1 files changed, 60 insertions, 27 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index ce7d115667e..fd31216b27b 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -7321,14 +7321,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		rawEnt = (RawColumnDefault *) palloc(sizeof(RawColumnDefault));
 		rawEnt->attnum = attribute->attnum;
 		rawEnt->raw_default = copyObject(colDef->raw_default);
-
-		/*
-		 * Attempt to skip a complete table rewrite by storing the specified
-		 * DEFAULT value outside of the heap.  This may be disabled inside
-		 * AddRelationNewConstraints if the optimization cannot be applied.
-		 */
-		rawEnt->missingMode = (colDef->generated != ATTRIBUTE_GENERATED_STORED);
-
+		rawEnt->missingMode = false;	/* XXX vestigial */
 		rawEnt->generated = colDef->generated;
 
 		/*
@@ -7340,13 +7333,6 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* Make the additional catalog changes visible */
 		CommandCounterIncrement();
-
-		/*
-		 * Did the request for a missing value work? If not we'll have to do a
-		 * rewrite
-		 */
-		if (!rawEnt->missingMode)
-			tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
 	}
 
 	/*
@@ -7363,9 +7349,9 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	 * rejects nulls.  If there are any domain constraints then we construct
 	 * an explicit NULL default value that will be passed through
 	 * CoerceToDomain processing.  (This is a tad inefficient, since it causes
-	 * rewriting the table which we really don't have to do, but the present
-	 * design of domain processing doesn't offer any simple way of checking
-	 * the constraints more directly.)
+	 * rewriting the table which we really wouldn't have to do; but we do it
+	 * to preserve the historical behavior that such a failure will be raised
+	 * only if the table currently contains some rows.)
 	 *
 	 * Note: we use build_column_default, and not just the cooked default
 	 * returned by AddRelationNewConstraints, so that the right thing happens
@@ -7384,6 +7370,9 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	 */
 	if (RELKIND_HAS_STORAGE(relkind))
 	{
+		bool		has_domain_constraints;
+		bool		has_missing = false;
+
 		/*
 		 * For an identity column, we can't use build_column_default(),
 		 * because the sequence ownership isn't set yet.  So do it manually.
@@ -7396,14 +7385,13 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			nve->typeId = attribute->atttypid;
 
 			defval = (Expr *) nve;
-
-			/* must do a rewrite for identity columns */
-			tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
 		}
 		else
 			defval = (Expr *) build_column_default(rel, attribute->attnum);
 
-		if (!defval && DomainHasConstraints(attribute->atttypid))
+		/* Build CoerceToDomain(NULL) expression if needed */
+		has_domain_constraints = DomainHasConstraints(attribute->atttypid);
+		if (!defval && has_domain_constraints)
 		{
 			Oid			baseTypeId;
 			int32		baseTypeMod;
@@ -7429,18 +7417,63 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		{
 			NewColumnValue *newval;
 
+			/* Prepare defval for execution, either here or in Phase 3 */
+			defval = expression_planner(defval);
+
+			/* Add the new default to the newvals list */
 			newval = (NewColumnValue *) palloc0(sizeof(NewColumnValue));
 			newval->attnum = attribute->attnum;
-			newval->expr = expression_planner(defval);
+			newval->expr = defval;
 			newval->is_generated = (colDef->generated != '\0');
 
 			tab->newvals = lappend(tab->newvals, newval);
-		}
 
-		if (DomainHasConstraints(attribute->atttypid))
-			tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
+			/*
+			 * Attempt to skip a complete table rewrite by storing the
+			 * specified DEFAULT value outside of the heap.  This is only
+			 * allowed for plain relations and non-generated columns, and the
+			 * default expression can't be volatile (stable is OK).  Note that
+			 * contain_volatile_functions deems CoerceToDomain immutable, but
+			 * here we consider that coercion to a domain with constraints is
+			 * volatile; else it might fail even when the table is empty.
+			 */
+			if (rel->rd_rel->relkind == RELKIND_RELATION &&
+				!colDef->generated &&
+				!has_domain_constraints &&
+				!contain_volatile_functions((Node *) defval))
+			{
+				EState	   *estate;
+				ExprState  *exprState;
+				Datum		missingval;
+				bool		missingIsNull;
+
+				/* Evaluate the default expression */
+				estate = CreateExecutorState();
+				exprState = ExecPrepareExpr(defval, estate);
+				missingval = ExecEvalExpr(exprState,
+										  GetPerTupleExprContext(estate),
+										  &missingIsNull);
+				/* If it turns out NULL, nothing to do; else store it */
+				if (!missingIsNull)
+				{
+					StoreAttrMissingVal(rel, attribute->attnum, missingval);
+					has_missing = true;
+				}
+				FreeExecutorState(estate);
+			}
+			else
+			{
+				/*
+				 * Failed to use missing mode.  We have to do a table rewrite
+				 * to install the value --- unless it's a virtual generated
+				 * column.
+				 */
+				if (colDef->generated != ATTRIBUTE_GENERATED_VIRTUAL)
+					tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
+			}
+		}
 
-		if (!TupleDescAttr(rel->rd_att, attribute->attnum - 1)->atthasmissing)
+		if (!has_missing)
 		{
 			/*
 			 * If the new column is NOT NULL, and there is no missing value,