Re: Patch: Write Amplification Reduction Method (WARM)

On Wed, Apr 5, 2017 at 2:32 PM, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com> wrote:
>> The only other idea that I have for a really clean solution here is to
>> support this only for index types that are amcanreturn, and actually
>> compare the value stored in the index tuple with the one stored in the
>> heap tuple, ensuring that new index tuples are inserted whenever they
>> don't match and then using the exact same test to determine the
>> applicability of a given index pointer to a given heap tuple.
>
> Just so that I understand, are you suggesting that while inserting WARM
> index pointers, we check if the new index tuple will look exactly the same
> as the old index tuple and not insert a duplicate pointer at all?

Yes.

> I considered that, but it will require us to do an index lookup during WARM
> index insert and for non-unique keys, that may or may not be exactly cheap.

I don't think it requires that. You should be able to figure out
based on the tuple being updated and the corresponding new tuple
whether this will bet true or not.

> Or we need something like what Claudio wrote to sort all index entries by
> heap TIDs. If we do that, then the recheck can be done just based on the
> index and heap flags (because we can then turn the old index pointer into a
> CLEAR pointer. Index pointer is set to COMMON during initial insert).

Yeah, I think that patch is going to be needed for some of the storage
work I'm interesting in doing, too, so I am tentatively in favor of
it, but I wasn't proposing using it here.

> The other way is to pass old tuple values along with the new tuple values to
> amwarminsert, build index tuples and then do a comparison. For duplicate
> index tuples, skip WARM inserts.

This is more what I was thinking. But maybe one of the other ideas
you wrote here is better; not sure.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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