Bug#52849 [Com]: datetime index not work

Problem: after introduction of "WL#2649 Number-to-string conversions"

This query:
  SET NAMES cp850; -- Or any other non-latin1 ASCII-based character set
  SELECT * FROM t1
  WHERE datetime_column='2010-01-01 00:00:00'
started to add extra character set conversion: 
  SELECT * FROM t1
  WHERE CONVERT(datetime_column USING cp850)='2010-01-01 00:00:00';

so index on DATETIME column was not used anymore.
Fix:
  avoid convertion of NUMERIC/DATETIME items
  (i.e. those with derivation DERIVATION_NUMERIC).

mysql-test/r/type_datetime.result

@@ -638,3 +638,22 @@ CAST(CAST('2008-07-29T10:42:51.1234567' AS DateTime) AS DECIMAL(30,7))
 Warnings:
 Warning	1292	Truncated incorrect datetime value: '2008-07-29T10:42:51.1234567'
 End of 5.1 tests
+#
+# Start of 5.5 tests
+#
+#
+# Bug#52849 datetime index not work
+#
+CREATE TABLE t1 (Id INT, AtTime DATETIME, KEY AtTime (AtTime));
+SET NAMES CP850;
+INSERT INTO t1 VALUES (1,'2010-04-12 22:30:12'), (2,'2010-04-12 22:30:12'), (3,'2010-04-12 22:30:12');
+EXPLAIN EXTENDED SELECT * FROM t1 FORCE INDEX(attime) WHERE AtTime = '2010-02-22 18:40:07';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ref	AtTime	AtTime	9	const	1	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`Id` AS `Id`,`test`.`t1`.`AtTime` AS `AtTime` from `test`.`t1` FORCE INDEX (`attime`) where (`test`.`t1`.`AtTime` = '2010-02-22 18:40:07')
+DROP TABLE t1;
+SET NAMES latin1;
+#
+# End of 5.5 tests
+#

mysql-test/t/type_datetime.test

@@ -446,3 +446,21 @@ SELECT CAST(CAST('00000002006-000008-0000010 000010:0000011:00000012.012345'  AS
 SELECT CAST(CAST('2008-07-29T10:42:51.1234567' AS DateTime) AS DECIMAL(30,7));
 
 --echo End of 5.1 tests
+
+--echo #
+--echo # Start of 5.5 tests
+--echo #
+
+--echo #
+--echo # Bug#52849 datetime index not work
+--echo #
+CREATE TABLE t1 (Id INT, AtTime DATETIME, KEY AtTime (AtTime));
+SET NAMES CP850;
+INSERT INTO t1 VALUES (1,'2010-04-12 22:30:12'), (2,'2010-04-12 22:30:12'), (3,'2010-04-12 22:30:12');
+EXPLAIN EXTENDED SELECT * FROM t1 FORCE INDEX(attime) WHERE AtTime = '2010-02-22 18:40:07';
+DROP TABLE t1;
+SET NAMES latin1;
+
+--echo #
+--echo # End of 5.5 tests
+--echo #

sql/item.cc

@@ -1796,6 +1796,24 @@ bool agg_item_set_converter(DTCollation &coll, const char *fname,
                                   &dummy_offset))
       continue;
 
+    /*
+      No needs to add converter if an "arg" is NUMERIC or DATETIME
+      value (which is pure ASCII) and at the same time target DTCollation
+      is ASCII-compatible. For example, no needs to rewrite:
+        SELECT * FROM t1 WHERE datetime_field = '2010-01-01';
+      to
+        SELECT * FROM t1 WHERE CONVERT(datetime_field USING cs) = '2010-01-01';
+      
+      TODO: avoid conversion of any values with
+      repertoire ASCII and 7bit-ASCII-compatible,
+      not only numeric/datetime origin.
+    */
+    if ((*arg)->collation.derivation == DERIVATION_NUMERIC &&
+        (*arg)->collation.repertoire == MY_REPERTOIRE_ASCII &&
+        !((*arg)->collation.collation->state & MY_CS_NONASCII) &&
+        !(coll.collation->state & MY_CS_NONASCII))
+      continue;
+
     if (!(conv= (*arg)->safe_charset_converter(coll.collation)) &&
         ((*arg)->collation.repertoire == MY_REPERTOIRE_ASCII))
       conv= new Item_func_conv_charset(*arg, coll.collation, 1);