MDEV-238: SHOW EXPLAIN: Server crashes in JOIN::print_explain with FROM subquery and GROUP BY

- Support SHOW EXPLAIN for selects that have "Using temporary; Using filesort".

mysql-test/r/show_explain.result

@@ -284,4 +284,29 @@ a
 7
 8
 9
+set debug='';
+#
+# MDEV-238: SHOW EXPLAIN: Server crashes in JOIN::print_explain with FROM subquery and GROUP BY
+#
+CREATE TABLE t2 ( a INT );
+INSERT INTO t2 VALUES (1),(2),(1),(4),(2);
+explain SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary; Using filesort
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using join buffer (flat, BNL join)
+set debug='d,show_explain_in_find_all_keys';
+SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
+# NOTE: current code will not show "Using join buffer":
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary; Using filesort
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	
+Warnings:
+Note	1003	SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a
+a
+1
+2
+4
+set debug='';
+DROP TABLE t2;
 drop table t0,t1;

mysql-test/t/show_explain.test

@@ -300,8 +300,30 @@ connection default;
 evalp show explain for $thr2;
 connection con1;
 reap;
+set debug='';
+
+--echo #
+--echo # MDEV-238: SHOW EXPLAIN: Server crashes in JOIN::print_explain with FROM subquery and GROUP BY
+--echo #
+CREATE TABLE t2 ( a INT ); 
+INSERT INTO t2 VALUES (1),(2),(1),(4),(2);
+explain SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
+
+set debug='d,show_explain_in_find_all_keys';
+send SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
+
+connection default;
+--source include/wait_condition.inc
+--echo # NOTE: current code will not show "Using join buffer":
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug='';
+
 
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.
 
+DROP TABLE t2;
+
 drop table t0,t1;

sql/filesort.cc

@@ -515,6 +515,12 @@ static ha_rows find_all_keys(SORTPARAM *param, SQL_SELECT *select,
   if (indexfile || flag)
     ref_pos= &file->ref[0];
   next_pos=ref_pos;
+
+  DBUG_EXECUTE_IF("show_explain_in_find_all_keys", 
+                  dbug_serve_apcs(thd, 1);
+                 );
+
+
   if (! indexfile && ! quick_select)
   {
     next_pos=(uchar*) 0;			/* Find records in sequence */

sql/sql_select.cc

@@ -7132,28 +7132,36 @@ prev_record_reads(POSITION *positions, uint idx, table_map found_ref)
   return found;
 }
 
+enum enum_exec_or_opt {WALK_OPTIMIZATION_TABS , WALK_EXECUTION_TABS};
 
 /*
   Enumerate join tabs in breadth-first fashion, including const tables.
 */
 
-JOIN_TAB *first_breadth_first_tab(JOIN *join)
+JOIN_TAB *first_breadth_first_tab(JOIN *join, enum enum_exec_or_opt tabs_kind)
 {
-  return join->join_tab; /* There's always one (i.e. first) table */
+  /* There's always one (i.e. first) table */
+  return (tabs_kind == WALK_EXECUTION_TABS)? join->join_tab:
+                                             join->table_access_tabs;
 }
 
 
-JOIN_TAB *next_breadth_first_tab(JOIN *join, JOIN_TAB *tab)
+JOIN_TAB *next_breadth_first_tab(JOIN *join, enum enum_exec_or_opt tabs_kind,
+                                 JOIN_TAB *tab)
 {
+  JOIN_TAB* const first_top_tab= first_breadth_first_tab(join, tabs_kind);
+  const uint n_top_tabs_count= (tabs_kind == WALK_EXECUTION_TABS)? 
+                                  join->top_join_tab_count:
+                                  join->top_table_access_tabs_count;
   if (!tab->bush_root_tab)
   {
     /* We're at top level. Get the next top-level tab */
     tab++;
-    if (tab < join->join_tab + join->top_join_tab_count)
+    if (tab < first_top_tab + n_top_tabs_count)
       return tab;
 
     /* No more top-level tabs. Switch to enumerating SJM nest children */
-    tab= join->join_tab;
+    tab= first_top_tab;
   }
   else
   {
@@ -7177,7 +7185,7 @@ JOIN_TAB *next_breadth_first_tab(JOIN *join, JOIN_TAB *tab)
     Ok, "tab" points to a top-level table, and we need to find the next SJM
     nest and enter it.
   */
-  for (; tab < join->join_tab + join->top_join_tab_count; tab++)
+  for (; tab < first_top_tab + n_top_tabs_count; tab++)
   {
     if (tab->bush_children)
       return tab->bush_children->start;
@@ -7201,7 +7209,7 @@ JOIN_TAB *first_top_level_tab(JOIN *join, enum enum_with_const_tables with_const
 
 JOIN_TAB *next_top_level_tab(JOIN *join, JOIN_TAB *tab)
 {
-  tab= next_breadth_first_tab(join, tab);
+  tab= next_breadth_first_tab(join, WALK_EXECUTION_TABS, tab);
   if (tab && tab->bush_root_tab)
     tab= NULL;
   return tab;
@@ -7501,6 +7509,13 @@ get_best_combination(JOIN *join)
 
   join->top_join_tab_count= join->join_tab_ranges.head()->end - 
                             join->join_tab_ranges.head()->start;
+  /*
+    Save pointers to select join tabs for SHOW EXPLAIN
+  */
+  join->table_access_tabs= join->join_tab;
+  join->top_table_access_tabs_count= join->top_join_tab_count;
+
+
   update_depend_map(join);
   DBUG_RETURN(0);
 }
@@ -7922,6 +7937,7 @@ JOIN::make_simple_join(JOIN *parent, TABLE *temp_table)
       !(parent->join_tab_reexec= (JOIN_TAB*) thd->alloc(sizeof(JOIN_TAB))))
     DBUG_RETURN(TRUE);                        /* purecov: inspected */
 
+  // psergey-todo: here, save the pointer for original join_tabs.
   join_tab= parent->join_tab_reexec;
   table= &parent->table_reexec[0]; parent->table_reexec[0]= temp_table;
   table_count= top_join_tab_count= 1;
@@ -10077,7 +10093,12 @@ void JOIN_TAB::cleanup()
   if (cache)
   {
     cache->free();
-    cache= 0;
+    cache= 0; // psergey: this is why we don't see "Using join cache" in SHOW EXPLAIN
+              //  when it is run for "Using temporary+filesort" queries while they 
+              //  are at reading-from-tmp-table phase.
+              //
+              //  TODO ask igor if this can be just moved to later phase
+              //  (JOIN_CACHE objects themselves are not big, arent they)
   }
   limit= 0;
   if (table)
@@ -21204,9 +21225,10 @@ int JOIN::print_explain(select_result_sink *result, bool on_the_fly,
 
     bool printing_materialize_nest= FALSE;
     uint select_id= join->select_lex->select_number;
+    JOIN_TAB* const first_top_tab= first_breadth_first_tab(join, WALK_OPTIMIZATION_TABS);
 
-    for (JOIN_TAB *tab= first_breadth_first_tab(join); tab;
-         tab= next_breadth_first_tab(join, tab))
+    for (JOIN_TAB *tab= first_breadth_first_tab(join, WALK_OPTIMIZATION_TABS); tab;
+         tab= next_breadth_first_tab(join, WALK_OPTIMIZATION_TABS, tab))
     {
       if (tab->bush_root_tab)
       {
@@ -21643,7 +21665,7 @@ int JOIN::print_explain(select_result_sink *result, bool on_the_fly,
           extra.append(STRING_WITH_LEN("; End temporary"));
         else if (tab->do_firstmatch)
         {
-          if (tab->do_firstmatch == join->join_tab - 1)
+          if (tab->do_firstmatch == /*join->join_tab*/ first_top_tab - 1)
             extra.append(STRING_WITH_LEN("; FirstMatch"));
           else
           {

sql/sql_select.h

@@ -896,6 +896,20 @@ protected:
 
 public:
   JOIN_TAB *join_tab, **best_ref;
+
+  /*
+    For "Using temporary+Using filesort" queries, JOIN::join_tab can point to
+    either: 
+    1. array of join tabs describing how to run the select, or
+    2. array of single join tab describing read from the temporary table.
+
+    SHOW EXPLAIN code needs to read/show #1. This is why two next members are
+    there for saving it.
+  */
+  JOIN_TAB *table_access_tabs;
+  uint     top_table_access_tabs_count;
+
+
   JOIN_TAB **map2table;    ///< mapping between table indexes and JOIN_TABs
   JOIN_TAB *join_tab_save; ///< saved join_tab for subquery reexecution
 

sql/table.h

@@ -850,7 +850,12 @@ struct st_table {
     See TABLE_LIST::process_index_hints().
   */
   bool force_index_group;
-  bool distinct,const_table,no_rows, used_for_duplicate_elimination;
+  /*
+    TRUE<=> this table was created with create_tmp_table(... distinct=TRUE..)
+    call
+  */
+  bool distinct;
+  bool const_table,no_rows, used_for_duplicate_elimination;
 
   /**
      If set, the optimizer has found that row retrieval should access index