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".

mysql-test/r/show_explain.result

@@ -414,4 +414,43 @@ Note	1003	select * from t0,t3 where t3.a=112233
 a	a
 set debug='';
 drop table t3;
+#
+# MDEV-270: SHOW EXPLAIN: server crashes in JOIN::print_explain on a query with 
+#           select tables optimized away
+#
+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)
+);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	Using where
+2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4	const	1	Using where
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+set @show_explain_probe_select_id=2;
+set debug='d,show_explain_probe_do_select';
+SELECT * FROM t2 WHERE a = 
+(SELECT MAX(a) FROM t2 
+WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
+);
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	Using where
+2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4		1	Using where
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+Warnings:
+Note	1003	SELECT * FROM t2 WHERE a = 
+(SELECT MAX(a) FROM t2 
+WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
+)
+pk	a
+3	7
+6	7
+7	7
+9	7
+set debug='';
+drop table t2;
 drop table t0,t1;

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.
 

sql/sql_lex.cc

@@ -3752,7 +3752,7 @@ int st_select_lex::print_explain(select_result_sink *output,
   int res;
   if (join && join->optimized == JOIN::OPTIMIZATION_DONE)
   {
-    if (!join->table_count)
+    if (!join->table_count || !join->tables_list)
     {
       /* It's a degenerate join */
       const char *cause= join->zero_result_cause ? join-> zero_result_cause : 

sql/sql_select.cc

@@ -15399,6 +15399,15 @@ do_select(JOIN *join,List<Item> *fields,TABLE *table,Procedure *procedure)
   else
   {
     DBUG_ASSERT(join->table_count);
+
+    THD *thd= join->thd;
+    DBUG_EXECUTE_IF("show_explain_probe_do_select", 
+                    if (dbug_user_var_equals_int(thd, 
+                                                 "show_explain_probe_select_id", 
+                                                 join->select_lex->select_number))
+                          dbug_serve_apcs(thd, 1);
+                   );
+
     if (join->outer_ref_cond && !join->outer_ref_cond->val_int())
       error= NESTED_LOOP_NO_MORE_ROWS;
     else