Massive slowdown when LIMIT applied

Hi,
I am using postgres 7.4.7 under Debian Sarge for RADACCT, this means
a fairly large table of data allowing us to hold approx 12 months of
data. I have used a trigger to break the system in 2 and keep the
live table which takes updates small, and all closed connections are
inserted into a large historical table. The histrorical table is
only ever inserted into and queried.

It is indexed on all mayor attributes for which it may be searched or
ordered.

The problem is this:

If I run the query:

SELECT username,acctstarttime FROM radacct WHERE username='user';

The results come back nice and quick (indexes are are username, and
acctstarttime BTW)

If I do the following:

SELECT username,acctstarttime FROM radacct WHERE username='user'
ORDER BY acctstarttime;

again the results come up slightly slower as would be expected but
still very quickly.. (a few hundred rows in this case)

Now, if I do the following:

SELECT username,acctstarttime FROM radacct WHERE username='user'
ORDER BY acctstarttime LIMIT 50;

I have to wait a very long time, IE instead of 2-3 seconds.. can be
as slow as 10+ minutes.

this seems very odd to me, any ideas?

Thanks in advance.

-----
Graeme Hinchliffe (BSc)
Core Systems Designer
Zen Internet (http://www.zen.co.uk/)

Direct: 0845 058 9074
Main : 0845 058 9000
Fax : 0845 058 9005