Problem with SQL query (eats swap)

Hi, I have two tables into which I dump mail statistics. The two tables are:

create table attachments (
id text,
attachment text
);
create table mail (
id text,
size int,
whofrom text,
subject text,
date datetime,
inout char
);

The table mail holds information about each mail message. The table attachments holds the name of any attachments. Linking the attachment(s) and the message is the id.

I want to delete all records relating to mail that is over two months old. I tried:

select count(*) from attachments a where a.id in (select m.id from mail m where m.date < now()-62);

but ran out of swap. The mail table is 19Mb, the attachment one 1Mb. I was up to 380Mb of swap used on a machine with 128Mb RAM, and over 15 minutes run time. At that point I killed the query.

I next tried is as follows:

drop table temp;
select id into temp from mail where date < now()-62;
delete from attachments where id in (select id from temp);
delete from mail where date < now()-62;

This worked fine and deleted the records as intended in a few minutes. Can anyone see why the initial query ate so much swap?

Cheers
Kevin
kevin(at)sartorelli(dot)gen(dot)nz