DRAFT: Pass sk_attno to consistent function

I realised it might be enough to pass sk_attno to consistent function as that should be enough to lookup metadata if needed.

Attached is a draft patch that does this.

—
Michal



> On 18 Mar 2024, at 14:31, Michał Kłeczek <michal(at)kleczek(dot)org> wrote:
>
> Hi All,
>
>> On 11 Mar 2024, at 18:58, Michał Kłeczek <michal(at)kleczek(dot)org> wrote:
>>
>> Hi All,
>>
>> During my journey of designing Pg based solution at my work I was severely hit by several shortcomings in GiST.
>> The most severe one is the lack of support for SAOP filters as it makes it difficult to have partition pruning and index (only) scans working together.
>>
>> To overcome the difficulties I implemented a simple extension:
>> https://github.com/mkleczek/btree_gist_extra
>>
>> Since it provides a separate operator class from btree_gist it requires re-indexing the data.
>> So I thought maybe it would be a good idea to incorporate it into btree_gist.
>>
>
> While working on supporting (sort of) SAOP support for Gist I was stuck with the interplay between Gist consistent function and partition pruning.
> Not sure how it applies to SAOP handling in general though.
>
> I’ve implemented an operator class/family that supports Gist index scan for the following query:
>
> SELECT * FROM tbl WHERE col ||= ARRAY[‘a’, ‘b’, ‘c’];
>
> It all works well except cases where tbl is partitioned by “col” column.
> In this case index scan unnecessarily scans pages for values that are not in the partition.
>
> I am not sure if it works as expected (ie. no unnecessary scans) in case of ANY = (ARRAY[]) and nbtree.
> In case of Gist the only place where pre-processing of queries can be performed is consistent function.
> But right now there is no possibility to access any scan related information as it is not passed to consistent function.
> The only thing available is GISTENTRY and it does not contain any metadata.
>
> As a workaround I’ve added options to the op family that allows (redundantly) specifying MODULUS/REMAINDER for the index:
>
> CREATE INDEX idx ON tbl_partition_01 USING gist ( col gist_text_extra_ops (modulus=4, remainder=2) )
>
> and use the values to filter out query array passed to consistent function.
>
> This is of course not ideal:
> - the information is redundant
> - changing partitioning scheme requires re-indexing
>
> I am wondering if it is possible to extend consistent function API so that either ScanKey or even the whole IndexScanDesc is passed as additional parameter.
>
> —
> Michal