Re: Page Scan Mode in Hash Index

From: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Page Scan Mode in Hash Index
Date: 2017-09-25 04:25:24
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On Fri, Sep 22, 2017 at 11:56 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Sep 21, 2017 at 3:08 AM, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com> wrote:
>> I have added a note for handling of logged and unlogged tables in
>> README file and also corrected the header comment for
>> hashbucketcleanup(). Please find the attached 0003*.patch having these
>> changes. Thanks.
> I committed 0001 and 0002 with some additional edits as
> 7c75ef571579a3ad7a1d3ee909f11dba5e0b9440.

I have noticed a typo in that commit (in Readme) and patch for the
same is attached.

> I also rebased 0003 and
> edited it a bit; see attached hash-cleanup-changes.patch.
> I'm not entirely sold on 0003. An alternative would be to rip the lsn
> stuff back out of HashScanPosData, and I think we ought to consider
> that. Basically, 0003 is betting that getting rid of the
> lock-chaining in hash index vacuum is more valuable than being able to
> kill dead items more aggressively. I bet that's a bad bet.
> In the case of btree indexes, since
> 2ed5b87f96d473962ec5230fd820abfeaccb2069, page-at-a-time scanning
> allows most btree index scans to avoid holding buffer pins when the
> scan is suspended, but we gain no such advantage here. We always have
> to hold a pin on the primary bucket page anyway, so even with this
> patch cleanup is going to block when it hits a bucket containing a
> suspended scan. 0003 helps if (a) the relation is permanent, (b) the
> bucket has overflow pages, and (c) the scan is moving faster than
> vacuum and can overtake it instead of waiting. But that doesn't seem
> like it will happen very often at all, whereas the LSN check will
> probably fail frequently and cause us to skip cleanup that we could
> usefully have done. So I propose the attached hashscan-no-lsn.patch
> as an alternative.

I think your proposal makes sense. Your patch looks good but you
might want to tweak the comments atop _hash_kill_items ("However,
having pin on the overflow page doesn't guarantee that vacuum won't
delete any items.). That part of the comment has been written to
indicate that we have to check LSN in this function unconditonally.

With Regards,
Amit Kapila.

Attachment Content-Type Size
typo_hash_readme_v1.patch application/octet-stream 775 bytes

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