Bug #17059925: UNIONS COMPUTES ROWS_EXAMINED INCORRECTLY

ISSUE:
------
For UNION of selects, rows examined by the query will be sum
of rows examined by individual select operations and rows
examined for union operation. The value of session level
global counter that is used to count the rows examined by a
select statement should be accumulated and reset before it
is used for next select statement. But we have missed to
reset the same. Because of this examined row count of a
select query is accounted more than once.

SOLUTION:
---------
In union reset the session level global counter used to
accumulate count of examined rows after its value is saved.

mysql-test/r/union.result:
  Expected output of testcase added.
mysql-test/t/union.test:
  Test to verify examined row count of Union operations.
sql/sql_union.cc:
  Reset the value of thd->examined_row_count after
  accumulating the value.

1 files changed, 31 insertions, 0 deletions

diff --git a/mysql-test/t/union.test b/mysql-test/t/union.test
index f5a5cad77e8..147c1d3834b 100644
--- a/mysql-test/t/union.test
+++ b/mysql-test/t/union.test
@@ -1225,3 +1225,34 @@ SELECT(SELECT 0 AS a FROM dual UNION SELECT 1 AS a FROM dual ORDER BY a DESC LIM
 SELECT(SELECT 1 AS a ORDER BY a) AS dev;
 SELECT(SELECT 1 AS a LIMIT 1) AS dev;
 SELECT(SELECT 1 AS a FROM dual ORDER BY a DESC LIMIT 1) AS dev;
+
+
+--echo #
+--echo # Bug #17059925 : UNIONS COMPUTES ROWS_EXAMINED INCORRECTLY
+--echo #
+
+## Save current state of slow log variables
+SET @old_slow_query_log= @@global.slow_query_log;
+SET @old_log_output= @@global.log_output;
+SET @old_long_query_time= @@long_query_time;
+SET GLOBAL log_output= "TABLE";
+SET GLOBAL slow_query_log= ON;
+SET SESSION long_query_time= 0;
+
+CREATE TABLE t17059925 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+INSERT INTO t17059925 VALUES (1), (2), (3);
+INSERT INTO t2 VALUES (4), (5), (6);
+INSERT INTO t3 VALUES (7), (8), (9);
+TRUNCATE table mysql.slow_log;
+--sorted_result
+SELECT * FROM t17059925 UNION SELECT * FROM t2 UNION SELECT * FROM t3;
+SELECT sql_text, rows_examined FROM mysql.slow_log WHERE sql_text LIKE '%SELECT%t17059925%';
+DROP TABLE t17059925, t2, t3;
+
+## Reset to initial values
+SET @@long_query_time= @old_long_query_time;
+SET @@global.log_output= @old_log_output;
+SET @@global.slow_query_log= @old_slow_query_log;
+