Fixed bug mdev-11081.

The idea of this fix was taken from the patch by Roy Lyseng
for mysql-5.6 bug iBug#14740889: "Wrong result for aggregate
functions when executing query through cursor".

Here's Roy's comment for his patch:
"
The problem was that a grouped query did not behave properly when
executed using a cursor. On further inspection, the query used one
intermediate temporary table for the grouping.
Then, Select_materialize::send_result_set_metadata created a temporary
table for storing the query result. Notice that get_unit_column_types()
is used to retrieve column meta-data for the query. The items contained
in this list are later modified so that their result_field points to
the row buffer of the materialized temporary table for the cursor.
But prior to this, these result_field objects have been prepared for
use in the grouping operation, by JOIN::make_tmp_tables_info(), hence
the grouping operation operates on wrong column buffers.

The problem is solved by using the list JOIN::fields when copying data
to the materialized table. This list is set by JOIN::make_tmp_tables_info()
and points to the columns of the last intermediate temporary table of
the executed query. For a UNION, it points to the temporary table
that is the result of the UNION query.

Notice that we have to assign a value to ::fields early in JOIN::optimize()
in case the optimization shortcuts due to a const plan detection.

A more optimal solution might be to avoid creating the final temporary
table when the query result is already stored in a temporary table.
"
The patch does not contain a test case, but the description of the
problem corresponds exactly what could be observed in the test
case for mdev-11081.

2 files changed, 77 insertions, 0 deletions

diff --git a/mysql-test/r/sp.result b/mysql-test/r/sp.result
index 130e789d3eb..aca9458f124 100644
--- a/mysql-test/r/sp.result
+++ b/mysql-test/r/sp.result
@@ -8001,3 +8001,42 @@ return 1;
 end |
 ERROR 0A000: Not allowed to return a result set from a function
 drop table t1,t2;
+#
+# MDEV-11081: CURSOR for query with GROUP BY
+#
+CREATE TABLE t1 (name VARCHAR(10), value INT);
+INSERT INTO t1 VALUES ('b',1);
+INSERT INTO t1 VALUES ('b',1);
+INSERT INTO t1 VALUES ('c',1);
+INSERT INTO t1 VALUES ('a',1);
+INSERT INTO t1 VALUES ('a',1);
+INSERT INTO t1 VALUES ('a',1);
+CREATE PROCEDURE p1 ()
+BEGIN
+DECLARE done INT DEFAULT FALSE;
+DECLARE v_name VARCHAR(10);
+DECLARE v_total INT;
+DECLARE c CURSOR FOR
+SELECT name, SUM(value) AS total FROM t1 GROUP BY name;
+DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
+OPEN c;
+read_loop:
+LOOP
+FETCH c INTO v_name, v_total;
+IF done THEN
+LEAVE read_loop;
+END IF;
+SELECT v_name, v_total;
+END LOOP;
+CLOSE c;
+END;
+|
+CALL p1();
+v_name	v_total
+a	3
+v_name	v_total
+b	2
+v_name	v_total
+c	1
+DROP PROCEDURE p1;
+DROP TABLE t1;
diff --git a/mysql-test/t/sp.test b/mysql-test/t/sp.test
index c80e1eaaa3e..aaab59bcb89 100644
--- a/mysql-test/t/sp.test
+++ b/mysql-test/t/sp.test
@@ -9446,3 +9446,41 @@ end |
 --delimiter ;
 
 drop table t1,t2;
+
+--echo #
+--echo # MDEV-11081: CURSOR for query with GROUP BY
+--echo #
+
+CREATE TABLE t1 (name VARCHAR(10), value INT);
+INSERT INTO t1 VALUES ('b',1);
+INSERT INTO t1 VALUES ('b',1);
+INSERT INTO t1 VALUES ('c',1);
+INSERT INTO t1 VALUES ('a',1);
+INSERT INTO t1 VALUES ('a',1);
+INSERT INTO t1 VALUES ('a',1);
+DELIMITER |;
+CREATE PROCEDURE p1 ()
+BEGIN
+  DECLARE done INT DEFAULT FALSE;
+  DECLARE v_name VARCHAR(10);
+  DECLARE v_total INT;
+  DECLARE c CURSOR FOR
+    SELECT name, SUM(value) AS total FROM t1 GROUP BY name;
+  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
+  OPEN c;
+read_loop:
+  LOOP
+    FETCH c INTO v_name, v_total;
+    IF done THEN
+      LEAVE read_loop;
+    END IF;
+    SELECT v_name, v_total;
+  END LOOP;
+  CLOSE c;
+END;
+|
+DELIMITER ;|
+CALL p1();
+DROP PROCEDURE p1;
+DROP TABLE t1;
+