From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Костя Кузнецов <chapaev28(at)ya(dot)ru>
Subject: Re: GiST VACUUM
Date: 2018-07-10 20:07:44
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I'm now looking at the first patch in this series, to allow completely
empty GiST pages to be recycled. I've got some questions:

> --- a/src/backend/access/gist/gist.c
> +++ b/src/backend/access/gist/gist.c
> @@ -700,6 +700,13 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
> GISTInsertStack *item;
> OffsetNumber downlinkoffnum;
> + if(GistPageIsDeleted(stack->page))
> + {
> + UnlockReleaseBuffer(stack->buffer);
> + xlocked = false;
> + state.stack = stack = stack->parent;
> + continue;
> + }
> downlinkoffnum = gistchoose(state.r, stack->page, itup, giststate);
> iid = PageGetItemId(stack->page, downlinkoffnum);
> idxtuple = (IndexTuple) PageGetItem(stack->page, iid);

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?

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.

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.

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.

- Heikki

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