Bug #46791: Assertion failed:(table->key_read==0),function unknown

  function,file sql_base.cc

When uncacheable queries are written to a temp table the optimizer must 
preserve the original JOIN structure, because it is re-using the JOIN 
structure to read from the resulting temporary table.
This was done only for uncacheable sub-queries. 
But top level queries can also benefit from this mechanism, specially if 
they're using index access and need a reset.
Fixed by not limiting the saving of JOIN structure to subqueries
exclusively.
Added a new test file to extend the existing (large) subquery.test.

mysql-test/r/subselect4.result

+#
+# Bug #46791: Assertion failed:(table->key_read==0),function unknown
+#    function,file sql_base.cc
+#
+CREATE TABLE t1 (a INT, b INT, KEY(a));
+INSERT INTO t1 VALUES (1,1),(2,2);
+CREATE TABLE t2 LIKE t1;
+INSERT INTO t2 VALUES (1,1),(2,2);
+CREATE TABLE t3 LIKE t1;
+# should have 1 impossible where and 2 dependent subqueries
+EXPLAIN
+SELECT 1 FROM t1
+WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE 1 = (SELECT MIN(t2.b) FROM t3))
+ORDER BY count(*);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	NULL	a	5	NULL	2	Using index; Using temporary
+2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+# should not crash the next statement
+SELECT 1 FROM t1
+WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE 1 = (SELECT MIN(t2.b) FROM t3))
+ORDER BY count(*);
+1
+1
+# should not crash: the crash is caused by the previous statement
+SELECT 1;
+1
+1
+DROP TABLE t1,t2,t3;
+End of 5.0 tests.

mysql-test/t/subselect4.test

+# General purpose bug fix tests go here : subselect.test too large
+
+
+--echo #
+--echo # Bug #46791: Assertion failed:(table->key_read==0),function unknown
+--echo #    function,file sql_base.cc
+--echo #
+
+CREATE TABLE t1 (a INT, b INT, KEY(a));
+INSERT INTO t1 VALUES (1,1),(2,2);
+CREATE TABLE t2 LIKE t1;
+INSERT INTO t2 VALUES (1,1),(2,2);
+CREATE TABLE t3 LIKE t1;
+
+--echo # should have 1 impossible where and 2 dependent subqueries
+EXPLAIN
+SELECT 1 FROM t1
+WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE 1 = (SELECT MIN(t2.b) FROM t3))
+ORDER BY count(*);
+
+--echo # should not crash the next statement
+SELECT 1 FROM t1
+WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE 1 = (SELECT MIN(t2.b) FROM t3))
+ORDER BY count(*);
+
+--echo # should not crash: the crash is caused by the previous statement
+SELECT 1;
+
+DROP TABLE t1,t2,t3;
+
+
+--echo End of 5.0 tests.

sql/sql_select.cc

@@ -1474,12 +1474,8 @@ JOIN::optimize()
       }
     }
 
-    /* 
-      If this join belongs to an uncacheable subquery save 
-      the original join 
-    */
-    if (select_lex->uncacheable && !is_top_level_join() &&
-        init_save_join_tab())
+    /* If this join belongs to an uncacheable query save the original join */
+    if (select_lex->uncacheable && init_save_join_tab())
       DBUG_RETURN(-1);                         /* purecov: inspected */
   }