Re: [HACKERS] HOT WIP Patch - version 2

There's a fair amount of added work to be done when updating tuples.
Will it be possible to postpone some of that to the bgwriter in a later
version? I realize that sometimes you'll still want to do the work up
front, like if it means we can stay on the same page instead of going
cold...

On Tue, Feb 20, 2007 at 12:08:14PM +0530, Pavan Deolasee wrote:
> Reposting - looks like the message did not get through in the first
> attempt. My apologies if multiple copies are received.
>
>
> This is the next version of the HOT WIP patch. Since the last patch that
> I sent out, I have implemented the HOT-update chain pruning mechanism.
>
> When following a HOT-update chain from the index fetch, if we notice that
> the root tuple is dead and it is HOT-updated, we try to prune the chain to
> the smallest possible length. To do that, the share lock is upgraded to an
> exclusive lock and the tuple chain is followed till we find a
> live/recently-dead
> tuple. At that point, the root t_ctid is made point to that tuple. In order
> to
> preserve the xmax/xmin chain, the xmax of the root tuple is also updated
> to xmin of the found tuple. Since this xmax is also < RecentGlobalXmin
> and is a committed transaction, the visibility of the root tuple still
> remains
> the same.
>
> The intermediate heap-only tuples are removed from the HOT-update chain.
> The HOT-updated status of these tuples is cleared and their respective
> t_ctid are made point to themselves. These tuples are not reachable now
> and ready for vacuuming. This entire action is logged in a single
> WAL record.
>
> During vacuuming, we keep track of number of root tuples vacuumed.
> If this count is zero, then the index cleanup step is skipped. This
> would avoid unnecessary index scans whenever possible.
>
> This patch should apply cleanly on current CVS head and pass all regression
> tests. I am still looking for review comments from the first WIP patch. If
> anyone
> has already looked through it and is interested in the incremental changes,
> please let me know. I can post that.
>
> Whats Next ?
> -----------------
>
> ISTM that the basic HOT-updates and ability to prune the HOT-update chain,
>
> should help us reduce the index bloat, limit the overhead of ctid following
> in
> index fetch and efficiently vacuum heap-only tuples. IMO the next important
> but rather less troublesome thing to tackle is to reuse space within a block
>
> without complete vacuum of the table. This would help us do much more
> HOT-updates and thus further reduce index/heap bloat.
>
> I am thinking of reusing the DEAD heap-only tuples which gets removed from
> the HOT-update chain as part of pruning operation. Since these tuples, once
> removed from the chain, are neither reachable nor have any index references,
> could be readily used for storing newer versions of the same or other rows
> in
> the block. How about setting LP_DELETE on these tuples as part of the
> prune operation ? LP_DELETE is unused for heap tuples, if I am not
> mistaken. Other information like length and offset are is maintained as it
> is.
> When we run out space for update-within-the-block, we traverse
> through all the line pointers looking for LP_DELETEd items. If any of these
> items have space large enough to store the new tuple, that item is reused.
> Does anyone see any issue with doing this ? Also, any suggestions
> about doing it in a better way ?
>
> If the page gets really fragmented, we can try to grab a VACUUM-strength
> lock on the page and de-fragment it. The lock is tried conditionally to
> avoid
> any deadlocks. This is done in the heap_update() code path, so would add
> some overhead, but may still prove better than putting the tuple in a
> different block and having corresponding index insert(s). Also, since we are
> more concerned about the large tables, the chances of being able to upgrade
> the exclusive lock to vacuum-strength lock are high. Comments ?
>
> If there are no objections, I am planning to work on the first part
> while Nikhil would take up the second task of block level retail-vacuum.
> Your comments on these issues and the patch are really appreciated.
>
> Thanks,
> Pavan
>
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>
> EnterpriseDB http://www.enterprisedb.com

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--
Jim Nasby jim(at)nasby(dot)net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)