Re: [PERFORMANCE] slow small delete on large table

On Monday February 23 2004 10:23, Tom Lane wrote:
> "Ed L." <pgsql@bluepolka.net> writes:
> > If I could say it the way I think for a simple example, it'd be
> > like this:
> >
> >     delete from mytable
> >     where posteddatetime < now() - '90 days'
> >     limit 100;
> >
> > Of course, that's not legal 7.3.4 syntax.
>
> Assuming you have a primary key on the table, consider this:
>
> CREATE TEMP TABLE doomed AS
>   SELECT key FROM mytable WHERE posteddatetime < now() - '90 days'
>   LIMIT 100;
>
> DELETE FROM mytable WHERE key = doomed.key;
>
> DROP TABLE doomed;
>
> Depending on the size of mytable, you might need an "ANALYZE doomed"
> in there, but I'm suspecting not.  A quick experiment suggests that
> you'll get a plan with an inner indexscan on mytable.key, which is
> exactly what you need.

I didn't mention I'd written a trigger to do delete N rows on each new
insert (with a delay governor preventing deletion avalanches).  The
approach looks a little heavy to be done from within a trigger with the
response time I need, but I'll try it.  Cantchajust toss in that "limit N"
functionality to delete clauses?  How hard could that be?  ;)

> See also Chris Browne's excellent suggestions nearby, if you are willing
> to make larger readjustments in your thinking...

I did a search for articles by Chris Browne, didn't see one that appeared
relevant.  What is the thread subject to which you refer?