Fix for #58422: Incorrect result when OUTER JOIN'ing with an empty table.

Fixed incorrect checks in join_read_const_table() for when to 
accept a non-existing, or empty const-row as a part of the const'ified 
set of tables.
      
Intention of this test is to only accept NULL-rows if this table is outer joined
into the resultset. (In case of an inner-join we can conclude at this point that 
resultset will be empty, end we want to return 'error' to signal this.)
      
Initially 'maybe_null' is set to the same value as 'outer_join' in 
setup_table_map(), mysql_priv.h ~line 2424. Later simplify_joins() will
attemp to replace outer joins by inner join whenever possible. This
will cause 'outer_join' to be updated. However, 'maybe_null' is *not* updated
to reflect this rewrite as this field is used to currectly set the 'nullability'
property for the columns in the resultset.
      
We should therefore change join_read_const_table() to check the 'outer_join'
property instead of 'maybe_null', as this correctly reflect the nullability of
the *execution plan* (not *resultset*).

1 files changed, 2 insertions, 2 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 26216225c3f..fdce2510df4 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -12040,7 +12040,7 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
       /* Mark for EXPLAIN that the row was not found */
       pos->records_read=0.0;
       pos->ref_depend_map= 0;
-      if (!table->maybe_null || error > 0)
+      if (!table->pos_in_table_list->outer_join || error > 0)
 	DBUG_RETURN(error);
     }
   }
@@ -12061,7 +12061,7 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
       /* Mark for EXPLAIN that the row was not found */
       pos->records_read=0.0;
       pos->ref_depend_map= 0;
-      if (!table->maybe_null || error > 0)
+      if (!table->pos_in_table_list->outer_join || error > 0)
 	DBUG_RETURN(error);
     }
   }