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change test_if_equality_guarantees_uniqueness()

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from an ad hoc set of limitations
to a correct rule
This commit is contained in:
Sergei Golubchik
2011-06-10 10:14:20 +02:00
parent fdfeb4bea3
commit 10fedf675a
3 changed files with 58 additions and 17 deletions
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30
sql/sql_select.cc
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@@ -12171,16 +12171,16 @@ remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value)
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.
Additionally, strings must have the same collation.
Or the *field* must be a datetime - if the constant is a datetime
and a field is not - this is not enough, consider:
create table t1 (a varchar(100));
insert t1 values ('2010-01-02'), ('2010-1-2'), ('20100102');
select distinct t1 from t1 where a=date('2010-01-02');
Arguments must be compared in the native type of the left argument
and (for strings) in the native collation of the left argument.
Otherwise, for example,
<string_field> = <int_const> may match more than 1 distinct value or
the <string_field>.
@note We don't need to aggregate l and r collations here, because r -
the constant item - has already been converted to a proper collation
for comparison. We only need to compare this collation with field's collation.
@retval true can be used
@retval false cannot be used
*/
@@ -12188,13 +12188,9 @@ static bool
test_if_equality_guarantees_uniqueness(Item *l, Item *r)
{
return r->const_item() &&
/* the field is a date (the const will be converted to a date) */
(l->cmp_type() == TIME_RESULT ||
/* or arguments are of the same result type */
(r->result_type() == l->result_type() &&
/* and must have the same collation if compared as strings */
(l->result_type() != STRING_RESULT ||
l->collation.collation == r->collation.collation)));
item_cmp_type(l->cmp_type(), r->cmp_type()) == l->cmp_type() &&
(l->cmp_type() != STRING_RESULT ||
l->collation.collation == r->collation.collation);
}
/**