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 <nikitamalya...@gmail.com> wrote: > > > On Wed, 4 Dec 2019 at 05:04, Sergei Golubchik <s...@mariadb.org> 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
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