Switch some system functions to use get_call_result_type()

This shaves some code by replacing the combinations of
CreateTemplateTupleDesc()/TupleDescInitEntry() hardcoding a mapping of
the attributes listed in pg_proc.dat by get_call_result_type() to build
the TupleDesc needed for the rows generated.

get_call_result_type() is more expensive than the former style, but this
removes some duplication with the lists of OUT parameters (pg_proc.dat
and the attributes hardcoded in these code paths).  This is applied to
functions that are not considered as critical (aka that could be called
repeatedly for monitoring purposes).

Author: Bharath Rupireddy
Reviewed-by: Robert Haas, Álvaro Herrera, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACV23HW5HP5hFjd89FNS-z5X8r2jNXdMXcpN2BgTtKd87w@mail.gmail.com

1 files changed, 4 insertions, 22 deletions

diff --git a/contrib/old_snapshot/time_mapping.c b/contrib/old_snapshot/time_mapping.c
index 2d8cb742c34..aa4901fa1c4 100644
--- a/contrib/old_snapshot/time_mapping.c
+++ b/contrib/old_snapshot/time_mapping.c
@@ -38,7 +38,6 @@ PG_MODULE_MAGIC;
 PG_FUNCTION_INFO_V1(pg_old_snapshot_time_mapping);
 
 static OldSnapshotTimeMapping *GetOldSnapshotTimeMapping(void);
-static TupleDesc MakeOldSnapshotTimeMappingTupleDesc(void);
 static HeapTuple MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc,
 												 OldSnapshotTimeMapping *mapping);
 
@@ -54,12 +53,15 @@ pg_old_snapshot_time_mapping(PG_FUNCTION_ARGS)
 	if (SRF_IS_FIRSTCALL())
 	{
 		MemoryContext oldcontext;
+		TupleDesc	tupdesc;
 
 		funcctx = SRF_FIRSTCALL_INIT();
 		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
 		mapping = GetOldSnapshotTimeMapping();
 		funcctx->user_fctx = mapping;
-		funcctx->tuple_desc = MakeOldSnapshotTimeMappingTupleDesc();
+		if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+			elog(ERROR, "return type must be a row type");
+		funcctx->tuple_desc = tupdesc;
 		MemoryContextSwitchTo(oldcontext);
 	}
 
@@ -102,26 +104,6 @@ GetOldSnapshotTimeMapping(void)
 }
 
 /*
- * Build a tuple descriptor for the pg_old_snapshot_time_mapping() SRF.
- */
-static TupleDesc
-MakeOldSnapshotTimeMappingTupleDesc(void)
-{
-	TupleDesc	tupdesc;
-
-	tupdesc = CreateTemplateTupleDesc(NUM_TIME_MAPPING_COLUMNS);
-
-	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "array_offset",
-					   INT4OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "end_timestamp",
-					   TIMESTAMPTZOID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "newest_xmin",
-					   XIDOID, -1, 0);
-
-	return BlessTupleDesc(tupdesc);
-}
-
-/*
  * Convert one entry from the old snapshot time mapping to a HeapTuple.
  */
 static HeapTuple