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MDEV-8553: Impossible where for a!=a, a<a, a>a

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For a table column `a`, the above expressions logically
equate to false in all cases.

With this patch the optimizer knows about this and queries
like:

SELECT * FROM t1 WHERE a!=a

no longer need to evaluate a!=a for every row.

The same applies if the expression was `a<a`, or `a>a`

An `EXPLAIN SELECT COOUNT(*) FROM t1 WHERE a<a` will show:

id     select_type     table   type    possible_keys   key     key_len ref     rows    Extra
1      SIMPLE  NULL    NULL    NULL    NULL    NULL    NULL    NULL    Impossible WHERE

Similarly `NOT (a!=a)` is always true.

EXPLAIN SELECT COUNT(*) FROM t1 WHERE not (a!=a);
id     select_type     table   type    possible_keys   key     key_len ref     rows    Extra
1      SIMPLE  NULL    NULL    NULL    NULL    NULL    NULL    NULL    Select tables optimized away
This commit is contained in:
Daniel Black
2018-12-16 17:57:47 +11:00
committed by Sergey Vojtovich
parent ad36d38024
commit d2fa5f8cfc
3 changed files with 126 additions and 1 deletions
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3
sql/sql_select.cc
View File

@ -17109,7 +17109,8 @@ Item_bool_func2::remove_eq_conds(THD *thd, Item::cond_result *cond_value,
{
if (args[0]->eq(args[1], true))
{
if (!args[0]->maybe_null || functype() == Item_func::EQUAL_FUNC)
if (*cond_value == Item::COND_FALSE ||
!args[0]->maybe_null || functype() == Item_func::EQUAL_FUNC)
return (COND*) 0; // Compare of identical items
}
}