This is very interesting. Sergey, is this engine-agnostic? I know that
Falcon has sparse bitmap indexing (already?) but I did not know if other
engines support the concept (if not, are we "emulating" it in the
optimizer?)
Baron, which engine are you testing against?
On a related note, I have been testing performance regressions of
subqueries in 5.2.6 (yes, I wrote regressions, not improvements...) and
have noticed another EXPLAIN phrase:
"Using index condition"
Sergey, what does this mean?
For reference, see the bug report I filed here:
http://bugs.mysql.com/?id=30659
Thanks!
Jay
Baron Schwartz wrote:
> Hi,
>
> Sergey Petrunia wrote:
>> Hi!
>>
>> On 8/29/07, Baron Schwartz <baron@stripped> wrote:
>>> Sometimes in EXPLAIN output, I see this in the Extra field:
>> ...
>>> explain select * from t1, t1 t2 where t1.y = 2 and t2.x between 7 and
>>> t1.y+0;
>>> id select_type table type possible_keys key
>>> key_len ref rows Extra
>>> 1 SIMPLE t1 ref y y 5 const 1 Using where
>>> 1 SIMPLE t2 range x x 5 NULL 4 Range
>>> checked for each record (index map: 0x1)
>>>
>>> I know what the rest of it means, but what does 0x1 mean?
>>
>> It is a bitmap of indexes which can be used by range/index_merge
>> access that is performed for each row combination of the preceding
>> tables.
>
> Thanks Sergey, Chad. Follow-up question: can I assume that in the
> output of SHOW CREATE TABLE, 0x1 is the first index, 0x2 is the second,
> 0x3 is the first and second -- in the order they appear in that output?
> If not, how would I be able to tell which indexes the bitmap refers to?
>
> Thanks
> Baron
>
