> 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
PostgreSQL Training, Services and Support
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