MDEV-15321:different results when using value of optimizer_use_condition_selectivity=4 and =1

To disallow equality propagation for DATETIME with non-zero YYYYMMDD part we were setting null_value to true.
This caused issues when we were calculating selectivity for a condition as this returned IMPOSSIBLE WHERE.

The issue is resolved by not setting null_value to true for DATETIME with non-zero YYYYMMDD.

mysql-test/r/type_time.result

@@ -1215,5 +1215,36 @@ MAX(a)	MAX(COALESCE(a))
 10:20:30	10:20:30
 DROP TABLE t1;
 #
+# MDEV-15321: different results when using value of optimizer_use_condition_selectivity=4 and =1
+#
+SET @save_old_mode=@@old_mode;
+SET @@old_mode=zero_date_time_cast;
+CREATE TABLE t1 (a TIME);
+INSERT INTO t1 VALUES ('0000-00-00 10:20:30'),('0000-00-00 10:20:31');
+INSERT INTO t1 VALUES ('0000-00-01 10:20:30'),('0000-00-01 10:20:31');
+INSERT INTO t1 VALUES ('31 10:20:30'),('32 10:20:30'),('33 10:20:30'),('34 10:20:30');
+SET @save_optimizer_use_condition_selectivity= @@optimizer_use_condition_selectivity;
+SET @@optimizer_use_condition_selectivity=1;
+SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+a
+34:20:30
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	8	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where ((`test`.`t1`.`a` = '0000-00-01 10:20:30') and (length(`test`.`t1`.`a`) = 8))
+SET @@optimizer_use_condition_selectivity=4;
+SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+a
+34:20:30
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	8	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where ((`test`.`t1`.`a` = '0000-00-01 10:20:30') and (length(`test`.`t1`.`a`) = 8))
+drop table t1;
+SET @@optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
+set @@old_mode= @save_old_mode;
+#
 # End of 10.1 tests
 #

mysql-test/t/type_time.test

@@ -723,6 +723,29 @@ INSERT INTO t1 VALUES ('10:10:10'),('10:20:30');
 SELECT MAX(a), MAX(COALESCE(a)) FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-15321: different results when using value of optimizer_use_condition_selectivity=4 and =1
+--echo #
+
+SET @save_old_mode=@@old_mode;
+SET @@old_mode=zero_date_time_cast;
+CREATE TABLE t1 (a TIME);
+INSERT INTO t1 VALUES ('0000-00-00 10:20:30'),('0000-00-00 10:20:31');
+INSERT INTO t1 VALUES ('0000-00-01 10:20:30'),('0000-00-01 10:20:31');
+INSERT INTO t1 VALUES ('31 10:20:30'),('32 10:20:30'),('33 10:20:30'),('34 10:20:30');
+
+SET @save_optimizer_use_condition_selectivity= @@optimizer_use_condition_selectivity;
+SET @@optimizer_use_condition_selectivity=1;
+SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+
+SET @@optimizer_use_condition_selectivity=4;
+SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;
+drop table t1;
+SET @@optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
+set @@old_mode= @save_old_mode;
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/item.cc

@@ -166,19 +166,21 @@ bool Item::get_time_with_conversion(THD *thd, MYSQL_TIME *ltime,
         - truncate the YYYYMMDD part
         - add (MM*33+DD)*24 to hours
         - add (MM*31+DD)*24 to hours
-        Let's return NULL here, to disallow equal field propagation.
+        Let's return TRUE here, to disallow equal field propagation.
         Note, If we start to use this method in more pieces of the code other
-        than eqial field propagation, we should probably return
-        NULL only if some flag in fuzzydate is set.
+        than equal field propagation, we should probably return
+        TRUE only if some flag in fuzzydate is set.
       */
-      return (null_value= true);
+      return true;
     }
     if (datetime_to_time_with_warn(thd, ltime, &ltime2, TIME_SECOND_PART_DIGITS))
     {
       /*
-        Time difference between CURRENT_DATE and ltime
-        did not fit into the supported TIME range
+        If the time difference between CURRENT_DATE and ltime
+        did not fit into the supported TIME range, then we set the
+        difference to the maximum possible value in the supported TIME range
       */
+      DBUG_ASSERT(0);
       return (null_value= true);
     }
     *ltime= ltime2;