Fixed a problem with unreferenced CTE:

explain for the query containing WITH clause
with an unreferenced CTE caused a crash.
Added a test covered this case.

Also added a test for usage CTE in different parts of union.

mysql-test/r/cte_nonrecursive.result

@@ -309,6 +309,41 @@ c	c
 1	1
 4	4
 4	4
+# t two references of t used in different parts of a union
+with t as (select a from t1 where b >= 'c')
+select * from t where a < 2
+union
+select * from t where a >= 4;
+a
+1
+4
+select * from (select a from t1 where b >= 'c') as t
+where t.a < 2
+union
+select * from (select a from t1 where b >= 'c') as t
+where t.a >= 4;
+a
+1
+4
+explain
+with t as (select a from t1 where b >= 'c')
+select * from t where a < 2
+union
+select * from t where a >= 4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	8	Using where
+3	UNION	t1	ALL	NULL	NULL	NULL	NULL	8	Using where
+NULL	UNION RESULT	<union1,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+explain
+select * from (select a from t1 where b >= 'c') as t
+where t.a < 2
+union
+select * from (select a from t1 where b >= 'c') as t
+where t.a >= 4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	8	Using where
+3	UNION	t1	ALL	NULL	NULL	NULL	NULL	8	Using where
+NULL	UNION RESULT	<union1,3>	ALL	NULL	NULL	NULL	NULL	NULL	
 # specification of t contains union
 with t as (select a from t1 where b >= 'f' 
 union
@@ -749,6 +784,19 @@ ERROR HY000: WITH column list and SELECT field list have different column counts
 with t(f1,f1) as (select * from t1 where b >= 'c') 
 select t1.b from t2,t1 where t1.a = t2.c;
 ERROR 42S21: Duplicate column name 'f1'
+# explain for query with unreferenced with table
+explain 
+with t as (select a from t1 where b >= 'c') 
+select t1.b from t2,t1 where t1.a = t2.c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	4	
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	8	Using where; Using join buffer (flat, BNL join)
+explain 
+with t as (select a, count(*) from t1 where b >= 'c' group by a)
+select t1.b from t2,t1 where t1.a = t2.c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	4	
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	8	Using where; Using join buffer (flat, BNL join)
 drop table t1,t2;
 #
 # Bug mdev-9937: View used in the specification of with table 

mysql-test/t/cte_nonrecursive.test

@@ -152,6 +152,28 @@ select * from (select * from t1 where b >= 'c') as r1,
 with t(c) as (select a from t1 where b >= 'c')
   select * from t r1, t r2 where r1.c=r2.c;
 
+--echo # t two references of t used in different parts of a union
+with t as (select a from t1 where b >= 'c')
+  select * from t where a < 2
+  union
+  select * from t where a >= 4;
+select * from (select a from t1 where b >= 'c') as t
+  where t.a < 2
+union
+select * from (select a from t1 where b >= 'c') as t
+  where t.a >= 4;
+explain
+with t as (select a from t1 where b >= 'c')
+  select * from t where a < 2
+  union
+  select * from t where a >= 4;
+explain
+select * from (select a from t1 where b >= 'c') as t
+  where t.a < 2
+union
+select * from (select a from t1 where b >= 'c') as t
+  where t.a >= 4;
+
 --echo # specification of t contains union
 with t as (select a from t1 where b >= 'f' 
            union
@@ -437,6 +459,16 @@ with t(f) as (select * from t1 where b >= 'c')
 with t(f1,f1) as (select * from t1 where b >= 'c') 
   select t1.b from t2,t1 where t1.a = t2.c;
 
+--echo # explain for query with unreferenced with table
+
+explain 
+with t as (select a from t1 where b >= 'c') 
+  select t1.b from t2,t1 where t1.a = t2.c;
+
+explain 
+with t as (select a, count(*) from t1 where b >= 'c' group by a)
+  select t1.b from t2,t1 where t1.a = t2.c;
+
 drop table t1,t2;
 
 --echo #

sql/sql_select.cc

@@ -24402,11 +24402,14 @@ 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
     */
     if (!(tmp_unit->item && tmp_unit->item->eliminated) &&    // (1)
         (!tmp_unit->derived ||
-         tmp_unit->derived->is_materialized_derived()))       // (2)
-    {
+         tmp_unit->derived->is_materialized_derived()) &&     // (2)
+        !(tmp_unit->with_element && 
+          !tmp_unit->with_element->is_referenced()))          // (3)
+   {
       explain->add_child(tmp_unit->first_select()->select_number);
     }
   }
@@ -24466,9 +24469,11 @@ 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.
     */
-    if (!(unit->item && unit->item->eliminated) &&             // (1)
-        !(unit->derived && unit->derived->merged_for_insert))  // (2)
+    if (!(unit->item && unit->item->eliminated) &&                     // (1)
+        !(unit->derived && unit->derived->merged_for_insert) &&        // (2)
+        !(unit->with_element && !unit->with_element->is_referenced())) // (3)  
     {
       if (mysql_explain_union(thd, unit, result))
         DBUG_VOID_RETURN;