From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Костя Кузнецов <chapaev28(at)ya(dot)ru>
Subject: Re: GiST VACUUM
Date: 2018-07-12 16:06:38
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PFA v5 of the patch series.

> 11 июля 2018 г., в 0:07, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> написал(а):
> This seems misplaced. This code deals with internal pages, and as far as I can see, this patch never marks internal pages as deleted, only leaf pages. However, we should have something like this in the leaf-page branch, to deal with the case that an insertion lands on a page that was concurrently deleted. Did you have any tests, where an insertion runs concurrently with vacuum, that would exercise this?

That bug could manifest only in case of crash between removing downlinks and marking pages deleted. I do not know how to test this reliably.
Internal pages are locked before leafs and locks are coupled. No cuncurrent backend can see downlinks to pages being deleted, unless crash happens.

I've replaced code covering this situation into leaf code path and added comment.

> The code in gistbulkdelete() seems pretty expensive. In the first phase, it records the parent of every empty leaf page it encounters. In the second phase, it scans every leaf page of that parent, not only those leaves that were seen as empty.

It is fixed in second patch of the series.

> I'm a bit wary of using pd_prune_xid for the checks to determine if a deleted page can be recycled yet. In heap pages, pd_prune_xid is just a hint, but here it's used for a critical check. This seems to be the same mechanism we use in B-trees, but in B-trees, we store the XID in BTPageOpaqueData.xact, not pd_prune_xid. Also, in B-trees, we use ReadNewTransactionId() to set it, not GetCurrentTransactionId(). See comments in _bt_unlink_halfdead_page() for explanation. This patch is missing any comments to explain how this works in GiST.

I've replaced usage of GetCurrentTransactionId() with ReadNewTransactionId() and added explanation of what is going on. Also, I've added comments about that pd_prune_xid usage. There is no other use in GiST for this field. And there is no other room to place this xid on a page without pg_upgrade.

> If you crash in the middle of gistbulkdelete(), after it has removed the downlink from the parent, but before it has marked the leaf page as deleted, the leaf page is "leaked". I think that's acceptable, but a comment at least would be good.

Added explanatory comment between WAL-logging downlink removal and marking pages deleted.

Thank you for reviewing the patch!

Best regards, Andrey Borodin.

Attachment Content-Type Size
0002-Physical-GiST-scan-during-VACUUM-v5.patch application/octet-stream 15.3 KB
0001-Delete-pages-during-GiST-VACUUM-v5.patch application/octet-stream 15.0 KB

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