Bug #55656: mysqldump can be slower after bug 39653 fix.

After fix for bug 39653 the shortest available secondary index was used for
full table scan. Primary clustered key was used only if no secondary index
can be used. However, when chosen secondary index includes all fields of the
table being scanned it's better to use primary index since the amount of
data to scan is the same but the primary index is clustered.
Now the find_shortest_key function takes this into account.


mysql-test/suite/innodb/r/innodb_mysql.result:
  Added a test case for the bug#55656.
mysql-test/suite/innodb/t/innodb_mysql.test:
  Added a test case for the bug#55656.
sql/sql_select.cc:
  Bug #55656: mysqldump can be slower after bug #39653 fix.
  The find_shortest_key function now prefers clustered primary key
  if found secondary key includes all fields of the table.

2 files changed, 83 insertions, 0 deletions

diff --git a/mysql-test/suite/innodb/r/innodb_mysql.result b/mysql-test/suite/innodb/r/innodb_mysql.result
index 23619b91fbc..ba8ac0ba86c 100644
--- a/mysql-test/suite/innodb/r/innodb_mysql.result
+++ b/mysql-test/suite/innodb/r/innodb_mysql.result
@@ -2529,4 +2529,62 @@ SELECT * FROM t1 FOR UPDATE;
 SELECT * FROM t1 GROUP BY (SELECT a FROM t2 LIMIT 1 FOR UPDATE) + t1.a;
 ERROR 40001: Deadlock found when trying to get lock; try restarting transaction
 DROP TABLE t1,t2;
+#
+# Bug#55656: mysqldump can be slower after bug #39653 fix
+#
+CREATE TABLE t1 (a INT , b INT, c INT, d INT,
+KEY (b), PRIMARY KEY (a,b)) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1,1,1,1), (2,2,2,2), (3,3,3,3);
+EXPLAIN SELECT COUNT(*) FROM t1;
+id	1
+select_type	SIMPLE
+table	t1
+type	index
+possible_keys	NULL
+key	b
+key_len	4
+ref	NULL
+rows	3
+Extra	Using index
+DROP INDEX b ON t1;
+CREATE INDEX b ON t1(a,b);
+EXPLAIN SELECT COUNT(*) FROM t1;
+id	1
+select_type	SIMPLE
+table	t1
+type	index
+possible_keys	NULL
+key	b
+key_len	8
+ref	NULL
+rows	3
+Extra	Using index
+DROP INDEX b ON t1;
+CREATE INDEX b ON t1(a,b,c);
+EXPLAIN SELECT COUNT(*) FROM t1;
+id	1
+select_type	SIMPLE
+table	t1
+type	index
+possible_keys	NULL
+key	b
+key_len	13
+ref	NULL
+rows	3
+Extra	Using index
+DROP INDEX b ON t1;
+CREATE INDEX b ON t1(a,b,c,d);
+EXPLAIN SELECT COUNT(*) FROM t1;
+id	1
+select_type	SIMPLE
+table	t1
+type	index
+possible_keys	NULL
+key	PRIMARY
+key_len	8
+ref	NULL
+rows	3
+Extra	Using index
+DROP TABLE t1;
+#
 End of 5.1 tests
diff --git a/mysql-test/suite/innodb/t/innodb_mysql.test b/mysql-test/suite/innodb/t/innodb_mysql.test
index d633cb2222e..4f13e15a627 100644
--- a/mysql-test/suite/innodb/t/innodb_mysql.test
+++ b/mysql-test/suite/innodb/t/innodb_mysql.test
@@ -781,5 +781,30 @@ disconnect con2;
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # Bug#55656: mysqldump can be slower after bug #39653 fix
+--echo #
+
+CREATE TABLE t1 (a INT , b INT, c INT, d INT,
+  KEY (b), PRIMARY KEY (a,b)) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1,1,1,1), (2,2,2,2), (3,3,3,3);
+--query_vertical EXPLAIN SELECT COUNT(*) FROM t1
+
+DROP INDEX b ON t1;
+CREATE INDEX b ON t1(a,b);
+--query_vertical EXPLAIN SELECT COUNT(*) FROM t1
+
+DROP INDEX b ON t1;
+CREATE INDEX b ON t1(a,b,c);
+--query_vertical EXPLAIN SELECT COUNT(*) FROM t1
+
+DROP INDEX b ON t1;
+CREATE INDEX b ON t1(a,b,c,d);
+--query_vertical EXPLAIN SELECT COUNT(*) FROM t1
+
+DROP TABLE t1;
+
+--echo #
+
 
 --echo End of 5.1 tests