Re: optimizing large query with IN (...)

On Wed, 10 Mar 2004, Marcus Andree S. Magalhaes wrote:

>
> Guys,
>
> I got a Java program to tune. It connects to a 7.4.1 postgresql server
> running Linux using JDBC.
>
> The program needs to update a counter on a somewhat large number of
> rows, about 1200 on a ~130k rows table. The query is something like
> the following:
>
> UPDATE table SET table.par = table.par + 1
> WHERE table.key IN ('value1', 'value2', ... , 'value1200' )
>
> This query runs on a  transaction (by issuing  a call to
> setAutoCommit(false)) and a commit() right after the query
> is sent to the backend.
>
> The process of committing and updating the values is painfully slow
> (no surprises here). Any ideas?

The problem, as I understand it, is that 7.4 introduced massive
improvements in handling moderately large in() clauses, as long as they
can fit in sort_mem, and are provided by a subselect.

So, creating a temp table with all the values in it and using in() on the
temp table may be a win:

begin;
create temp table t_ids(id int);
insert into t_ids(id) values (123); <- repeat a few hundred times
select * from maintable where id in (select id from t_ids);
...