maria-developers team mailing list archive

[Nikita Malyavin <nikitamalyavin@xxxxxxxxx>]

Heh, unfortunately comparing refs will not work in conjunction with KEYREAD:

for innodb ref can be a primary key, if it has it, so index is not
clustered. The index we're evaluating can be different from a primary key.

In that case some values can be left unfetched with KEYREAD.

All that is because if innodb has primary key, it just does key_copy

On Wed, 4 Dec 2019 at 17:14, Nikita Malyavin <nikitamalyavin@xxxxxxxxx>
wrote:

>
>
> On Wed, 4 Dec 2019 at 05:04, Sergei Golubchik <serg@xxxxxxxxxxx> wrote:
>
>> > > > > two rows are the same, if their "positions" are equal, not if
>> > > > > their column values are equal. Also positions are much shorter
>> > > > > to compare.
>> > > > >
>> > > > > after ha_index_read_map or ha_index_next you do
>> > > > >
>> > > > >   handler->position(record_buffer)
>> > > > >
>> > > > > and then you have a "position" stored in handler->ref, and it
>> > > > > has the length of handler->ref_length bytes. For MyISAM it's
>> > > > > usually the file offset, for InnoDB - PK value.
>> > > > >
>> > > > > For UPDATE you can, I suppose, call this->position(old_data) to
>> > > > > get the position.
>> > > >
>> > old_data could have been even not fetched, and in some cases handler
>> > is not cloned, so it can't work in general.
>>
>> In what cases? We're talking about UPDATE here, there's `this` handler
>> that has already read old_data, so `this->position()` is well defined.
>>
>> Hmm yes... I guess ha_update_row wouldn't even work correctly if old_data
> is not the last fetched row. I remember that was true for myisam at least
> -- ha_write_row was spoiling ref (when history row was inserted, or period
> row were split in FOR PORTION case), and that was worked around -- either
> by saving/restoring positions, or by having separate file with rowids
>
>
>> So both positions exist, can be read and compared. What's not general
>> here, that it doesn't work for INSERT? It doesn't have to, this is
>> special logic for UPDATE only.
>>
> Ok, I still think it is a little bit a hack but we can try and see if
> it'll work in long term.
>
> --
> Yours truly,
> Nikita Malyavin
>


-- 
Yours truly,
Nikita Malyavin