From: | "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> |
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To: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | "Pavan Deolasee" <pavan(dot)deolasee(at)gmail(dot)com>, "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Bruce Momjian" <bruce(at)momjian(dot)us>, "PostgreSQL-patches" <pgsql-patches(at)postgresql(dot)org> |
Subject: | Re: HOT patch - version 15 |
Date: | 2007-09-06 16:26:45 |
Message-ID: | 46E02A45.3010705@enterprisedb.com |
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Lists: | pgsql-patches |
Me & Greg just had a little chat, and came up with this scheme:
1. on heap_update, if the page is full, you prune the page (but don't
defragment it, because you can't get the vacuum lock). That hopefully
leaves behind a large enough gap to put the new tuple in. Insert the new
tuple in the gap, and mark the page as Fragmented. Also make a note in
some backend-private data structure that we've left that page in
fragmented state.
2. In UnpinBuffer, if the pin count falls to zero and it's a page we've
pruned (check the backend-private data structure), defragment it.
Under little contention, all the cost of pruning will be carried by
transactions that do updates. Whether we need to prune in heap_fetch in
addition to that to keep the chains short, I don't know.
One problem with this scheme is that when the page gets full, you have
to hope that you can create a wide enough gap by pruning. It will work
well with fixed-size tuples, but not so well otherwise.
Hmm. I wonder if we could prune/defragment in bgwriter?
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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