Improve efficiency in pg_lock_status()

Allocate memory of CdbPgResults.pg_results with palloc0() instead
of calloc(), and free the memory afer use.

The CdbPgResults.pg_results array that is returned from various dispatch
functions is allocated by cdbdisp_returnResults() via calloc(), but in
most cases the memory is not free()-ed after use.

To avoid memory leak, the array is now allocated with palloc0() and
recycled with pfree().

Track which row and which result set is being processed in function context in pg_lock_status(), 
so that an ineffient inner loop can be eliminated.

Author: Fang Zheng <zhengfang.xjtu@gmail.com>

src/backend/utils/adt/lockfuncs.c

@@ -57,6 +57,8 @@ typedef struct
 
 	int			numSegLocks;	/* Total number of locks being reported back to client */
 	int			numsegresults;	/* If we dispatch to segDBs, the number of segresults */
+	int			whichResultset; /* which result set is being processed */
+	int			whichRow;	/* which row in current result set is being processed */
 	struct pg_result **segresults;	/* pg_result for each segDB */
 } PG_Lock_Status;
 
@@ -166,6 +168,8 @@ pg_lock_status(PG_FUNCTION_ARGS)
 
 		mystatus->numSegLocks = 0;
 		mystatus->numsegresults = 0;
+		mystatus->whichResultset = 0;
+		mystatus->whichRow = 0;
 		mystatus->segresults = NULL;
 
 		/*
@@ -500,35 +504,37 @@ pg_lock_status(PG_FUNCTION_ARGS)
 		Datum		values[NUM_LOCK_STATUS_COLUMNS];
 		bool		nulls[NUM_LOCK_STATUS_COLUMNS];
 		int i;
-		int whichresultset = 0;
-		int whichelement = mystatus->currIdx - lockData->nelements;
-		int whichrow = whichelement;
+		int whichresultset = mystatus->whichResultset;
+		int whichrow = mystatus->whichRow;
 
 		Assert(Gp_role == GP_ROLE_DISPATCH);
 
 		/*
 		 * Because we have one result set per segDB (rather than one big result set with everything),
-		 * we need to figure out which result set we are on, and which row within that result set
-		 * we are returning.
-		 *
-		 * So, we walk through all the result sets and all the rows in each one, in order.
+		 * we use mystatus->whichResultset and mystatus->whichRow to track which result set we are on,
+		 * and which row within that result set we are returning, respectively.
 		 */
-
-		while(whichrow >= PQntuples(mystatus->segresults[whichresultset]))
+		if (whichrow < PQntuples(mystatus->segresults[whichresultset]))
+		{
+			/* Advance to next row in current result set. */
+			mystatus->whichRow++;
+		}
+		else
 		{
-			whichrow -= PQntuples(mystatus->segresults[whichresultset]);
+			/* Advance to next result set. */
+			mystatus->whichResultset++;
+			mystatus->whichRow = 0;
 			whichresultset++;
+			whichrow = 0;
+
+			/*
+			 * If this condition is true, we have already sent everything back,
+			 * and we just want to do the SRF_RETURN_DONE
+			 */
 			if (whichresultset >= mystatus->numsegresults)
 				break;
 		}
 
-		/*
-		 * If this condition is true, we have already sent everything back,
-		 * and we just want to do the SRF_RETURN_DONE
-		 */
-		if (whichresultset >= mystatus->numsegresults)
-			break;
-
 		mystatus->currIdx++;
 
 		/*