Re: Forcing using index instead of sequential scan?

> The real issue here is caching across successive queries, an effect that
> Postgres doesn't deal with very well at the moment. If you run these
> queries from a standing start (freshly booted machine) you'll likely
> find that the indexscan plan is indeed slower than the seqscan/hash
> plan, just like the planner thinks.

Here's a little trick I learned to speed up this test.

find / -type f -exec grep foobar {} \;

This causes massive file-system activity and flushes all files that the kernel has cached. If you run this between each Postgres test (let it run for a couple minutes), it gives you an apples-to-apples comparison between successive benchmarks, and eliminates the effects of caching.

If you run this as a regular user (NOT super-user or 'postgres'), you won't have permission to access your Postgres files, so you're guaranteed they'll be flushed from the cache.

Craig