Remove rd_issyscat from RelationData.

Greenplum added rd_isyscat to Relation structure. Only usage of the same is in
markDirty() to decide if buffer should be marked dirty or not. The setting of
rd_issyscat is based in checking if relation name starts with "pg_" then set it
else not. Which anyways is very loose.

Modified instead to just make check based on if oid < FirstNormalObjectId or to
cover for pg_aoseg tables RelationGetNamespace(relation) ==
PG_AOSEGMENT_NAMESPACE. So, this allows us to remove the extra variable.

This patch is not trying to change the intent of GUC
`gp_disable_tuple_hints`. That's all together different discussion.

src/backend/utils/cache/relcache.c

@@ -866,7 +866,6 @@ RelationBuildDesc(Oid targetRelId, bool insertIt)
 	relation->rd_isnailed = false;
 	relation->rd_createSubid = InvalidSubTransactionId;
 	relation->rd_newRelfilenodeSubid = InvalidSubTransactionId;
-	relation->rd_issyscat = (strncmp(relation->rd_rel->relname.data, "pg_", 3) == 0);
 
 	switch (relation->rd_rel->relpersistence)
 	{
@@ -1414,7 +1413,6 @@ formrdesc(const char *relationName, Oid relationReltype,
 	relation->rd_createSubid = InvalidSubTransactionId;
 	relation->rd_newRelfilenodeSubid = InvalidSubTransactionId;
 	relation->rd_backend = InvalidBackendId;
-	relation->rd_issyscat = (strncmp(relationName, "pg_", 3) == 0);	/* GP */
 
 	/*
 	 * initialize relation tuple form
@@ -2636,9 +2634,6 @@ RelationBuildLocalRelation(const char *relname,
 	/* must flag that we have rels created in this transaction */
 	need_eoxact_work = true;
 
-	/* is it a system catalog? */
-	rel->rd_issyscat = (strncmp(relname, "pg_", 3) == 0);
-
 	/*
 	 * create a new tuple descriptor from the one passed in.  We do this
 	 * partly to copy it into the cache context, and partly because the new

src/backend/utils/time/tqual.c

@@ -65,6 +65,7 @@
 #include "storage/procarray.h"
 #include "utils/tqual.h"
 
+#include "catalog/pg_namespace.h"
 #include "cdb/cdbtm.h"
 #include "cdb/cdbvars.h"
 
@@ -100,12 +101,13 @@ markDirty(Buffer buffer, Relation relation, HeapTupleHeader tuple, bool isXmin)
 	}
 
 	/*
-	 * The GUC gp_disable_tuple_hints is on.  Do further evaluation whether we want to write out the
-	 * buffer or not.
+	 * The GUC gp_disable_tuple_hints is on. Do further evaluation whether we
+	 * want to write out the buffer or not.
 	 */
 	Assert(relation != NULL);
 
-	if (relation->rd_issyscat)
+	if (RelationGetRelid(relation) < FirstNormalObjectId ||
+		RelationGetNamespace(relation) == PG_AOSEGMENT_NAMESPACE)
 	{
 		/* Assume we want to always mark the buffer dirty */
 		MarkBufferDirtyHint(buffer);

src/include/utils/rel.h

@@ -136,8 +136,6 @@ typedef struct RelationData
 	struct SMgrRelationData *rd_smgr;	/* cached file handle, or NULL */
 	int			rd_refcnt;		/* reference count */
 	BackendId	rd_backend;		/* owning backend id, if temporary relation */
-	// GPDB_91_MERGE_FIXME: is rd_issyscat still needed?
-	bool		rd_issyscat;	/* GP: true => system catalog table (has "pg_" prefix) */
 	bool		rd_isnailed;	/* rel is nailed in cache */
 	bool		rd_isvalid;		/* relcache entry is valid */
 	char		rd_indexvalid;	/* state of rd_indexlist: 0 = not valid, 1 =