Bug#50939: Loose Index Scan unduly relies on engine to

remember range endpoints

The Loose Index Scan optimization keeps track of a sequence
of intervals. For the current interval it maintains the
current interval's endpoints. But the maximum endpoint was
not stored in the SQL layer; rather, it relied on the
storage engine to retain this value in-between reads. By
coincidence this holds for MyISAM and InnoDB. Not for the
partitioning engine, however.

Fixed by making the key values iterator 
(QUICK_RANGE_SELECT) keep track of the current maximum endpoint.
This is also more efficient as we save a call through the
handler API in case of open-ended intervals.

The code to calculate endpoints was extracted into 
separate methods in QUICK_RANGE_SELECT, and it was possible to
get rid of some code duplication as part of fix.

1 files changed, 44 insertions, 0 deletions

diff --git a/mysql-test/r/range.result b/mysql-test/r/range.result
index 64e00521cd2..ad8aec7383a 100644
--- a/mysql-test/r/range.result
+++ b/mysql-test/r/range.result
@@ -1653,4 +1653,48 @@ a	b
 0	0
 1	1
 DROP TABLE t1;
+#
+# Bug#50939: Loose Index Scan unduly relies on engine to remember range 
+# endpoints
+#
+CREATE TABLE t1 (
+a INT,
+b INT,
+KEY ( a, b )
+) PARTITION BY HASH (a) PARTITIONS 1;
+CREATE TABLE t2 (
+a INT,
+b INT,
+KEY ( a, b )
+);
+INSERT INTO t1 VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5);
+INSERT INTO t1 SELECT a +  5, b +  5 FROM t1;
+INSERT INTO t1 SELECT a + 10, b + 10 FROM t1;
+INSERT INTO t1 SELECT a + 20, b + 20 FROM t1;
+INSERT INTO t1 SELECT a + 40, b + 40 FROM t1;
+INSERT INTO t2 SELECT * FROM t1;
+# plans should be identical
+EXPLAIN SELECT a, MAX(b) FROM t1 WHERE a IN (10,100) GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	a	a	5	NULL	1	Using where; Using index for group-by
+EXPLAIN SELECT a, MAX(b) FROM t2 WHERE a IN (10,100) GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	range	a	a	5	NULL	2	Using where; Using index for group-by
+FLUSH status;
+SELECT a, MAX(b) FROM t1 WHERE a IN (10, 100) GROUP BY a;
+a	MAX(b)
+10	10
+# Should be no more than 4 reads.
+SHOW status LIKE 'handler_read_key';
+Variable_name	Value
+Handler_read_key	4
+FLUSH status;
+SELECT a, MAX(b) FROM t2 WHERE a IN (10, 100) GROUP BY a;
+a	MAX(b)
+10	10
+# Should be no more than 4 reads.
+SHOW status LIKE 'handler_read_key';
+Variable_name	Value
+Handler_read_key	4
+DROP TABLE t1, t2;
 End of 5.1 tests