MDEV-20371: Invalid reads at plan refinement stage: join->positions...

best_access_path() is called from two optimization phases:

1. Plan choice phase, in choose_plan(). Here, the join prefix being
   considered is in join->positions[]

2. Plan refinement stage, in fix_semijoin_strategies_for_picked_join_order
   Here, the join prefix is in join->best_positions[]

It used to access join->positions[] from stage #2. This didnt cause any
valgrind or asan failures (as join->positions[] has been written-to before)
but the effect was similar to that of reading the random data:
The join prefix we've picked (in join->best_positions) could have
nothing in common with the join prefix that was last to be considered
(in join->positions).

sql/opt_subselect.cc

@@ -456,11 +456,6 @@ bool find_eq_ref_candidate(TABLE *table, table_map sj_inner_tables);
 static SJ_MATERIALIZATION_INFO *
 at_sjmat_pos(const JOIN *join, table_map remaining_tables, const JOIN_TAB *tab,
              uint idx, bool *loose_scan);
-void best_access_path(JOIN *join, JOIN_TAB *s, 
-                             table_map remaining_tables, uint idx, 
-                             bool disable_jbuf, double record_count,
-                             POSITION *pos, POSITION *loose_scan_pos);
-
 static Item *create_subq_in_equalities(THD *thd, SJ_MATERIALIZATION_INFO *sjm, 
                                 Item_in_subselect *subq_pred);
 static void remove_sj_conds(THD *thd, Item **tree);
@@ -2756,6 +2751,13 @@ void advance_sj_state(JOIN *join, table_map remaining_tables, uint idx,
     NULL,
   };
 
+#ifdef HAVE_valgrind
+  new (&pos->firstmatch_picker) Firstmatch_picker;
+  new (&pos->loosescan_picker) LooseScan_picker;
+  new (&pos->sjmat_picker) Sj_materialization_picker;
+  new (&pos->dups_weedout_picker) Duplicate_weedout_picker;
+#endif
+
   if (join->emb_sjm_nest)
   {
     /* 
@@ -3045,7 +3047,8 @@ bool Sj_materialization_picker::check_qep(JOIN *join,
     bool disable_jbuf= (join->thd->variables.join_cache_level == 0);
     for (i= first_tab + mat_info->tables; i <= idx; i++)
     {
-      best_access_path(join, join->positions[i].table, rem_tables, i,
+      best_access_path(join, join->positions[i].table, rem_tables,
+                       join->positions, i,
                        disable_jbuf, prefix_rec_count, &curpos, &dummy);
       prefix_rec_count= COST_MULT(prefix_rec_count, curpos.records_read);
       prefix_cost= COST_ADD(prefix_cost, curpos.read_time);
@@ -3661,7 +3664,8 @@ void fix_semijoin_strategies_for_picked_join_order(JOIN *join)
       join->cur_sj_inner_tables= 0;
       for (i= first + sjm->tables; i <= tablenr; i++)
       {
-        best_access_path(join, join->best_positions[i].table, rem_tables, i, 
+        best_access_path(join, join->best_positions[i].table, rem_tables,
+                         join->best_positions, i,
                          FALSE, prefix_rec_count,
                          join->best_positions + i, &dummy);
         prefix_rec_count *= join->best_positions[i].records_read;
@@ -3693,8 +3697,9 @@ void fix_semijoin_strategies_for_picked_join_order(JOIN *join)
       {
         if (join->best_positions[idx].use_join_buffer)
         {
-           best_access_path(join, join->best_positions[idx].table, 
-                            rem_tables, idx, TRUE /* no jbuf */,
+           best_access_path(join, join->best_positions[idx].table,
+                            rem_tables, join->best_positions, idx,
+                            TRUE /* no jbuf */,
                             record_count, join->best_positions + idx, &dummy);
         }
         record_count *= join->best_positions[idx].records_read;
@@ -3724,7 +3729,8 @@ void fix_semijoin_strategies_for_picked_join_order(JOIN *join)
         if (join->best_positions[idx].use_join_buffer || (idx == first))
         {
            best_access_path(join, join->best_positions[idx].table,
-                            rem_tables, idx, TRUE /* no jbuf */,
+                            rem_tables, join->best_positions, idx,
+                            TRUE /* no jbuf */,
                             record_count, join->best_positions + idx,
                             &loose_scan_pos);
            if (idx==first)

sql/opt_subselect.h

@@ -88,6 +88,7 @@ class Loose_scan_opt
   KEYUSE *best_loose_scan_start_key;
 
   uint best_max_loose_keypart;
+  table_map best_ref_depend_map;
 
 public:
   Loose_scan_opt():
@@ -250,13 +251,14 @@ class Loose_scan_opt
           best_loose_scan_records= records;
           best_max_loose_keypart= max_loose_keypart;
           best_loose_scan_start_key= start_key;
+          best_ref_depend_map= 0;
         }
       }
     }
   }
   
   void check_ref_access_part2(uint key, KEYUSE *start_key, double records, 
-                              double read_time)
+                              double read_time, table_map ref_depend_map_arg)
   {
     if (part1_conds_met && read_time < best_loose_scan_cost)
     {
@@ -266,6 +268,7 @@ class Loose_scan_opt
       best_loose_scan_records= records;
       best_max_loose_keypart= max_loose_keypart;
       best_loose_scan_start_key= start_key;
+      best_ref_depend_map= ref_depend_map_arg;
     }
   }
 
@@ -281,6 +284,7 @@ class Loose_scan_opt
       best_loose_scan_records= rows2double(quick->records);
       best_max_loose_keypart= quick_max_loose_keypart;
       best_loose_scan_start_key= NULL;
+      best_ref_depend_map= 0;
     }
   }
 
@@ -296,7 +300,7 @@ class Loose_scan_opt
       pos->loosescan_picker.loosescan_parts= best_max_loose_keypart + 1;
       pos->use_join_buffer= FALSE;
       pos->table=           tab;
-      // todo need ref_depend_map ?
+      pos->ref_depend_map=  best_ref_depend_map;
       DBUG_PRINT("info", ("Produced a LooseScan plan, key %s, %s",
                           tab->table->key_info[best_loose_scan_key].name,
                           best_loose_scan_start_key? "(ref access)":

sql/sql_select.cc

@@ -97,10 +97,6 @@ static int sort_keyuse(KEYUSE *a,KEYUSE *b);
 static bool are_tables_local(JOIN_TAB *jtab, table_map used_tables);
 static bool create_ref_for_key(JOIN *join, JOIN_TAB *j, KEYUSE *org_keyuse,
 			       bool allow_full_scan, table_map used_tables);
-void best_access_path(JOIN *join, JOIN_TAB *s, 
-                             table_map remaining_tables, uint idx, 
-                             bool disable_jbuf, double record_count,
-                             POSITION *pos, POSITION *loose_scan_pos);
 static void optimize_straight_join(JOIN *join, table_map join_tables);
 static bool greedy_search(JOIN *join, table_map remaining_tables,
                           uint depth, uint prune_level,
@@ -4571,6 +4567,13 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
     {
       if (choose_plan(join, all_table_map & ~join->const_table_map))
         goto error;
+
+#ifdef HAVE_valgrind
+      // JOIN::positions holds the current query plan. We've already
+      // made the plan choice, so we should only use JOIN::best_positions
+      for (uint k=join->const_tables; k < join->table_count; k++)
+        MEM_UNDEFINED(&join->positions[k], sizeof(join->positions[k]));
+#endif
     }
     else
     {
@@ -6285,6 +6288,7 @@ void
 best_access_path(JOIN      *join,
                  JOIN_TAB  *s,
                  table_map remaining_tables,
+                 const POSITION *join_positions,
                  uint      idx,
                  bool      disable_jbuf,
                  double    record_count,
@@ -6388,7 +6392,7 @@ best_access_path(JOIN      *join,
             if (!(keyuse->used_tables & ~join->const_table_map))
               const_part|= keyuse->keypart_map;
 
-            double tmp2= prev_record_reads(join->positions, idx,
+            double tmp2= prev_record_reads(join_positions, idx,
                                            (found_ref | keyuse->used_tables));
             if (tmp2 < best_prev_record_reads)
             {
@@ -6429,7 +6433,7 @@ best_access_path(JOIN      *join,
           Really, there should be records=0.0 (yes!)
           but 1.0 would be probably safer
         */
-        tmp= prev_record_reads(join->positions, idx, found_ref);
+        tmp= prev_record_reads(join_positions, idx, found_ref);
         records= 1.0;
       }
       else
@@ -6445,7 +6449,7 @@ best_access_path(JOIN      *join,
           if ((key_flags & (HA_NOSAME | HA_NULL_PART_KEY)) == HA_NOSAME ||
               MY_TEST(key_flags & HA_EXT_NOSAME))
           {
-            tmp = prev_record_reads(join->positions, idx, found_ref);
+            tmp = prev_record_reads(join_positions, idx, found_ref);
             records=1.0;
           }
           else
@@ -6689,7 +6693,8 @@ best_access_path(JOIN      *join,
         }
 
         tmp= COST_ADD(tmp, s->startup_cost);
-        loose_scan_opt.check_ref_access_part2(key, start_key, records, tmp);
+        loose_scan_opt.check_ref_access_part2(key, start_key, records, tmp,
+                                              found_ref);
       } /* not ft_key */
       if (tmp + 0.0001 < best_time - records/(double) TIME_FOR_COMPARE)
       {
@@ -7367,7 +7372,8 @@ optimize_straight_join(JOIN *join, table_map join_tables)
   for (JOIN_TAB **pos= join->best_ref + idx ; (s= *pos) ; pos++)
   {
     /* Find the best access method from 's' to the current partial plan */
-    best_access_path(join, s, join_tables, idx, disable_jbuf, record_count,
+    best_access_path(join, s, join_tables, join->positions, idx,
+                     disable_jbuf, record_count,
                      join->positions + idx, &loose_scan_pos);
 
     /* compute the cost of the new plan extended with 's' */
@@ -8285,8 +8291,9 @@ best_extension_by_limited_search(JOIN      *join,
 
       /* Find the best access method from 's' to the current partial plan */
       POSITION loose_scan_pos;
-      best_access_path(join, s, remaining_tables, idx, disable_jbuf,
-                       record_count, join->positions + idx, &loose_scan_pos);
+      best_access_path(join, s, remaining_tables, join->positions, idx,
+                       disable_jbuf, record_count, join->positions + idx,
+                       &loose_scan_pos);
 
       /* Compute the cost of extending the plan with 's' */
       current_record_count= COST_MULT(record_count, position->records_read);
@@ -8672,11 +8679,11 @@ cache_record_length(JOIN *join,uint idx)
 */
 
 double
-prev_record_reads(POSITION *positions, uint idx, table_map found_ref)
+prev_record_reads(const POSITION *positions, uint idx, table_map found_ref)
 {
   double found=1.0;
-  POSITION *pos_end= positions - 1;
-  for (POSITION *pos= positions + idx - 1; pos != pos_end; pos--)
+  const POSITION *pos_end= positions - 1;
+  for (const POSITION *pos= positions + idx - 1; pos != pos_end; pos--)
   {
     if (pos->table->table->map & found_ref)
     {
@@ -15400,7 +15407,8 @@ void optimize_wo_join_buffering(JOIN *join, uint first_tab, uint last_tab,
     if ((i == first_tab && first_alt) || join->positions[i].use_join_buffer)
     {
       /* Find the best access method that would not use join buffering */
-      best_access_path(join, rs, reopt_remaining_tables, i, 
+      best_access_path(join, rs, reopt_remaining_tables,
+                       join->positions, i,
                        TRUE, rec_count,
                        &pos, &loose_scan_pos);
     }

sql/sql_select.h

@@ -792,6 +792,7 @@ class LooseScan_picker : public Semi_join_strategy_picker
   friend void best_access_path(JOIN      *join,
                                JOIN_TAB  *s,
                                table_map remaining_tables,
+                               const struct st_position *join_positions,
                                uint      idx,
                                bool      disable_jbuf,
                                double    record_count,
@@ -1960,6 +1961,11 @@ class store_key_const_item :public store_key_item
   }
 };
 
+void best_access_path(JOIN *join, JOIN_TAB *s,
+                      table_map remaining_tables,
+                      const POSITION *join_positions, uint idx,
+                      bool disable_jbuf, double record_count,
+                      POSITION *pos, POSITION *loose_scan_pos);
 bool cp_buffer_from_ref(THD *thd, TABLE *table, TABLE_REF *ref);
 bool error_if_full_join(JOIN *join);
 int report_error(TABLE *table, int error);
@@ -2277,7 +2283,7 @@ bool instantiate_tmp_table(TABLE *table, KEY *keyinfo,
                            ulonglong options);
 bool open_tmp_table(TABLE *table);
 void setup_tmp_table_column_bitmaps(TABLE *table, uchar *bitmaps);
-double prev_record_reads(POSITION *positions, uint idx, table_map found_ref);
+double prev_record_reads(const POSITION *positions, uint idx, table_map found_ref);
 void fix_list_after_tbl_changes(SELECT_LEX *new_parent, List<TABLE_LIST> *tlist);
 
 struct st_cond_statistic