Question about pg_dump

Well, all you're doing with pg_dump is getting a snapshot of the database at
a single point in time -- as pg_dump reads the data out of the tables, I'm
sure that it won't go back and re-read tables that have changed since it was
first started. On the other hand, if pg_dump is busy on table A and table Z
changes, even though it's after the pg_dump process starts, I would assume
the changes to table Z would get dumped.

I'm assuming a lot -- all of that is based on the fact that I've never seen
pg_dump lock the whole database down (though I do think it locks the table
it's dumping while it's dumping it)..

-Mitch

----- Original Message -----
From: "Vilson farias" <vilson.farias@digitro.com.br>
To: <pgsql-general@postgresql.org>
Cc: "SIMONE Carla MOSENA" <simone.mosena@digitro.com.br>
Sent: Friday, October 13, 2000 11:53 AM
Subject: [GENERAL] Question about pg_dump

Hi.

I started testing pg_dump and I have a little question :

What does pg_dump with data arrived while its running?

Lets suppose I'd started a pg_dump mybase > /home/postgres/text.txt and
while its running, new data was inserted into its tables.

All data that cames after the beginning of processing are ignored ? How

can

"Mitch Vincent" <mitch@venux.net> writes:

Well, all you're doing with pg_dump is getting a snapshot of the database at
a single point in time -- as pg_dump reads the data out of the tables, I'm
sure that it won't go back and re-read tables that have changed since it was
first started. On the other hand, if pg_dump is busy on table A and table Z
changes, even though it's after the pg_dump process starts, I would assume
the changes to table Z would get dumped.

I'm assuming a lot -- all of that is based on the fact that I've never seen
pg_dump lock the whole database down (though I do think it locks the table
it's dumping while it's dumping it)..

Under MVCC, pg_dump doesn't lock much of anything --- ordinary read
and write transactions can proceed with abandon. You would see some
interlocking behavior with schema-altering commands (eg, ALTER or DROP
TABLE) but not with updates of user data. The general rule for
SELECT-only transactions, which is what a pg_dump run is, is that the
transaction sees all and only that data written by transactions that
committed before it started.

I've just finished putting together a talk about MVCC transaction
processing --- both how it behaves and how it's implemented --- for
the upcoming OSDN database conference. I think the slides for that
talk are fairly self-contained and might be of general interest;
shall I brace Vince about putting them up on the website?

regards, tom lane

On Sat, 14 Oct 2000, Tom Lane wrote:

I've just finished putting together a talk about MVCC transaction
processing --- both how it behaves and how it's implemented --- for
the upcoming OSDN database conference. I think the slides for that
talk are fairly self-contained and might be of general interest;
shall I brace Vince about putting them up on the website?

I would certainly be interested in them, FWIW...

--
Dominic J. Eidson
"Baruk Khazad! Khazad ai-menu!" - Gimli
-------------------------------------------------------------------------------
http://www.the-infinite.org/ http://www.the-infinite.org/~dominic/

At 16:53 13/10/00 -0200, Vilson farias wrote:

Hi.

I started testing pg_dump and I have a little question :

What does pg_dump with data arrived while its running?

pg_dump runs in a single transaction, so in theory it dumps a consistent
view of the data, not necessarily the absolute latest data.

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.B.N. 75 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 0500 83 82 82 | ___________ |
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| --________--
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and from pgp5.ai.mit.edu:11371 |/

At 00:10 14/10/00 -0400, Tom Lane wrote:

I think the slides for that
talk are fairly self-contained and might be of general interest;
shall I brace Vince about putting them up on the website?

This would be good to see. Would it fit in the documentation hierarchy
somewhere?

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.B.N. 75 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 0500 83 82 82 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
PGP key available upon request, | /
and from pgp5.ai.mit.edu:11371 |/

On Sat, 14 Oct 2000, Philip Warner wrote:

At 00:10 14/10/00 -0400, Tom Lane wrote:

I think the slides for that
talk are fairly self-contained and might be of general interest;
shall I brace Vince about putting them up on the website?

This would be good to see. Would it fit in the documentation hierarchy
somewhere?

Ok, what did I miss?

Vince.
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