MDEV-17871 Crash when running explain with CTE

When the with clause of a query contains a recursive CTE that is not used
then processing of EXPLAIN for this query does not require optimization
of the unit specifying this CTE. In this case if 'derived' is the
TABLE_LIST object created for this CTE then derived->derived_result is NULL
and any assignment to derived->derived_result->table causes a crash.
After fixing this problem in the code of st_select_lex_unit::prepare()
EXPLAIN for such a query worked without crashes. Yet an execution
plan for the recursive CTE appeared there. The cause of this problem was
an incorrect condition used in JOIN::save_explain_data_intern() that
determined whether CTE was to be optimized or not. A similar condition was
used in select_describe() and this patch has corrected it as well.

mysql-test/r/cte_recursive.result

@@ -3600,3 +3600,51 @@ Mandelbrot Set
                 ..............................................................................        
                   ...........................................................................         
                    ........................................................................           
+#
+# MDEV-17871: EXPLAIN for query with not used recursive cte
+#
+create table t1 (a int);
+insert into t1 values (2), (1), (4), (3);
+explain extended
+with recursive cte as
+(select * from t1 where a=1 union select a+1 from cte where a<3)
+select * from cte as t;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	4	100.00	
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	4	100.00	Using where
+3	RECURSIVE UNION	<derived2>	ALL	NULL	NULL	NULL	NULL	4	100.00	Using where
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	
+Warnings:
+Note	1003	with recursive cte as (select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` = 1 union select `cte`.`a` + 1 AS `a+1` from `cte` where `cte`.`a` < 3)select `t`.`a` AS `a` from `cte` `t`
+with recursive cte as
+(select * from t1 where a=1 union select a+1 from cte where a<3)
+select * from cte as t;
+a
+1
+2
+3
+explain extended
+with recursive cte as
+(select * from t1 where a=1 union select a+1 from cte where a<3)
+select * from t1 as t;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t	ALL	NULL	NULL	NULL	NULL	4	100.00	
+Warnings:
+Note	1003	with recursive cte as (select `*` AS `*` from `test`.`t1` where `a` = 1 union select `a` + 1 AS `a+1` from `cte` where `a` < 3)select `test`.`t`.`a` AS `a` from `test`.`t1` `t`
+with recursive cte as
+(select * from t1 where a=1 union select a+1 from cte where a<3)
+select * from t1 as t;
+a
+2
+1
+4
+3
+create table t2 ( i1 int, i2 int);
+insert into t2 values (1,1),(2,2);
+explain
+with recursive cte as
+( select * from t1 union select s1.* from t1 as s1, cte where s1.i1 = cte.i2 )
+select * from t1 as t;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t	ALL	NULL	NULL	NULL	NULL	4	
+drop table t1,t2;

mysql-test/t/cte_recursive.test

@@ -2495,3 +2495,42 @@ SELECT GROUP_CONCAT(
   FROM Zt
 GROUP BY Iy
 ORDER BY Iy;
+
+--echo #
+--echo # MDEV-17871: EXPLAIN for query with not used recursive cte
+--echo #
+
+create table t1 (a int);
+insert into t1 values (2), (1), (4), (3);
+
+let $rec_cte =
+with recursive cte as
+  (select * from t1 where a=1 union select a+1 from cte where a<3);
+
+eval
+explain extended
+$rec_cte
+select * from cte as t;
+
+eval
+$rec_cte
+select * from cte as t;
+
+eval
+explain extended
+$rec_cte
+select * from t1 as t;
+
+eval
+$rec_cte
+select * from t1 as t;
+
+create table t2 ( i1 int, i2 int);
+insert into t2 values (1,1),(2,2);
+
+explain
+with recursive cte as
+ ( select * from t1 union select s1.* from t1 as s1, cte where s1.i1 = cte.i2 )
+select * from t1 as t;
+
+drop table t1,t2;

sql/sql_select.cc

@@ -25041,13 +25041,13 @@ int JOIN::save_explain_data_intern(Explain_query *output,
       (1) they are not parts of ON clauses that were eliminated by table 
           elimination.
       (2) they are not merged derived tables
-      (3) they are not unreferenced CTE
+      (3) they are not hanging CTEs (they are needed for execution)
     */
     if (!(tmp_unit->item && tmp_unit->item->eliminated) &&    // (1)
         (!tmp_unit->derived ||
          tmp_unit->derived->is_materialized_derived()) &&     // (2)
-        !(tmp_unit->with_element && 
-          !tmp_unit->with_element->is_referenced()))          // (3)
+        !(tmp_unit->with_element &&
+          (!tmp_unit->derived || !tmp_unit->derived->derived_result))) // (3)
    {
       explain->add_child(tmp_unit->first_select()->select_number);
     }
@@ -25108,11 +25108,12 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
       Save plans for child subqueries, when
       (1) they are not parts of eliminated WHERE/ON clauses.
       (2) they are not VIEWs that were "merged for INSERT".
-      (3) they are not unreferenced CTE.
+      (3) they are not hanging CTEs (they are needed for execution)
     */
     if (!(unit->item && unit->item->eliminated) &&                     // (1)
         !(unit->derived && unit->derived->merged_for_insert) &&        // (2)
-        !(unit->with_element && !unit->with_element->is_referenced())) // (3)  
+        !(unit->with_element &&
+          (!unit->derived || !unit->derived->derived_result)))         // (3)
     {
       if (mysql_explain_union(thd, unit, result))
         DBUG_VOID_RETURN;

sql/sql_union.cc

@@ -694,8 +694,11 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
                                               instantiate_tmp_table, false))
           goto err;
         if (!derived->table)
-          derived->table= derived->derived_result->table= 
-            with_element->rec_result->rec_tables.head();
+        {
+          derived->table= with_element->rec_result->rec_tables.head();
+          if (derived->derived_result)
+            derived->derived_result->table= derived->table;
+        }
         with_element->mark_as_with_prepared_anchor();
         is_rec_result_table_created= true;
       }