From: Simon Riggs <simon@2ndquadrant.com>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Robert Treat <xzilla@users.sourceforge.net>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Decibel! <decibel@decibel.org>
Cc: Andrew Dunstan <andrew@dunslane.net>
Cc: pgsql-hackers@postgresql.org
Subject: Re: pg_dump restore time and Foreign Keys
Date: Tue, 10 Jun 2008 01:14:25 +0100
Message-ID: <1213056865.12046.167.camel@ebony.site> (raw)
In-Reply-To: <20080609180728.GA10034@alvh.no-ip.org>
References: <1212647003.19964.19.camel@ebony.site>
<1213026485.12046.123.camel@ebony.site>
<20406.1213027167@sss.pgh.pa.us>
<200806091237.49153.xzilla@users.sourceforge.net>
<1213034097.12046.150.camel@ebony.site>
<20080609180728.GA10034@alvh.no-ip.org>
On Mon, 2008-06-09 at 14:07 -0400, Alvaro Herrera wrote:
> Simon Riggs wrote:
>
> > If we break down the action into two parts.
> >
> > ALTER TABLE ... ADD CONSTRAINT foo FOREIGN KEY ... NOVALIDATE;
> > which holds exclusive lock, but only momentarily
> > After this runs any new data is validated at moment of data change, but
> > the older data has yet to be validated.
> >
> > ALTER TABLE ... VALIDATE CONSTRAINT foo
> > which runs lengthy check, though only grabs lock as last part of action
>
> The problem I see with this approach in general (two-phase FK creation)
> is that you have to keep the same transaction for the first and second
> command, but you really want concurrent backends to see the tuple for
> the not-yet-validated constraint row.
Well, they *must* be in separate transactions if we are to avoid holding
an AccessExclusiveLock while we perform the check. Plus the whole idea
is to perform the second part at some other non-critical time, though we
all agree that never performing the check at all is foolhardy.
Maybe we say that you can defer the check, but after a while autovacuum
runs it for you if you haven't done so. It would certainly be useful to
run the VALIDATE part as a background task with vacuum wait enabled.
> Another benefit that could arise from this is that the hypothetical
> VALIDATE CONSTRAINT step could validate more than one constraint at a
> time, possibly processing all the constraints with a single table scan.
Good thought, though not as useful for FK checks.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
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To: pgsql-hackers@postgresql.org
Cc: simon@2ndquadrant.com, alvherre@commandprompt.com, xzilla@users.sourceforge.net, tgl@sss.pgh.pa.us, decibel@decibel.org, andrew@dunslane.net
Subject: Re: pg_dump restore time and Foreign Keys
In-Reply-To: <1213056865.12046.167.camel@ebony.site>
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