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, 5 insertions, 1 deletions

diff --git a/sql/item.h b/sql/item.h
index adbf7103de1..748a5258b1b 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -544,7 +544,11 @@ public:
                                            of a query with ROLLUP */ 
   bool null_value;			/* if item is null */
   bool unsigned_flag;
-  bool with_sum_func;
+  bool with_sum_func;                   /* True if item contains a sum func */
+  /**
+    True if any item except Item_sum_func contains a field. Set during parsing.
+  */
+  bool with_field;
   bool fixed;                           /* If item fixed with fix_fields */
   bool is_autogenerated_name;           /* indicate was name of this Item
                                            autogenerated or set by user */