MDEV-30721: Assertion `fixed()' failed in Item_cond_and::val_bool()

When processing a degenerate JTBM semi-join (a subquery that produces
0 or 1 rows), `execute_degenerate_jtbm_semi_join()` creates equality
conditions and adds them to `eq_list`. These equalities are then merged
with existing conditions via `and_new_conditions_to_optimized_cond()`,
creating an `Item_cond_and`.

When multiple conditions need to be combined, a new `Item_cond_and` is
created and populated. However, this item was not fixed before being
walked by the `is_simplified_cond_processor`, which calls `val_bool()`
to check if the condition is a known constant. Calling `val_bool()`
on an unfixed item triggers the assertion.

Solution:
Fix the newly created `Item_cond_and` immediately after it's populated
ensuring it's ready for any subsequent operations including evaluation.

Author: mariadb-OlegSmirnov

mysql-test/main/subselect_mat.result

@@ -2744,6 +2744,17 @@ ORDER BY l_orderkey, l_linenumber;
 l_orderkey	l_linenumber	l_quantity
 DROP TABLE t1;
 # end of 10.6 tests
+#
+# MDEV-30721: Assertion `fixed()' failed in Item_cond_and::val_bool()
+# with degenerate JTBM semi-join
+#
+CREATE TABLE t1 (c INT KEY);
+INSERT INTO t1 (c) VALUES (0);
+SELECT * FROM t1 WHERE (0,0) IN (SELECT MAX(c),MIN(c) FROM t1);
+c
+0
+DROP TABLE t1;
+# end of 10.11 tests
 set @subselect_mat_test_optimizer_switch_value=null;
 set @@optimizer_switch='materialization=on,in_to_exists=off,semijoin=off';
 set optimizer_switch='mrr=on,mrr_sort_keys=on,index_condition_pushdown=on';

mysql-test/main/subselect_sj_mat.result

@@ -2786,3 +2786,14 @@ ORDER BY l_orderkey, l_linenumber;
 l_orderkey	l_linenumber	l_quantity
 DROP TABLE t1;
 # end of 10.6 tests
+#
+# MDEV-30721: Assertion `fixed()' failed in Item_cond_and::val_bool()
+# with degenerate JTBM semi-join
+#
+CREATE TABLE t1 (c INT KEY);
+INSERT INTO t1 (c) VALUES (0);
+SELECT * FROM t1 WHERE (0,0) IN (SELECT MAX(c),MIN(c) FROM t1);
+c
+0
+DROP TABLE t1;
+# end of 10.11 tests

mysql-test/main/subselect_sj_mat.test

@@ -2456,3 +2456,20 @@ ORDER BY l_orderkey, l_linenumber;
 DROP TABLE t1;
 
 --echo # end of 10.6 tests
+
+--echo #
+--echo # MDEV-30721: Assertion `fixed()' failed in Item_cond_and::val_bool()
+--echo # with degenerate JTBM semi-join
+--echo #
+
+CREATE TABLE t1 (c INT KEY);
+INSERT INTO t1 (c) VALUES (0);
+
+# This query triggered an assertion because the Item_cond_and created
+# during condition merging in and_new_conditions_to_optimized_cond()
+# was evaluated before being fixed.
+SELECT * FROM t1 WHERE (0,0) IN (SELECT MAX(c),MIN(c) FROM t1);
+
+DROP TABLE t1;
+
+--echo # end of 10.11 tests

sql/opt_subselect.cc

@@ -6138,7 +6138,7 @@ Item *and_new_conditions_to_optimized_cond(THD *thd, Item *cond,
     }
   }
 
-  if (!cond)
+  if (!cond || cond->fix_fields_if_needed(thd, &cond))
     return NULL;
 
   if (*cond_eq)
@@ -6171,9 +6171,6 @@ Item *and_new_conditions_to_optimized_cond(THD *thd, Item *cond,
   if (cond && is_simplified_cond)
     cond= cond->remove_eq_conds(thd, cond_value, true);
 
-  if (cond && cond->fix_fields_if_needed(thd, NULL))
-    return NULL;
-
   return cond;
 }