diff options
Diffstat (limited to 'sql/sql_select.cc')
| -rw-r--r-- | sql/sql_select.cc | 44 |
1 files changed, 42 insertions, 2 deletions
diff --git a/sql/sql_select.cc b/sql/sql_select.cc index 5da48c69e9e..3c1c2fe4778 100644 --- a/sql/sql_select.cc +++ b/sql/sql_select.cc @@ -8593,6 +8593,46 @@ remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value) return cond; // Point at next and level } +/* + Check if equality can be used in removing components of GROUP BY/DISTINCT + + SYNOPSIS + test_if_equality_guarantees_uniqueness() + l the left comparison argument (a field if any) + r the right comparison argument (a const of any) + + DESCRIPTION + Checks if an equality predicate can be used to take away + DISTINCT/GROUP BY because it is known to be true for exactly one + distinct value (e.g. <expr> == <const>). + Arguments must be of the same type because e.g. + <string_field> = <int_const> may match more than 1 distinct value from + the column. + We must take into consideration and the optimization done for various + string constants when compared to dates etc (see Item_int_with_ref) as + well as the collation of the arguments. + + RETURN VALUE + TRUE can be used + FALSE cannot be used +*/ +static bool +test_if_equality_guarantees_uniqueness(Item *l, Item *r) +{ + return r->const_item() && + /* elements must be of the same result type */ + (r->result_type() == l->result_type() || + /* or dates compared to longs */ + (((l->type() == Item::FIELD_ITEM && + ((Item_field *)l)->field->can_be_compared_as_longlong()) || + (l->type() == Item::FUNC_ITEM && + ((Item_func *)l)->result_as_longlong())) && + r->result_type() == INT_RESULT)) + /* and must have the same collation if compared as strings */ + && (l->result_type() != STRING_RESULT || + l->collation.collation == r->collation.collation); +} + /* Return 1 if the item is a const value in all the WHERE clause */ @@ -8629,7 +8669,7 @@ const_expression_in_where(COND *cond, Item *comp_item, Item **const_item) Item *right_item= ((Item_func*) cond)->arguments()[1]; if (left_item->eq(comp_item,1)) { - if (right_item->const_item()) + if (test_if_equality_guarantees_uniqueness (left_item, right_item)) { if (*const_item) return right_item->eq(*const_item, 1); @@ -8639,7 +8679,7 @@ const_expression_in_where(COND *cond, Item *comp_item, Item **const_item) } else if (right_item->eq(comp_item,1)) { - if (left_item->const_item()) + if (test_if_equality_guarantees_uniqueness (right_item, left_item)) { if (*const_item) return left_item->eq(*const_item, 1); |