System catalog bloat removing safety

Hi,

On 2013-09-17 23:12:24 -0700, Sergey Konoplev wrote:

How safe is it to use the technique described by the link below with
system catalog tables to remove bloat?

(in a couple of words it is about moving tuples to the beginning of
the table with a special way of updating)

http://www.depesz.com/2010/10/17/reduce-bloat-of-table-without-longexclusive-locks/

Are there any caveats?

At least for pg_index it's not safe, we use the xmin of rows when
indcheckxmin = true.
I am not aware of other cases, but I'd be hesitant to do so.

You have bloat because of lots of temporary tables IIRC?

Greetings,

Andres Freund

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On Wed, Sep 18, 2013 at 2:06 AM, Andres Freund <andres@2ndquadrant.com> wrote:

On 2013-09-17 23:12:24 -0700, Sergey Konoplev wrote:

How safe is it to use the technique described by the link below with
system catalog tables to remove bloat?
(in a couple of words it is about moving tuples to the beginning of
the table with a special way of updating)
http://www.depesz.com/2010/10/17/reduce-bloat-of-table-without-longexclusive-locks/

At least for pg_index it's not safe, we use the xmin of rows when
indcheckxmin = true.
I am not aware of other cases, but I'd be hesitant to do so.

Thank you, Andres, I did not know this. Just wondering, why it was
made of type bool and not xid?

You have bloat because of lots of temporary tables IIRC?

Actually I am developing a bloat removing tool based on the updating technique:

https://code.google.com/p/pgtoolkit/source/browse/trunk/README

Now the --system-catalog feature marked as experimental. About
pg_index - I think the solution is to exclude it from processing
strictly.

Anyone has other ideas of what system tables it might be affected
except pg_index?

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Kind regards,
Sergey Konoplev
PostgreSQL Consultant and DBA

http://www.linkedin.com/in/grayhemp
+1 (415) 867-9984, +7 (901) 903-0499, +7 (988) 888-1979
gray.ru@gmail.com

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