Re: PATCH: index-only scans with partial indexes

Hello Horiguchi-san,

On 09/17/2015 12:45 PM, Kyotaro HORIGUCHI wrote:
>
> After all, what should be done to properly ignore useless
> conditions would be,
>
>   1. Make create_index_paths() to make the list of restrict
>      clauses which are implied by the index predicate of the index
>      in focus. The list would be stored as a member in
>      IndexOptInfo. Then create index clauses excluding the listed
>      clauses and call get_index_paths using them. Modify
>      check_index_only() to ignore the listed clauses when pulling
>      varattnos. This is similar but different a bit to what I said
>      in the previous mail.
>
>   2. Pass the listed clauses to generated IndexPath.
>
>   3. Modify extract_nonindex_conditions to be called with the
>      exclusion list and the cluases are exluded from the result of
>      the function.
>
>   4. Make create_indexscan_plan to extract qpqual excluding the
>      exclusion list.
>
> The same result could be obtained by more smarter way but the
> steps above will archive right decision on whether to do index
> only scan on partial index and correct cost estimate propery
> ignoring unused restrictions.
>
> Does this make sense for you?

Yes, this seems sane. I've been poking at this a bit too, and I came to 
the same plan in general, except that I think it's better to build list 
of clauses that are *not* implied by the index, because that's what we 
need both in cost_index and check_index_only.

It also seems to me that this change (arguably a bug fix) should pretty 
much make the original patch irrelevant, because check_index_only can 
simply walk over the new list.

regards

-- 
Tomas Vondra                   http://www.2ndQuadrant.com/
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