Support domains over composite types.

This is the last major omission in our domains feature: you can now
make a domain over anything that's not a pseudotype.

The major complication from an implementation standpoint is that places
that might be creating tuples of a domain type now need to be prepared
to apply domain_check().  It seems better that unprepared code fail
with an error like "<type> is not composite" than that it silently fail
to apply domain constraints.  Therefore, relevant infrastructure like
get_func_result_type() and lookup_rowtype_tupdesc() has been adjusted
to treat domain-over-composite as a distinct case that unprepared code
won't recognize, rather than just transparently treating it the same
as plain composite.  This isn't a 100% solution to the possibility of
overlooked domain checks, but it catches most places.

In passing, improve typcache.c's support for domains (it can now cache
the identity of a domain's base type), and rewrite the argument handling
logic in jsonfuncs.c's populate_record[set]_worker to reduce duplicative
per-call lookups.

I believe this is code-complete so far as the core and contrib code go.
The PLs need varying amounts of work, which will be tackled in followup
patches.

Discussion: https://postgr.es/m/4206.1499798337@sss.pgh.pa.us

1 files changed, 13 insertions, 2 deletions

diff --git a/src/backend/executor/functions.c b/src/backend/executor/functions.c
index 42a4ca94e97..98eb777421b 100644
--- a/src/backend/executor/functions.c
+++ b/src/backend/executor/functions.c
@@ -1665,7 +1665,15 @@ check_sql_fn_retval(Oid func_id, Oid rettype, List *queryTreeList,
 	}
 	else if (fn_typtype == TYPTYPE_COMPOSITE || rettype == RECORDOID)
 	{
-		/* Returns a rowtype */
+		/*
+		 * Returns a rowtype.
+		 *
+		 * Note that we will not consider a domain over composite to be a
+		 * "rowtype" return type; it goes through the scalar case above.  This
+		 * is because SQL functions don't provide any implicit casting to the
+		 * result type, so there is no way to produce a domain-over-composite
+		 * result except by computing it as an explicit single-column result.
+		 */
 		TupleDesc	tupdesc;
 		int			tupnatts;	/* physical number of columns in tuple */
 		int			tuplogcols; /* # of nondeleted columns in tuple */
@@ -1711,7 +1719,10 @@ check_sql_fn_retval(Oid func_id, Oid rettype, List *queryTreeList,
 			}
 		}
 
-		/* Is the rowtype fixed, or determined only at runtime? */
+		/*
+		 * Is the rowtype fixed, or determined only at runtime?  (Note we
+		 * cannot see TYPEFUNC_COMPOSITE_DOMAIN here.)
+		 */
 		if (get_func_result_type(func_id, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
 		{
 			/*