Of course, back up the data first.

It might be faster to simply migrate the data you want to keep and drop
the entire old table (and recreate it afterwards with only the migrated
data if desired).

On Sat, 2006-03-25 at 16:17 +0100, Aage Johansen wrote:
> k_smith_000 wrote:
> > I've a table with a large number (200 000 000) of records. I'd like
> to
> > remove ~90% of records from table that fulfill given criteria.
> > I know such operation can take a long time and I should backup and
> > restore database after operation has finished.
> > My question is, what is the most efficient way of performing such
> > operations. Should I put one delete statement into one transaction,
> or
> > divide it into few statements with seperate transactions?
> >
>
>
> One possibility is to create a new (empty) database with the same
> metadata, and then copy records from the old database into the new
> one.
> You could start with a metadata-only backup. Restoring this gives
> you the new (empty) database. Watch out for generator values, and
> you might choose to deactive triggers in the new one during transfer.
>
> If you do one big delete (or several small ones) you should probably
> make a backup (without garbage collection), and restore this. And
> hope that no user issues a "select count(*) ..." before the new
> database is ready.
>
> If you make many small deletes you might do a "select count(*) ..."
> on the parts where records are deleted (delete+commit+select), but
> this could mean the that database is inconsistent until you are
> finished.
>
>
> --
> Aage J.
>
>
>
>
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