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Re: Compressing the AFTER TRIGGER queue

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Compressing the AFTER TRIGGER queue
Date: 2011-08-01 20:02:45
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Mon, Aug 1, 2011 at 7:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> However, this means that Dean is not simply adding some self-contained
> compression logic; he's eliminating information from the data structure
> on the grounds that he can get it from elsewhere.  I think that that
> ought to be treated as a separate patch, and that the costs/benefits
> of the "compressed" data structure ought to be evaluated relative to the
> situation with the second CTID removed but the data structure not
> otherwise changed.

I would prefer an approach where we store the events in a buffer,
which gets added to the main event queue when it fills/block number
changes/etc. That way we can apply intelligence to the actual
compression format used, yet retain all required info in the queued
event. We might also be able to ensure the event queue is ordered more
naturally by block number, to ensure we have reasonable readahead
while re-traversing the event queue - its not just the size of the
event queue that is the problem here.

I'm also nervous of compression formats that rely too heavily on
earlier data, so any corner case bug in one of the formats will screw
the whole data structure. I'd prefer a more modular approach that is
easier to test in regression tests.

The cases we have problems with are the bulk cases, so we should be
specifically optimising for that. For example, if we are running a
COPY/INSERT SELECT or a seqscan DELETE then we will queue a trigger
for every tuple in a block. The best compression and flexibility in
that case is to store a bitmap since that will average out at about 1
bit per row, with variable length bitmaps. Which is about 8 times
better compression ratio than originally suggested, without any loss
of metadata.

We should also optimise the HOT UPDATE case.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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