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

1 files changed, 31 insertions, 2 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index d3e0f882438..b4e61e2777b 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -194,13 +194,15 @@ 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;
 
 /*
@@ -4960,7 +4962,9 @@ 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;
 
@@ -4968,6 +4972,9 @@ 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.
 				 */
@@ -5891,6 +5918,7 @@ 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);
 		}
@@ -10407,6 +10435,7 @@ 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))