Re: exceptionally large UPDATE

Ivan Sergio Borgonovo wrote:
> I've to make large UPDATE to a DB.
> The largest UPDATE involve a table that has triggers and a gin
> index on a computed tsvector.
> The table is 1.5M records with about 15 fields of different types.
> I've roughly 2.5-3Gb of ram dedicated to postgres.
>
> UPDATE queries are simple, few of them use join and mainly consist
> of updating records from temporary tables that contains a very
> similar structure to the target.
>
> This updates are rare so I can afford to tune postgresql just for
> this large update and then return to a more balanced configuration.
> I can even afford to be the only user of the DB so responsiveness of
> the application using the DB is not an issue. Duration of the update
> is.
>
> Anything I can tune in postgresql.conf to speed up the UPDATE?
>
> I'm increasing maintenance_work_mem to 180MB just before recreating
> the gin index. Should it be more?
> The update should be monolithic and it is inside a single
> transaction. Since I can afford to be the only user of the DB for a
> while, is there anything I can tune to take advantage of it?
> What else could I change to speed up the update?
>
> The triggers recreate the tsvector. One of the component of the
> tsvector is taken from a join table.
>
> I'll surely drop the gin index and recreate it when everything is
> over.
> I'm not sure if it's a good idea to drop the triggers since I'll
> have to update the tsvectr later and I suspect this will cause twice
> the disk IO.
>
> thanks
>
>

Is there an inherent value in a single transaction for such an update?
By that I mean Do all the updates actually pertain to a single event?
Nice as it is to get a clean slate if the a single record has a problem,
it's also nice when N-1 of N batches succeed in a realistic amount of
time and you're left hunting for the problematic record in one Nth of
the records.

Corollary: if you can afford to be the only user for a while perhaps you
can afford to reload from dump if you need to get back to ground zero.