Bug#46454: MySQL wrong index optimisation leads to incorrect result & crashes

Problem 1:
When the 'Using index' optimization is used, the optimizer may still - after
cost-based optimization - decide to use another index in order to avoid using
a temporary table. But when this happens, the flag to the storage engine to 
read index only (not table) was still set. Fixed by resetting the flag in the 
storage engine and TABLE structure in the above scenario, unless the new index
allows for the same optimization.
Problem 2:
When a 'ref' access method was employed by cost-based optimizer, (when the column
is non-NULLable), it was assumed that it needed no initialization if 'quick' access
methods (since they are based on range scan). When ORDER BY optimization overrides 
the decision, however, it expects to have this initialized and hence crashes. 
Fixed in 5.1 (was fixed in 6.0 already) by initializing 'quick' even when there's 
'ref' access. 

mysql-test/r/order_by.result:
  Bug#46454: Test result.
mysql-test/t/order_by.test:
  Bug#46454: Test case.
sql/sql_select.cc:
  Bug#46454: 
  Problem 1 fixed in make_join_select()
  Problem 2 fixed in test_if_skip_sort_order()
sql/table.h:
  Bug#46454: Added comment to field.

1 files changed, 41 insertions, 0 deletions

diff --git a/mysql-test/t/order_by.test b/mysql-test/t/order_by.test
index f09c1aa7bd4..cca1e3209cc 100644
--- a/mysql-test/t/order_by.test
+++ b/mysql-test/t/order_by.test
@@ -1361,3 +1361,44 @@ DROP TABLE t1;
 
 
 
+#
+# Bug#46454: MySQL wrong index optimisation leads to incorrect result & crashes 
+#
+CREATE TABLE t1 (
+  a INT,
+  b INT NOT NULL,
+  c char(100),
+  KEY (b, c),
+  KEY (b, a, c)
+)
+DEFAULT CHARSET = utf8;
+
+INSERT INTO t1 VALUES 
+(1,  1, 1),
+(2,  2, 2),
+(3,  3, 3),
+(4,  4, 4),
+(5,  5, 5),
+(6,  6, 6),
+(7,  7, 7),
+(8,  8, 8),
+(9,  9, 9);
+
+INSERT INTO t1 SELECT a + 10,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 20,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 40,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 80,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 160, b, c FROM t1;
+INSERT INTO t1 SELECT a + 320, b, c FROM t1;
+INSERT INTO t1 SELECT a + 640, b, c FROM t1;
+INSERT INTO t1 SELECT a + 1280, b, c FROM t1 LIMIT 80;
+
+EXPLAIN
+SELECT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 9;
+SELECT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 9;
+
+EXPLAIN
+SELECT DISTINCT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 0, 9;
+SELECT DISTINCT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 0, 9;
+
+DROP TABLE t1;