Slow queries after vacuum analyze

Hello,

I have a customer that experience a strange behaviour related to statictics.

Threre is a vacuum analyze planned during the night.

The morning, 1 day out of 2, there are some extremely slow queries.

Those queries lasts more than 5 minutes (never waited more and cancelled them) whereas when everything is OK they last less than 300ms.

In order to resolve this i have to execute a least one analyze, sometimes more.

My Configuration:

Windows

PostgreSQL 8.4.8

default_statistics_target = 100

In addition to an increase for shared_buffers, work_mem, ..., i changed the default_statistics_target to 500 with no effect.

It was even worse as i never managed to get rid of the slow queries after running many analyze.

So i fell back to default_statistics_target=100 in order to get rid of those slow queries.

I have no idea of what i can do to solve this issue.

Any help would be greatly appreciated.