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MDEV-7149 Constant propagation erroneously applied for LIKE

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Simply disallowing equality propagation into LIKE.
A more delicate fix is be possible, but it would need too many changes,
which is not desirable in 10.0 at this point.
This commit is contained in:
Alexander Barkov 2014-11-28 18:11:58 +04:00
commit f8e1952be4
13 changed files with 1110 additions and 1 deletions
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37
sql/item_cmpfunc.h
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@ -1486,7 +1486,42 @@ public:
longlong val_int();
enum Functype functype() const { return LIKE_FUNC; }
optimize_type select_optimize() const;
cond_result eq_cmp_result() const { return COND_TRUE; }
cond_result eq_cmp_result() const
{
/**
We cannot always rewrite conditions as follows:
from: WHERE expr1=const AND expr1 LIKE expr2
to: WHERE expr1=const AND const LIKE expr2
or
from: WHERE expr1=const AND expr2 LIKE expr1
to: WHERE expr1=const AND expr2 LIKE const
because LIKE works differently comparing to the regular "=" operator:
1. LIKE performs a stricter one-character-to-one-character comparison
and does not recognize contractions and expansions.
Replacing "expr1" to "const in LIKE would make the condition
stricter in case of a complex collation.
2. LIKE does not ignore trailing spaces and thus works differently
from the "=" operator in case of "PAD SPACE" collations
(which are the majority in MariaDB). So, for "PAD SPACE" collations:
- expr1=const - ignores trailing spaces
- const LIKE expr2 - does not ignore trailing spaces
- expr2 LIKE const - does not ignore trailing spaces
Allow only "binary" for now.
It neither ignores trailing spaces nor has contractions/expansions.
TODO:
We could still replace "expr1" to "const" in "expr1 LIKE expr2"
in case of a "PAD SPACE" collation, but only if "expr2" has '%'
at the end.
*/
return ((Item_func_like *)this)->compare_collation() == &my_charset_bin ?
COND_TRUE : COND_OK;
}
const char *func_name() const { return "like"; }
bool fix_fields(THD *thd, Item **ref);
void cleanup();