Re: optimal insert

On 10/8/06, Dirk Jagdmann <jagdmann(at)gmail(dot)com> wrote:
>
> Hello experts,
>
> I have a database that contains three tables:
>
> create table a (
> id serial primary key,
> ... -- some more fields not relevant for my question
> );
> create table b (
> id serial primary key,
> ... -- some more fields not relevant for my question
> );
> create table a_b (
> a int not null references a,
> b int not null references b
> );
>
> Tables a and b have already been filled with lots of rows. Now my
> application needs to insert the relationship of a to b into table a_b
> and is currently doing it with inserts like the following:
>
> insert into a_b(a,b) values(1,100);
> insert into a_b(a,b) values(1,200);
> insert into a_b(a,b) values(1,54);
> insert into a_b(a,b) values(1,4577);
>
> So for a batch of inserts the value of a stays the same, while for by
> arbitrary values are inserted. Now I have wondered if PostreSQL offers
> a smarter way to insert those values? A solution can contains usage of
> some plpgsql code.

It depends on your logic. If you can write a query that selects out the a
and b records, there is a smart way:

insert into a_b(a, b)
select 1, b.id from b where b.id in (100, 200, 54, 4577);

This is not really smart because you already have the id values - the select
may diminish your performance rather than help. But if the select is
"smarter" then you don't even need to know what the b.id values are:

insert into a_b(a, b)
select 1, b.id from b where b.somecolumn = 'somevalue';

Hope this helps.

> ==================================================================
> Aaron Bono
> Aranya Software Technologies, Inc.
> http://www.aranya.com
> http://codeelixir.com
> ==================================================================