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.
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