MDEV-267: SHOW EXPLAIN: Server crashes in JOIN::print_explain on most of queries

- Make SHOW EXPLAIN code handle degenerate subquery cases (No tables, impossible where, etc)

mysql-test/r/show_explain.result

@@ -370,4 +370,48 @@ a	b
 set debug='';
 DROP VIEW v1;
 DROP TABLE t2, t3;
+#
+# MDEV-267: SHOW EXPLAIN: Server crashes in JOIN::print_explain on most of queries
+#
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_join_exec_end';
+select sleep(1);
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1003	select sleep(1)
+sleep(1)
+0
+set debug='';
+#
+# Same as above, but try another reason for JOIN to be degenerate
+#
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_join_exec_end';
+select * from t0 where 1>10;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+Warnings:
+Note	1003	select * from t0 where 1>10
+a
+set debug='';
+#
+# Same as above, but try another reason for JOIN to be degenerate (2)
+#
+create table t3(a int primary key);
+insert into t3 select a from t0;
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_join_exec_end';
+select * from t0,t3 where t3.a=112233;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t3	const	PRIMARY	PRIMARY	4		0	unique row not found
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	10	
+Warnings:
+Note	1003	select * from t0,t3 where t3.a=112233
+a	a
+set debug='';
+drop table t3;
 drop table t0,t1;

mysql-test/t/show_explain.test

@@ -371,6 +371,49 @@ set debug='';
 DROP VIEW v1;
 DROP TABLE t2, t3;
 
+--echo #
+--echo # MDEV-267: SHOW EXPLAIN: Server crashes in JOIN::print_explain on most of queries
+--echo #
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_join_exec_end';
+send select sleep(1);
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug='';
+
+
+--echo #
+--echo # Same as above, but try another reason for JOIN to be degenerate
+--echo #
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_join_exec_end';
+send select * from t0 where 1>10;
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug='';
+
+--echo #
+--echo # Same as above, but try another reason for JOIN to be degenerate (2)
+--echo #
+create table t3(a int primary key);
+insert into t3 select a from t0;
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_join_exec_end';
+send select * from t0,t3 where t3.a=112233;
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug='';
+drop table t3;
+
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.
 

sql/sql_lex.cc

@@ -3751,12 +3751,23 @@ int st_select_lex::print_explain(select_result_sink *output,
 {
   int res;
   if (join && join->optimized == JOIN::OPTIMIZATION_DONE)
+  {
+    if (!join->table_count)
+    {
+      /* It's a degenerate join */
+      const char *cause= join->zero_result_cause ? join-> zero_result_cause : 
+                                                   "No tables used";
+      res= join->print_explain(output, explain_flags, TRUE, FALSE, FALSE, 
+                               FALSE, cause);
+    }
+    else
     {
       res= join->print_explain(output, explain_flags, TRUE,
                                join->need_tmp, // need_tmp_table
                                (join->order != 0 && !join->skip_sort_order), // bool need_order
                                join->select_distinct, // bool distinct
                                NULL); //const char *message
+    }
     if (res)
       goto err;