MDEV-298: SHOW EXPLAIN: Plan returned by SHOW EXPLAIN only contains 'Using temporary' ...

- Correct the way SHOW EXPLAIN code calculates 'need_order' parameter 
  for JOIN::print_explain(). 
  The calculation is still an approximation (see MDEV entry for details) (even EXPLAIN itself
  is wrong in certain cases), but now it's a better approximation than before.

mysql-test/r/show_explain.result

@@ -888,4 +888,42 @@ a+SLEEP(0.01)
 0
 set debug_dbug='';
 drop table t1;
+# 
+# MDEV-298: SHOW EXPLAIN: Plan returned by SHOW EXPLAIN only contains 
+#   'Using temporary' while the standard EXPLAIN says 'Using temporary; Using filesort'
+# 
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9),
+(10),(11),(12),(13),(14),(15),(16);
+INSERT INTO t1 SELECT t11.a FROM t1 t11, t1 t12, t1 t13;
+EXPLAIN SELECT a FROM t1 GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4112	Using temporary; Using filesort
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+SELECT a FROM t1 GROUP BY a;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4112	Using temporary; Using filesort
+Warnings:
+Note	1003	SELECT a FROM t1 GROUP BY a
+a
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+15
+16
+set debug_dbug='';
+drop table t1;
 drop table t0;

mysql-test/t/show_explain.test

@@ -919,5 +919,33 @@ reap;
 set debug_dbug='';
 
 drop table t1;
+
+--echo # 
+--echo # MDEV-298: SHOW EXPLAIN: Plan returned by SHOW EXPLAIN only contains 
+--echo #   'Using temporary' while the standard EXPLAIN says 'Using temporary; Using filesort'
+--echo # 
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9),
+  (10),(11),(12),(13),(14),(15),(16);
+INSERT INTO t1 SELECT t11.a FROM t1 t11, t1 t12, t1 t13;
+
+EXPLAIN SELECT a FROM t1 GROUP BY a;
+
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+--send
+SELECT a FROM t1 GROUP BY a;
+
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+
+connection con1;
+reap;
+
+set debug_dbug='';
+
+drop table t1;
+
 drop table t0;
 

sql/sql_lex.cc

@@ -4100,7 +4100,8 @@ int st_select_lex::print_explain(select_result_sink *output,
     {
       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->skip_sort_order && !join->no_order &&
+                               (join->order || join->group_list), // bool need_order
                                join->select_distinct, // bool distinct
                                NULL); //const char *message
     }