MDEV-273: SHOW EXPLAIN: server crashes in JOIN::print_explain on a query with impossible WHERE

- It turns out, there is a case where the join is degenerate, but 
  join->table_count!= && join->tables_list!=NULL. Need to also check 
  if join->zero_result_cause!=NULL, too.

- There is a slight problem: The code sets
    zero_result_cause= "no matching row in const table"
  when NOT running EXPLAIN. The result is that SHOW EXPLAIN will show this line while 
  regular EXPLAIN will not.

mysql-test/r/show_explain.result

-drop table if exists t0, t1, t2;
+drop table if exists t0, t1, t2, t3, t4;
 drop view if exists v1;
 create table t0 (a int);
 insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
@@ -407,8 +407,7 @@ 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	
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 Warnings:
 Note	1003	select * from t0,t3 where t3.a=112233
 a	a
@@ -453,4 +452,55 @@ pk	a
 9	7
 set debug='';
 drop table t2;
+#
+# MDEV-273: SHOW EXPLAIN: server crashes in JOIN::print_explain on a query with impossible WHERE
+#
+CREATE TABLE t2 (a1 INT, KEY(a1)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES
+(4),(6),(7),(1),(0),(7),(7),(1),(7),(1),
+(5),(2),(0),(1),(8),(1),(1),(9),(1),(5);
+CREATE TABLE t3 (b1 INT) ENGINE=MyISAM;
+INSERT INTO t3 VALUES
+(4),(5),(8),(4),(8),(2),(9),(6),(4),(8),
+(3),(5),(9),(6),(8),(3),(2),(6),(3),(1),
+(4),(3),(1),(7),(0),(0),(9),(5),(9),(0),
+(2),(2),(5),(9),(1),(4),(8),(6),(5),(5),
+(1),(7),(2),(8),(9),(3),(2),(6),(6),(5),
+(4),(3),(2),(7),(4),(6),(0),(8),(5),(8),
+(2),(9),(7),(5),(7),(0),(4),(3),(1),(0),
+(6),(2),(8),(3),(7),(3),(5),(5),(1),(2),
+(1),(7),(1),(9),(9),(8),(3);
+CREATE TABLE t4 (c1 INT) ENGINE=MyISAM;
+EXPLAIN
+SELECT count(*) FROM t2, t3
+WHERE a1 < ALL ( 
+SELECT a1 FROM t2
+WHERE a1 IN ( SELECT a1 FROM t2, t4 )
+);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	index	NULL	a1	5	NULL	20	Using where; Using index
+1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	87	Using join buffer (flat, BNL join)
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_do_select';
+SELECT count(*) FROM t2, t3
+WHERE a1 < ALL ( 
+SELECT a1 FROM t2
+WHERE a1 IN ( SELECT a1 FROM t2, t4 )
+);
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	index	NULL	a1	5	NULL	20	Using where; Using index
+1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	87	Using join buffer (flat, BNL join)
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+Warnings:
+Note	1003	SELECT count(*) FROM t2, t3
+WHERE a1 < ALL ( 
+SELECT a1 FROM t2
+WHERE a1 IN ( SELECT a1 FROM t2, t4 )
+)
+count(*)
+1740
+set debug='';
+drop table t2, t3, t4;
 drop table t0,t1;

mysql-test/t/show_explain.test

@@ -4,7 +4,7 @@
 --source include/have_debug.inc
 
 --disable_warnings
-drop table if exists t0, t1, t2;
+drop table if exists t0, t1, t2, t3, t4;
 drop view if exists v1;
 --enable_warnings
 
@@ -444,9 +444,50 @@ set debug='';
 drop table t2;
 
 
+--echo #
+--echo # MDEV-273: SHOW EXPLAIN: server crashes in JOIN::print_explain on a query with impossible WHERE
+--echo #
+CREATE TABLE t2 (a1 INT, KEY(a1)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES
+  (4),(6),(7),(1),(0),(7),(7),(1),(7),(1),
+  (5),(2),(0),(1),(8),(1),(1),(9),(1),(5);
+
+CREATE TABLE t3 (b1 INT) ENGINE=MyISAM;
+INSERT INTO t3 VALUES
+  (4),(5),(8),(4),(8),(2),(9),(6),(4),(8),
+  (3),(5),(9),(6),(8),(3),(2),(6),(3),(1),
+  (4),(3),(1),(7),(0),(0),(9),(5),(9),(0),
+  (2),(2),(5),(9),(1),(4),(8),(6),(5),(5),
+  (1),(7),(2),(8),(9),(3),(2),(6),(6),(5),
+  (4),(3),(2),(7),(4),(6),(0),(8),(5),(8),
+  (2),(9),(7),(5),(7),(0),(4),(3),(1),(0),
+  (6),(2),(8),(3),(7),(3),(5),(5),(1),(2),
+  (1),(7),(1),(9),(9),(8),(3);
+CREATE TABLE t4 (c1 INT) ENGINE=MyISAM;
+
+EXPLAIN
+SELECT count(*) FROM t2, t3
+WHERE a1 < ALL ( 
+  SELECT a1 FROM t2
+  WHERE a1 IN ( SELECT a1 FROM t2, t4 )
+);
 
+set @show_explain_probe_select_id=1;
+set debug='d,show_explain_probe_do_select';
+send
+SELECT count(*) FROM t2, t3
+WHERE a1 < ALL ( 
+  SELECT a1 FROM t2
+  WHERE a1 IN ( SELECT a1 FROM t2, t4 )
+);
 
-
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug='';
+drop table t2, t3, t4;
 
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.

sql/sql_lex.cc

@@ -3752,7 +3752,11 @@ int st_select_lex::print_explain(select_result_sink *output,
   int res;
   if (join && join->optimized == JOIN::OPTIMIZATION_DONE)
   {
-    if (!join->table_count || !join->tables_list)
+    /*
+      There is a number of reasons join can be marked as degenerate, so all
+      three conditions below can happen simultaneously, or individually:
+    */
+    if (!join->table_count || !join->tables_list || join->zero_result_cause)
     {
       /* It's a degenerate join */
       const char *cause= join->zero_result_cause ? join-> zero_result_cause :