" does not exist
+ALTER TABLE gtest25 ADD COLUMN c int DEFAULT 42,
+  ADD COLUMN x int GENERATED ALWAYS AS (c * 4) STORED;
+ALTER TABLE gtest25 ADD COLUMN d int DEFAULT 101;
+ALTER TABLE gtest25 ALTER COLUMN d SET DATA TYPE float8,
+  ADD COLUMN y float8 GENERATED ALWAYS AS (d * 4) STORED;
+SELECT * FROM gtest25 ORDER BY a;
+ a | b  | c  |  x  |  d  |  y  
+---+----+----+-----+-----+-----
+ 3 |  9 | 42 | 168 | 101 | 404
+ 4 | 12 | 42 | 168 | 101 | 404
+(2 rows)
+
+\d gtest25
+                                         Table "public.gtest25"
+ Column |       Type       | Collation | Nullable |                       Default                        
+--------+------------------+-----------+----------+------------------------------------------------------
+ a      | integer          |           | not null | 
+ b      | integer          |           |          | generated always as (a * 3) stored
+ c      | integer          |           |          | 42
+ x      | integer          |           |          | generated always as (c * 4) stored
+ d      | double precision |           |          | 101
+ y      | double precision |           |          | generated always as (d * 4::double precision) stored
+Indexes:
+    "gtest25_pkey" PRIMARY KEY, btree (a)
+
 -- ALTER TABLE ... ALTER COLUMN
 CREATE TABLE gtest27 (
     a int,
-    b int GENERATED ALWAYS AS (a * 2) STORED
+    b int,
+    x int GENERATED ALWAYS AS ((a + b) * 2) STORED
 );
-INSERT INTO gtest27 (a) VALUES (3), (4);
+INSERT INTO gtest27 (a, b) VALUES (3, 7), (4, 11);
 ALTER TABLE gtest27 ALTER COLUMN a TYPE text;  -- error
 ERROR:  cannot alter type of a column used by a generated column
-DETAIL:  Column "a" is used by generated column "b".
-ALTER TABLE gtest27 ALTER COLUMN b TYPE numeric;
+DETAIL:  Column "a" is used by generated column "x".
+ALTER TABLE gtest27 ALTER COLUMN x TYPE numeric;
 \d gtest27
                                 Table "public.gtest27"
  Column |  Type   | Collation | Nullable |                  Default                   
---------+---------+-----------+----------+--------------------------------------
+--------+---------+-----------+----------+--------------------------------------------
  a      | integer |           |          | 
- b      | numeric |           |          | generated always as ((a * 2)) stored
+ b      | integer |           |          | 
+ x      | numeric |           |          | generated always as (((a + b) * 2)) stored
 
 SELECT * FROM gtest27;
- a | b 
----+---
- 3 | 6
- 4 | 8
+ a | b  | x  
+---+----+----
+ 3 |  7 | 20
+ 4 | 11 | 30
 (2 rows)
 
-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
+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
+-- It's possible to alter the column types this way:
+ALTER TABLE gtest27
+  DROP COLUMN x,
+  ALTER COLUMN a TYPE bigint,
+  ALTER COLUMN b TYPE bigint,
+  ADD COLUMN x bigint GENERATED ALWAYS AS ((a + b) * 2) STORED;
 \d gtest27
                               Table "public.gtest27"
  Column |  Type  | Collation | Nullable |                 Default                  
---------+---------+-----------+----------+--------------------------------------
- a      | integer |           |          | 
- b      | numeric |           |          | generated always as ((a * 2)) stored
+--------+--------+-----------+----------+------------------------------------------
+ a      | bigint |           |          | 
+ b      | bigint |           |          | 
+ x      | bigint |           |          | generated always as ((a + b) * 2) stored
+
+-- Ideally you could just do this, but not today (and should x change type?):
+ALTER TABLE gtest27
+  ALTER COLUMN a TYPE float8,
+  ALTER COLUMN b TYPE float8;  -- error
+ERROR:  cannot alter type of a column used by a generated column
+DETAIL:  Column "a" is used by generated column "x".
+\d gtest27
+                              Table "public.gtest27"
+ Column |  Type  | Collation | Nullable |                 Default                  
+--------+--------+-----------+----------+------------------------------------------
+ a      | bigint |           |          | 
+ b      | bigint |           |          | 
+ x      | bigint |           |          | generated always as ((a + b) * 2) stored
+
+SELECT * FROM gtest27;
+ a | b  | x  
+---+----+----
+ 3 |  7 | 20
+ 4 | 11 | 30
+(2 rows)
 
 -- triggers
 CREATE TABLE gtest26 (

src/test/regress/sql/generated.sql

@@ -316,21 +316,40 @@ ALTER TABLE gtest25 ADD COLUMN b int GENERATED ALWAYS AS (a * 3) STORED;
 SELECT * FROM gtest25 ORDER BY a;
 ALTER TABLE gtest25 ADD COLUMN x int GENERATED ALWAYS AS (b * 4) STORED;  -- error
 ALTER TABLE gtest25 ADD COLUMN x int GENERATED ALWAYS AS (z * 4) STORED;  -- error
+ALTER TABLE gtest25 ADD COLUMN c int DEFAULT 42,
+  ADD COLUMN x int GENERATED ALWAYS AS (c * 4) STORED;
+ALTER TABLE gtest25 ADD COLUMN d int DEFAULT 101;
+ALTER TABLE gtest25 ALTER COLUMN d SET DATA TYPE float8,
+  ADD COLUMN y float8 GENERATED ALWAYS AS (d * 4) STORED;
+SELECT * FROM gtest25 ORDER BY a;
+\d gtest25
 
 -- ALTER TABLE ... ALTER COLUMN
 CREATE TABLE gtest27 (
     a int,
-    b int GENERATED ALWAYS AS (a * 2) STORED
+    b int,
+    x int GENERATED ALWAYS AS ((a + b) * 2) STORED
 );
-INSERT INTO gtest27 (a) VALUES (3), (4);
+INSERT INTO gtest27 (a, b) VALUES (3, 7), (4, 11);
 ALTER TABLE gtest27 ALTER COLUMN a TYPE text;  -- error
-ALTER TABLE gtest27 ALTER COLUMN b TYPE numeric;
+ALTER TABLE gtest27 ALTER COLUMN x TYPE numeric;
 \d gtest27
 SELECT * FROM gtest27;
-ALTER TABLE gtest27 ALTER COLUMN b TYPE boolean USING b <> 0;  -- error
-
-ALTER TABLE gtest27 ALTER COLUMN b DROP DEFAULT;  -- error
+ALTER TABLE gtest27 ALTER COLUMN x TYPE boolean USING x <> 0;  -- error
+ALTER TABLE gtest27 ALTER COLUMN x DROP DEFAULT;  -- error
+-- It's possible to alter the column types this way:
+ALTER TABLE gtest27
+  DROP COLUMN x,
+  ALTER COLUMN a TYPE bigint,
+  ALTER COLUMN b TYPE bigint,
+  ADD COLUMN x bigint GENERATED ALWAYS AS ((a + b) * 2) STORED;
+\d gtest27
+-- Ideally you could just do this, but not today (and should x change type?):
+ALTER TABLE gtest27
+  ALTER COLUMN a TYPE float8,
+  ALTER COLUMN b TYPE float8;  -- error
 \d gtest27
+SELECT * FROM gtest27;
 
 -- triggers
 CREATE TABLE gtest26 (