Re: properly detecting cases where handler can silently overwrite rows

I have, but we decided not to use it back in 5.1 because it was
causing us problems. Perhaps that is different now. Thank you for the
feedback. I will dig into old emails and try to remember what the
issues were, and then will try it.

Thanks
-Zardosht

On Wed, Mar 6, 2013 at 1:06 AM, Dmitry Lenev <[email protected]> wrote:
> Hello Zardosht!
>
> * Zardosht Kasheff <[email protected]> [13/03/06 06:27]:
>> On Tue, Feb 26, 2013 at 4:47 PM, Zardosht Kasheff <[email protected]> wrote:
>> > Hello all,
>> >
>> > For a bunch of statements, "replace into", "replace into ... select",
>> > "load data... replace", we sometimes do not follow the protocol of
>> > return an error if we see a duplicate key so that a subsequent update
>> > can overwrite the row. Instead, we have handler::write_row silently
>> > overwrite the data and report success.
>> >
>> > The trouble we have is properly detecting when we are indeed allowed
>> > to silently overwrite the row. What we have been doing is checking
>> > (thd->lex->duplicates == DUP_REPLACE).
>> >
>> > We recently found what seems like a corner case where this does not
>> > work. During replication on a slave, if slave_exec_mode is set to
>> > IDEMPOTENT, then thd->lex->duplicates is set to DUP_REPLACE, but the
>> > operation is not just a silent overwrite. The operation works like an
>> > insert on duplicate key update, where the updated row is dependent on
>> > the existing row and the inserted row.
>> >
>> > My questions:
>> >  - why is DUP_REPLACE being set in this case in replication?
>> >  - What is the proper way to determine when we can safely silently
>> > overwrite an existing row. We already check for having a binary log in
>> > row format and for triggers. It seems checking thd->lex->DUP_REPLACE
>> > is not sufficient.
>
> Have you seen HA_EXTRA_WRITE_CAN_REPLACE ?
>
> AFAIR it was added to implement optimization similar
> to the one you describe for NDB cluster.
>
> Best regards,
> Dmitry
>
> --
> Dmitry Lenev, Software Developer
> Oracle Development SPB/MySQL, www.mysql.com
>
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