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This commit is contained in:
Sergei Golubchik
2011-05-21 18:41:56 +02:00
parent 7c459960ec
commit dda9577d55
7 changed files with 43 additions and 28 deletions
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40
sql/sql_select.cc
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@ -9417,36 +9417,36 @@ 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)
@param l the left comparison argument (a field if any)
@param 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.
@details
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');
RETURN VALUE
TRUE can be used
FALSE cannot be used
@retval true can be used
@retval false cannot be used
*/
static bool
test_if_equality_guarantees_uniqueness(Item *l, Item *r)
{
return r->const_item() &&
/* elements must be compared as dates */
/* the field is a date (the const will be converted to a date) */
(l->cmp_type() == TIME_RESULT ||
/* or of the same result type */
/* 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 ||