MDEV-21318: Wrong results with window functions and implicit grouping

The issue here is for degenerate joins we should execute the window
function but it is not getting executed in all the cases.

To get the window function values window function needs to be executed
always. This currently does not happen in few cases
where the join would return 0 or 1 row like
  1) IMPOSSIBLE WHERE
  2) MIN/MAX optimization
  3) EMPTY CONST TABLE

The fix is to make sure that window functions get executed
and the temporary table is setup for the execution of window functions

2 files changed, 31 insertions, 1 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 1bf8addbcf0..72c1e876359 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -1447,6 +1447,7 @@ JOIN::optimize_inner()
         zero_result_cause= "Zero limit";
       }
       table_count= top_join_tab_count= 0;
+      handle_implicit_grouping_with_window_funcs();
       error= 0;
       goto setup_subq_exit;
     }
@@ -1502,6 +1503,7 @@ JOIN::optimize_inner()
 	zero_result_cause= "No matching min/max row";
         table_count= top_join_tab_count= 0;
 	error=0;
+        handle_implicit_grouping_with_window_funcs();
         goto setup_subq_exit;
       }
       if (res > 1)
@@ -1517,6 +1519,7 @@ JOIN::optimize_inner()
       tables_list= 0;				// All tables resolved
       select_lex->min_max_opt_list.empty();
       const_tables= top_join_tab_count= table_count;
+      handle_implicit_grouping_with_window_funcs();
       /*
         Extract all table-independent conditions and replace the WHERE
         clause with them. All other conditions were computed by opt_sum_query
@@ -1615,6 +1618,7 @@ JOIN::optimize_inner()
     zero_result_cause= "no matching row in const table";
     DBUG_PRINT("error",("Error: %s", zero_result_cause));
     error= 0;
+    handle_implicit_grouping_with_window_funcs();
     goto setup_subq_exit;
   }
   if (!(thd->variables.option_bits & OPTION_BIG_SELECTS) &&
@@ -1639,6 +1643,7 @@ JOIN::optimize_inner()
     zero_result_cause=
       "Impossible WHERE noticed after reading const tables";
     select_lex->mark_const_derived(zero_result_cause);
+    handle_implicit_grouping_with_window_funcs();
     goto setup_subq_exit;
   }
 
@@ -1781,6 +1786,7 @@ JOIN::optimize_inner()
     zero_result_cause=
       "Impossible WHERE noticed after reading const tables";
     select_lex->mark_const_derived(zero_result_cause);
+    handle_implicit_grouping_with_window_funcs();
     goto setup_subq_exit;
   }
 
@@ -18225,7 +18231,8 @@ void set_postjoin_aggr_write_func(JOIN_TAB *tab)
     }
   }
   else if (join->sort_and_group && !tmp_tbl->precomputed_group_by &&
-           !join->sort_and_group_aggr_tab && join->tables_list)
+           !join->sort_and_group_aggr_tab && join->tables_list &&
+           join->top_join_tab_count)
   {
     DBUG_PRINT("info",("Using end_write_group"));
     aggr->set_write_func(end_write_group);
@@ -26925,6 +26932,28 @@ Item *remove_pushed_top_conjuncts(THD *thd, Item *cond)
   return cond;
 }
 
+/*
+  There are 5 cases in which we shortcut the join optimization process as we
+  conclude that the join would be a degenerate one
+    1) IMPOSSIBLE WHERE
+    2) MIN/MAX optimization (@see opt_sum_query)
+    3) EMPTY CONST TABLE
+  If a window function is present in any of the above cases then to get the
+  result of the window function, we need to execute it. So we need to
+  create a temporary table for its execution. Here we need to take in mind
+  that aggregate functions and non-aggregate function need not be executed.
+
+*/
+
+
+void JOIN::handle_implicit_grouping_with_window_funcs()
+{
+  if (select_lex->have_window_funcs() && send_row_on_empty_set())
+  {
+    const_tables= top_join_tab_count= table_count= 0;
+  }
+}
+
 /**
   @} (end of group Query_Optimizer)
 */
diff --git a/sql/sql_select.h b/sql/sql_select.h
index fe44f448446..ffc9eba1186 100644
--- a/sql/sql_select.h
+++ b/sql/sql_select.h
@@ -1057,6 +1057,7 @@ protected:
   void restore_query_plan(Join_plan_state *restore_from);
   /* Choose a subquery plan for a table-less subquery. */
   bool choose_tableless_subquery_plan();
+  void handle_implicit_grouping_with_window_funcs();
 
 public:
   JOIN_TAB *join_tab, **best_ref;