| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Edmund Horner <ejrh00(at)gmail(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, tomas(dot)vondra(at)2ndquadrant(dot)com, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Tid scan improvements |
| Date: | 2018-12-20 23:06:41 |
| Message-ID: | 10208.1545347201@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-12-20 17:21:07 -0500, Tom Lane wrote:
>> I'm having a hard time wrapping my mind around why you'd bother with
>> backwards TID scans.
> I've not followed this thread, but wouldn't that be quite useful to be
> able to move old tuples to free space earlier in the table?
> I've written multiple scripts that update the later pages in a table, to
> force reuse of earlier free pages (in my case by generating ctid = ANY()
> style queries with all possible tids for the last few pages, the most
> efficient way I could think of).
Sure, but wouldn't you now write those using something on the order of
WHERE ctid >= '(cutoff_page_here, 1)'
? I don't see that you'd want to write "ORDER BY ctid DESC LIMIT n"
because you wouldn't know what value of n to use to get all the
tuples on some-number-of-ending-pages.
regards, tom lane
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