MDEV-270: SHOW EXPLAIN: server crashes in JOIN::print_explain on a query with

 select tables optimized away
- Take into account, that for some degenerate joins instead of "join->table_count=0"
  the code sets "join->tables_list=0".

1 files changed, 34 insertions, 0 deletions

diff --git a/mysql-test/t/show_explain.test b/mysql-test/t/show_explain.test
index c9f715bbb67..d813956a1fe 100644
--- a/mysql-test/t/show_explain.test
+++ b/mysql-test/t/show_explain.test
@@ -414,6 +414,40 @@ reap;
 set debug='';
 drop table t3;
 
+--echo #
+--echo # MDEV-270: SHOW EXPLAIN: server crashes in JOIN::print_explain on a query with 
+--echo #           select tables optimized away
+--echo #
+
+CREATE TABLE t2 (pk INT PRIMARY KEY, a INT ) ENGINE=MyISAM; 
+INSERT INTO t2 VALUES 
+  (1,4),(2,62),(3,7),(4,1),(5,0),(6,7),(7,7),(8,1),(9,7),(10,1),
+  (11,5),(12,2),(13,0),(14,1),(15,8),(16,1),(17,1),(18,9),(19,1),(20,5) ;
+
+explain SELECT * FROM t2 WHERE a = 
+       (SELECT MAX(a) FROM t2 
+        WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
+       );
+
+set @show_explain_probe_select_id=2;
+set debug='d,show_explain_probe_do_select';
+send SELECT * FROM t2 WHERE a = 
+       (SELECT MAX(a) FROM t2 
+        WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
+       );
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug='';
+drop table t2;
+
+
+
+
+
+
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.