Indexes where not used for sorting in sub queries

This happens when the subquery marks some index fields as constant
but the fields are still present in GROUP BY

Fixed by checking if the 'constant field' is still part of GROUP BY before
skipping it.

Other things:
- Added Item_field::contains() to make it easier to check if a field
  is equal to a Item_field or part of Item_equal.

1 files changed, 18 insertions, 0 deletions

diff --git a/sql/item.cc b/sql/item.cc
index 5bcddb5cd67..aa0553228b3 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -6348,6 +6348,24 @@ Item_equal *Item_field::find_item_equal(COND_EQUAL *cond_equal)
 }
 
 
+/*
+  Check if field is is equal to current field or any of the fields in
+  item_equal
+*/
+
+bool Item_field::contains(Field *field_arg)
+{
+  if (field == field_arg)
+    return 1;
+  /*
+    Check if there is a multiple equality that allows to infer that field
+    (see also: compute_part_of_sort_key_for_equals)
+  */
+  if (item_equal && item_equal->contains(field_arg))
+    return 1;
+  return 0;
+}
+
 /**
   Set a pointer to the multiple equality the field reference belongs to
   (if any).