From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Jeff Davis <pgsql(at)j-davis(dot)com> |
Cc: | Mats Kindahl <mats(at)timescale(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: RFC: Table access methods and scans |
Date: | 2021-06-11 19:09:08 |
Message-ID: | 20210611190908.srxx4hp7io6hrkkc@alap3.anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On 2021-06-03 17:52:24 -0700, Jeff Davis wrote:
> I agree that would be very conventient for non-heap AMs. There's a very
> old commit[3] that says:
>
> + /*
> + * Note that unlike IndexScan, SeqScan never use keys
> + * in heap_beginscan (and this is very bad) - so, here
> + * we have not check are keys ok or not.
> + */
>
> and that language has just been carried forward for decades. I wonder
> if there's any major reason this hasn't been done yet. Does it just not
> improve performance for a heap, or is there some other reason?
It's not actually a good idea in general:
- Without substantial refactoring more work is done while holding the
content lock on the page. Whereas doing it as part of a seqscan only
requires a buffer pin (and thus allows for concurrent writes to the
same page)
- It's hard to avoid repeated work with expressions that can't fully be
evaluated as part of the ScanKey. Expression evaluation generally can
be a bit smarter about evaluation, e.g. not deforming the tuple
one-by-one.
Greetings,
Andres Freund
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