MDEV-6859: scalar subqueries in a comparison produced unexpected result

When one evaluates row-based comparison like (X, Y) = (A,B), one should
first call bring_value() for the Item that returns row value. If you
don't do that and just attempt to read values of X and Y, you get stale
values.
Semi-join/Materialization can take a row-based comparison apart and
make ref access from it. In that case, we need to call bring_value()
to get the index lookup components.

1 files changed, 11 insertions, 0 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index f43f506692c..a9b86b5e76b 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -17580,7 +17580,18 @@ int join_read_key2(THD *thd, JOIN_TAB *tab, TABLE *table, TABLE_REF *table_ref)
     }
   }
 
+  /*
+    The following is needed when one makes ref (or eq_ref) access from row
+    comparisons: one must call row->bring_value() to get the new values.
+  */
+  if (tab && tab->bush_children)
+  {
+    TABLE_LIST *emb_sj_nest= tab->bush_children->start->emb_sj_nest;
+    emb_sj_nest->sj_subq_pred->left_expr->bring_value();
+  }
+
   /* TODO: Why don't we do "Late NULLs Filtering" here? */
+
   if (cmp_buffer_with_ref(thd, table, table_ref) ||
       (table->status & (STATUS_GARBAGE | STATUS_NO_PARENT | STATUS_NULL_ROW)))
   {