Fix LP BUG#715738

Analysis:
A query with implicit grouping is one with aggregate functions and
no GROUP BY clause. MariaDB inherits from MySQL an SQL extenstion
that allows mixing aggregate functions with non-aggregate fields.
If a query with such mixed select clause produces an empty result
set, the meaning of aggregate functions is well defined - either
NULL (MIN, MAX, etc.), or 0 (count(*)). However the non-aggregated
fields must also have some value, and the only reasonable value in
the case of empty result is NULL.

The cause of the many wrong results was that if a field is declared
as non-nullable (e.g. because it is a PK or NOT NULL), the semantic
analysis and the optimization phases treat this field as non-nullable,
and generate all related query plan elements based on this assumption.

Later during execution, these incorrectly configured/generated query
plan elements result in a wrong result because the selected fields
are not null due to the not-null assumption during optimization.

Solution:
Detect before the context analysys phase that a query uses implicit
grouping with mixed aggregates/non-aggregates, and set all fields
as nullable. The parser already walks the SELECT clause, and
already sets Item::with_sum_func for Items that reference aggreagate
functions. The patch adds a symmetric Item::with_field so that all
Items that reference an Item_field are marked during their
construction at parse time in the same way as with aggregate function
use.

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/item_row.cc b/sql/item_row.cc
index 22421199b76..99a1644cc48 100644
--- a/sql/item_row.cc
+++ b/sql/item_row.cc
@@ -86,6 +86,7 @@ bool Item_row::fix_fields(THD *thd, Item **ref)
     }
     maybe_null|= item->maybe_null;
     with_sum_func= with_sum_func || item->with_sum_func;
+    with_field= with_field || item->with_field;
   }
   fixed= 1;
   return FALSE;