Bug #24714857: FOUND_ROWS() RETURNS 1 WHEN NO ROWS FOUND

This patch is specific to mysql-5.7

ISSUE: FOUND_ROWS() returned 1 even though the previous
       select did not return any rows. This was happening
       because the statistics returned by InnoDB returned
       1 for the select even though there are no rows.
       The same count was further returned by FOUND_ROWS().

FIX: In InnoDB, while determining statistics, check whether
     its for temporary table. If its temporary table,
     do not increment the row count.

Author: Kailasnath Nagarkar

mysql-test/r/func_found_rows.result

@@ -0,0 +1,16 @@
+#
+# Bug #24714857 FOUND_ROWS() RETURNS 1 WHEN NO ROWS FOUND
+#
+CREATE TABLE tbl (
+col_text TEXT
+);
+SELECT SQL_CALC_FOUND_ROWS col_text FROM tbl AS tbl1
+UNION ALL
+SELECT col_text FROM tbl AS tbl2
+ORDER BY col_text
+LIMIT 0, 2;
+col_text
+SELECT FOUND_ROWS();
+FOUND_ROWS()
+0
+DROP TABLE tbl;

mysql-test/t/func_found_rows.test

@@ -0,0 +1,17 @@
+--echo #
+--echo # Bug #24714857 FOUND_ROWS() RETURNS 1 WHEN NO ROWS FOUND
+--echo #
+
+CREATE TABLE tbl (
+  col_text TEXT
+);
+
+SELECT SQL_CALC_FOUND_ROWS col_text FROM tbl AS tbl1
+UNION ALL
+SELECT col_text FROM tbl AS tbl2
+ORDER BY col_text
+LIMIT 0, 2;
+
+SELECT FOUND_ROWS();
+
+DROP TABLE tbl;

storage/innobase/handler/ha_innodb.cc

@@ -13883,9 +13883,12 @@ ha_innobase::info_low(
 		HA_STATUS_TIME flag set, while the left join optimizer does not
 		set that flag, we add one to a zero value if the flag is not
 		set. That way SHOW TABLE STATUS will show the best estimate,
-		while the optimizer never sees the table empty. */
+		while the optimizer never sees the table empty.
+		However, if it is internal temporary table used by optimizer,
+		the count should be accurate */
 
-		if (n_rows == 0 && !(flag & HA_STATUS_TIME)) {
+		if (n_rows == 0 && !(flag & HA_STATUS_TIME)
+                  && table_share->table_category != TABLE_CATEGORY_TEMPORARY) {
 			n_rows++;
 		}