Bug#37870: Usage of uninitialized value caused failed assertion.

The convert_constant_item function converts a constant to integer using
field for condition like 'field = a_constant'. When the convert_constant_item
is called for a subquery the outer select is already being executed, so
convert_constant_item saves field's value to prevent its corruption.
For EXPLAIN field's value isn't initialized thus when convert_constant_item
tries to restore saved value it fails assertion.
      
Now the convert_constant_item doesn't save/restore field's value
for EXPLAIN.

mysql-test/r/explain.result

@@ -107,3 +107,18 @@ X	X	X	X	X	X	X	X	X
 X	X	X	X	X	X	X	X	X	Range checked for each record (index map: 0xFFFFFFFFFF)
 DROP TABLE t2;
 DROP TABLE t1;
+#
+# Bug#37870: Usage of uninitialized value caused failed assertion.
+#
+create table t1 (dt datetime not null);
+create table t2 (dt datetime not null);
+insert into t1 values ('2001-01-01 1:1:1'), ('2001-01-01 1:1:1');
+insert into t2 values ('2001-01-01 1:1:1'), ('2001-01-01 1:1:1');
+flush tables;
+EXPLAIN SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN (SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.dt IS NULL );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	OUTR	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	INNR	ALL	NULL	NULL	NULL	NULL	2	Using where
+SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN (SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.dt IS NULL );
+dt
+drop tables t1, t2;

mysql-test/t/explain.test

@@ -94,4 +94,16 @@ EXPLAIN SELECT 1 FROM
 DROP TABLE t2;
 DROP TABLE t1;
 
+--echo #
+--echo # Bug#37870: Usage of uninitialized value caused failed assertion.
+--echo #
+create table t1 (dt datetime not null);
+create table t2 (dt datetime not null);
+insert into t1 values ('2001-01-01 1:1:1'), ('2001-01-01 1:1:1');
+insert into t2 values ('2001-01-01 1:1:1'), ('2001-01-01 1:1:1');
+flush tables;
+EXPLAIN SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN (SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.dt IS NULL );
+SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN (SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.dt IS NULL );
+drop tables t1, t2;
+
 # End of 5.0 tests.

sql/item_cmpfunc.cc

@@ -415,8 +415,9 @@ static bool convert_constant_item(THD *thd, Item_field *field_item,
     /*
       Store the value of the field if it references an outer field because
       the call to save_in_field below overrides that value.
+      Don't store it for EXPLAIN since it's not initialized.
     */
-    if (field_item->depended_from)
+    if (field_item->depended_from && !thd->lex->describe)
       orig_field_val= field->val_int();
     if (!(*item)->is_null() && !(*item)->save_in_field(field, 1))
     {
@@ -427,7 +428,7 @@ static bool convert_constant_item(THD *thd, Item_field *field_item,
       result= 1;					// Item was replaced
     }
     /* Restore the original field value. */
-    if (field_item->depended_from)
+    if (field_item->depended_from && !thd->lex->describe)
     {
       result= field->store(orig_field_val, TRUE);
       /* orig_field_val must be a valid value that can be restored back. */