MDEV-19429: Wrong query result with EXISTS and LIMIT 0bb-5.5-MDEV-19429

Check EXISTS LIMIT before rewriting.

1 files changed, 14 insertions, 7 deletions

diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index afc42dc08d5..9d6fe5ac5ab 100644
--- a/sql/item_subselect.cc
+++ b/sql/item_subselect.cc
@@ -1432,13 +1432,20 @@ void Item_exists_subselect::fix_length_and_dec()
 {
   DBUG_ENTER("Item_exists_subselect::fix_length_and_dec");
   init_length_and_dec();
-  /*
-    We need only 1 row to determine existence (i.e. any EXISTS that is not
-    an IN always requires LIMIT 1)
-  */
-  thd->change_item_tree(&unit->global_parameters->select_limit,
-                        new Item_int((int32) 1));
-  DBUG_PRINT("info", ("Set limit to 1"));
+  // If limit is not set or it is constant more than 1
+  if (!unit->global_parameters->select_limit ||
+      (unit->global_parameters->select_limit->basic_const_item() &&
+       unit->global_parameters->select_limit->val_int() > 1))
+  {
+    /*
+       We need only 1 row to determine existence (i.e. any EXISTS that is not
+       an IN always requires LIMIT 1)
+     */
+    thd->change_item_tree(&unit->global_parameters->select_limit,
+        new Item_int((int32) 1));
+    unit->global_parameters->explicit_limit= 1; // we set the limit
+    DBUG_PRINT("info", ("Set limit to 1"));
+  }
   DBUG_VOID_RETURN;
 }