Store number of tuples in WalRcvExecResult

It seems to be a useful information while allocating memory for queries
that returns more than one row. It reduces memory allocation
for initial table synchronization.

While in it, since we have the number of columns, allocate only nfields
for cstrs instead of MaxTupleAttributeNumber.

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -853,6 +853,7 @@ libpqrcv_processTuples(PGresult *pgres, WalRcvExecResult *walres,
 				 errdetail("Expected %d fields, got %d fields.",
 						   nRetTypes, nfields)));
 
+	walres->ntuples = PQntuples(pgres);
 	walres->tuplestore = tuplestore_begin_heap(true, false, work_mem);
 
 	/* Create tuple descriptor corresponding to expected result. */
@@ -863,7 +864,7 @@ libpqrcv_processTuples(PGresult *pgres, WalRcvExecResult *walres,
 	attinmeta = TupleDescGetAttInMetadata(walres->tupledesc);
 
 	/* No point in doing more here if there were no tuples returned. */
-	if (PQntuples(pgres) == 0)
+	if (walres->ntuples == 0)
 		return;
 
 	/* Create temporary context for local allocations. */
@@ -872,9 +873,9 @@ libpqrcv_processTuples(PGresult *pgres, WalRcvExecResult *walres,
 									   ALLOCSET_DEFAULT_SIZES);
 
 	/* Process returned rows. */
-	for (tupn = 0; tupn < PQntuples(pgres); tupn++)
+	for (tupn = 0; tupn < walres->ntuples; tupn++)
 	{
-		char	   *cstrs[MaxTupleAttributeNumber];
+		char	   *cstrs[nfields];
 
 		ProcessWalRcvInterrupts();
 

src/backend/replication/logical/tablesync.c

@@ -720,9 +720,8 @@ fetch_remote_table_info(char *nspname, char *relname,
 				(errmsg("could not fetch table info for table \"%s.%s\": %s",
 						nspname, relname, res->err)));
 
-	/* We don't know the number of rows coming, so allocate enough space. */
-	lrel->attnames = palloc0(MaxTupleAttributeNumber * sizeof(char *));
-	lrel->atttyps = palloc0(MaxTupleAttributeNumber * sizeof(Oid));
+	lrel->attnames = palloc0(res->ntuples * sizeof(char *));
+	lrel->atttyps = palloc0(res->ntuples * sizeof(Oid));
 	lrel->attkeys = NULL;
 
 	natt = 0;

src/include/replication/walreceiver.h

@@ -189,6 +189,7 @@ typedef struct WalRcvExecResult
 	char	   *err;
 	Tuplestorestate *tuplestore;
 	TupleDesc	tupledesc;
+	int			ntuples;
 } WalRcvExecResult;
 
 /* libpqwalreceiver hooks */