Don't count zero-filled buffers as 'read' in EXPLAIN.

If you extend a relation, it should count as a block written, not
read (we write a zero-filled block).  If you ask for a zero-filled
buffer, it shouldn't be counted as read or written.

Later we might consider counting zero-filled buffers with a separate
counter, if they become more common due to future work.

Author: Thomas Munro
Reviewed-by: Haribabu Kommi, Kyotaro Horiguchi, David Rowley
Discussion: https://postgr.es/m/CAEepm%3D3JytB3KPpvSwXzkY%2Bdwc5zC8P8Lk7Nedkoci81_0E9rA%40mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -733,7 +733,10 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
 		if (found)
 			pgBufferUsage.local_blks_hit++;
-		else
+		else if (isExtend)
+			pgBufferUsage.local_blks_written++;
+		else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
+				 mode == RBM_ZERO_ON_ERROR)
 			pgBufferUsage.local_blks_read++;
 	}
 	else
@@ -746,7 +749,10 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 							 strategy, &found);
 		if (found)
 			pgBufferUsage.shared_blks_hit++;
-		else
+		else if (isExtend)
+			pgBufferUsage.shared_blks_written++;
+		else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
+				 mode == RBM_ZERO_ON_ERROR)
 			pgBufferUsage.shared_blks_read++;
 	}