domain over virtual generated column

domains that don't have constraints work just fine.  domain constraints check
happen within ExecComputeGenerated.  we compute the virtual generated column in
ExecComputeGenerated and discarded the value.  if value cannot coerce to domain
then we will error out.

domain_with_constaint can be element type of range, multirange, array,
composite. To be bulletproof, if this column type is not type of TYPTYPE_BASE or
it's an array type, then we do actually compute the virtual generated
expression.  This can have negative performance for some INSERTs.  The following
two query shows in pg_catalog, most of the system type is TYPTYPE_BASE.  So I
guess this compromise is fine.

select count(*),typtype from pg_type
where typnamespace = 'pg_catalog'::regnamespace
group by 2 order by 1 desc;

ALTER DOMAIN ADD CONSTRAINT is supported for virtual generated column.

ALTER TABLE ADD VIRTUAL GENERATED COLUMN.
need table scan to verify new column generation expression value
satisfied the domain constraints. but no need table rewrite.

discussion: https://postgr.es/m/CACJufxFNUHxuSE=g20C2aLO3d+4T_j9h0x8esMirzcC4FrLbBg@mail.gmail.com

Author: jianhe-fun

src/backend/catalog/heap.c

@@ -583,17 +583,6 @@ CheckAttributeType(const char *attname,
 	}
 	else if (att_typtype == TYPTYPE_DOMAIN)
 	{
-		/*
-		 * Prevent virtual generated columns from having a domain type.  We
-		 * would have to enforce domain constraints when columns underlying
-		 * the generated column change.  This could possibly be implemented,
-		 * but it's not.
-		 */
-		if (flags & CHKATYPE_IS_VIRTUAL)
-			ereport(ERROR,
-					errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					errmsg("virtual generated column \"%s\" cannot have a domain type", attname));
-
 		/*
 		 * If it's a domain, recurse to check its base type.
 		 */

src/backend/commands/copyfrom.c

@@ -1343,9 +1343,10 @@ CopyFrom(CopyFromState cstate)
 			}
 			else
 			{
-				/* Compute stored generated columns */
+				/* Compute generated columns */
 				if (resultRelInfo->ri_RelationDesc->rd_att->constr &&
-					resultRelInfo->ri_RelationDesc->rd_att->constr->has_generated_stored)
+					(resultRelInfo->ri_RelationDesc->rd_att->constr->has_generated_stored ||
+					 resultRelInfo->ri_RelationDesc->rd_att->constr->has_generated_virtual))
 					ExecComputeGenerated(resultRelInfo, estate, myslot,
 										 CMD_INSERT);
 

src/backend/commands/tablecmds.c

@@ -6009,7 +6009,7 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 			 * rebuild data.
 			 */
 			if (tab->constraints != NIL || tab->verify_new_notnull ||
-				tab->partition_constraint != NULL)
+				tab->newvals || tab->partition_constraint != NULL)
 				ATRewriteTable(tab, InvalidOid);
 
 			/*
@@ -6129,6 +6129,7 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap)
 	BulkInsertState bistate;
 	int			ti_options;
 	ExprState  *partqualstate = NULL;
+	List		*domain_virtual_attrs = NIL;
 
 	/*
 	 * Open the relation(s).  We have surely already locked the existing
@@ -6204,6 +6205,16 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap)
 
 		/* expr already planned */
 		ex->exprstate = ExecInitExpr((Expr *) ex->expr, NULL);
+		if (tab->rewrite == 0 && ex->is_generated)
+		{
+			Form_pg_attribute attr = TupleDescAttr(newTupDesc, ex->attnum - 1);
+			if (attr->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL &&
+				DomainHasConstraints(attr->atttypid))
+			{
+				Assert(!attr->attisdropped);
+				domain_virtual_attrs = lappend_int(domain_virtual_attrs, attr->attnum);
+			}
+		}
 	}
 
 	notnull_attrs = notnull_virtual_attrs = NIL;
@@ -6239,6 +6250,13 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap)
 			needscan = true;
 	}
 
+	/*
+	 * We need verify domain constraints on the virtual generated column are
+	 * satisfied, which requires table scan.
+	*/
+	if (domain_virtual_attrs != NIL)
+		needscan = true;
+
 	if (newrel || needscan)
 	{
 		ExprContext *econtext;
@@ -6497,6 +6515,35 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap)
 				}
 			}
 
+			/*
+			 * domain_virtual_attrs is a list of newly added columns that are
+			 * virtual generated columns with a domain data type and associated
+			 * constraints. We need to verify that these domain constraints are
+			 * satisfied.
+			*/
+			if (domain_virtual_attrs != NIL)
+			{
+				Assert(tab->rewrite == 0);
+
+				foreach(l, tab->newvals)
+				{
+					NewColumnValue *ex = lfirst(l);
+
+					if (!ex->is_generated)
+						continue;
+
+					if(list_member_int(domain_virtual_attrs, ex->attnum) &&
+					   !ExecCheck(ex->exprstate, econtext))
+					{
+						Form_pg_attribute attr = TupleDescAttr(newTupDesc, ex->attnum - 1);
+						ereport(ERROR,
+								errcode(ERRCODE_CHECK_VIOLATION),
+								errmsg("value for domain %s is violated by some row",
+								format_type_be(attr->atttypid)));
+					}
+				}
+			}
+
 			if (partqualstate && !ExecCheck(partqualstate, econtext))
 			{
 				if (tab->validate_default)