Horton

Re: heap vacuum & cleanup locks

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndQuadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2011-11-09T21:48:46Z
Lists: pgsql-hackers

Commits

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  1. Make VACUUM avoid waiting for a cleanup lock, where possible.

On Wed, Nov 9, 2011 at 9:12 PM, Robert Haas <robertmhaas@gmail.com> wrote:

> Well, I'm not sure of the details of how page-at-a-time mode works for
> seq scans, but I am absolutely 100% sure that you can reproduce this
> problem using a cursor over a sequential scan.  Just do this:
>
> create table test (a text);
> insert into test values ('aaa'), ('bbb');
> delete from test where a = 'aaa';
> begin;
> declare x cursor for select * from test;
> fetch next from x;

That's a bug. heapam.c line 1202 says

	/*
	 * we can use page-at-a-time mode if it's an MVCC-safe snapshot
	 */
	scan->rs_pageatatime = IsMVCCSnapshot(snapshot);

So either the comment or the code is wrong.

Can't see where, as yet.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
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