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Fix LP BUG#715738

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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.
This commit is contained in:
unknown 2011-03-24 16:34:06 +02:00
commit ec23949158
11 changed files with 670 additions and 5 deletions
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4
sql/item_func.cc
View file

@ -79,6 +79,7 @@ void Item_func::set_arguments(List<Item> &list)
{
*(save_args++)= item;
with_sum_func|=item->with_sum_func;
with_field|= item->with_field;
}
}
list.empty(); // Fields are used
@ -129,6 +130,7 @@ Item_func::Item_func(THD *thd, Item_func *item)
Sets as a side effect the following class variables:
maybe_null Set if any argument may return NULL
with_sum_func Set if any of the arguments contains a sum function
with_field Set if any of the arguments contains or is a field
used_tables_cache Set to union of the tables used by arguments
str_value.charset If this is a string function, set this to the
@ -198,6 +200,7 @@ Item_func::fix_fields(THD *thd, Item **ref)
maybe_null=1;
with_sum_func= with_sum_func || item->with_sum_func;
with_field= with_field || item->with_field;
used_tables_cache|= item->used_tables();
not_null_tables_cache|= item->not_null_tables();
const_item_cache&= item->const_item();
@ -2939,6 +2942,7 @@ udf_handler::fix_fields(THD *thd, Item_result_field *func,
if (item->maybe_null)
func->maybe_null=1;
func->with_sum_func= func->with_sum_func || item->with_sum_func;
func->with_field= func->with_field || item->with_field;
used_tables_cache|=item->used_tables();
const_item_cache&=item->const_item();
f_args.arg_type[i]=item->result_type();