Fixed bug #18206.

The bug report revealed two problems related to min/max optimization:
1. If the length of a constant key used in a SARGable condition for
for the MIN/MAX fields is greater than the length of the field an 
unwanted warning on key truncation is issued;
2. If MIN/MAX optimization is applied to a partial index, like INDEX(b(4))
than can lead to returning a wrong result set.


mysql-test/r/func_group.result:
  Added test cases for bug #18206.
mysql-test/t/func_group.test:
  Added test cases for bug #18206.
sql/opt_sum.cc:
  Fixed bug #18206.
  Suppressed the warning about data truncation when store_val_in_field
  was used to store keys for the field used in MIN/MAX optimization.
  Blocked MIN/MAX optimization for partial keys, such as in INDEX(b(4)).
sql/sql_select.cc:
  Fixed bug #18206.
  Added a parameter for the function store_val_in_field allowing to
  control setting warnings about data truncation in the function.
sql/sql_select.h:
  Fixed bug #18206.
  Added a parameter for the function store_val_in_field allowing to
  control setting warnings about data truncation in the function.

1 files changed, 19 insertions, 0 deletions

diff --git a/mysql-test/t/func_group.test b/mysql-test/t/func_group.test
index dc647dbcfeb..f8a3ed0f25e 100644
--- a/mysql-test/t/func_group.test
+++ b/mysql-test/t/func_group.test
@@ -598,4 +598,23 @@ select count(*), min(7), max(7) from t2m, t1i;
 
 drop table t1m, t1i, t2m, t2i;
 
+#
+# Bug #18206: min/max optimization cannot be applied to partial index
+#
+
+CREATE TABLE t1 (id int PRIMARY KEY, b char(3), INDEX(b));
+INSERT INTO t1 VALUES (1,'xx'), (2,'aa');
+SELECT * FROM t1;
+
+SELECT MAX(b) FROM t1 WHERE b < 'ppppp';
+SHOW WARNINGS;
+SELECT MAX(b) FROM t1 WHERE b < 'pp';
+DROP TABLE t1;
+
+CREATE TABLE t1 (id int PRIMARY KEY, b char(16), INDEX(b(4)));
+INSERT INTO t1 VALUES (1, 'xxxxbbbb'), (2, 'xxxxaaaa');
+SELECT MAX(b) FROM t1;
+EXPLAIN SELECT MAX(b) FROM t1;
+DROP TABLE t1;
+
 # End of 4.1 tests