From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
---|---|
To: | Mats Kindahl <mats(at)timescale(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: RFC: Table access methods and scans |
Date: | 2021-06-04 00:52:24 |
Message-ID: | 6d74b91d43479dc90c401e7ed7906258a1e42341.camel@j-davis.com |
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Lists: | pgsql-hackers |
Hi,
On Wed, 2021-03-31 at 22:10 +0200, Mats Kindahl wrote:
> As an example of how this is useful, I noticed the work by Heikki and
> Ashwin [1], where they return a `TableScanDesc` that contains
> information about what columns to scan, which looks very useful.
> Since
> the function `table_beginscan` in `src/include/access/tableam.h`
> accept a `ScanKey` as input, this is (AFAICT) what Heikki and Ashwin
> was exploiting to create a specialized scan for a columnar store.
I don't think ScanKeys are the right place to store information about
what columns would be useful. See another thread[2] about that topic.
> Another example of where this can be useful is to optimize access
> during a sequential scan when you can handle some specific scans very
> efficiently and can "skip ahead" many tuples if you know what is
> being
> looked for instead of filtering "late". Two examples of where this
> could be useful are:
>
> - An access method that reads data from a remote system and doesn't
> want
> to transfer all tuples unless necessary.
> - Some sort of log-structured storage with Bloom filters that allows
> you to quickly skip suites that do not have a key.
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?
Regards,
Jeff Davis
[3]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=e3a1ab764ef2
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