Don't exaggerate the number of temporary blocks read.

A read that returns zero bytes (or an error) should not increment
the number of temporary blocks read.

Thomas Munro

Discussion: http://postgr.es/m/CAEepm=21xgihg=WaG+O5MFotEZfN6kFETpfw+RkSnEqNQqGn2Q@mail.gmail.com

src/backend/storage/file/buffile.c

@@ -264,7 +264,8 @@ BufFileLoadBuffer(BufFile *file)
 	file->offsets[file->curFile] += file->nbytes;
 	/* we choose not to advance curOffset here */
 
-	pgBufferUsage.temp_blks_read++;
+	if (file->nbytes > 0)
+		pgBufferUsage.temp_blks_read++;
 }
 
 /*