Re: Tuning planner cost estimates

Jim,

> I've been doing some work to try and identify the actual costs
> associated with an index scan with some limited sucess. What's been run
> so far can be seen at http://stats.distributed.net/~decibel. But there's
> a couple problems. First, I can't use the box exclusively for this
> testing, which results in some result inconsistencies.

I can get you access to boxes.  Chat on IRC?

> Second, I've been
> using a dataset that I can't make public, which means no one else can
> run these tests on different hardware.

Then use one of the DBT databases.

> In the
> case of testing index scans, we need to be able to vary correlation,
> which so far I've been doing by ordering by different columns. I suspect
> it will also be important to test with different tuple sizes. There's
> also the question of whether or not the cache should be flushed for each
> run or not.
>
> Does this sound like a good way to determine actual costs for index
> scans (and hopefully other access methods in the future)? If so, what
> would be a good way to implement this?

Well, the problem is that what we need to index scans is a formula, rather
than a graph.   The usefulness of benchmarking index scan cost is so that we
can test our formula for accuracy and precision.  However, such a formula
*does* need to take into account concurrent activity, updates, etc ... that
is, it needs to approximately estimate the relative cost on a live database,
not a test one.

This is also going to be a moving target because Tom's in-memory-bitmapping
changes relative cost equations.

I think a first step would be, in fact, to develop a tool that allows us to
put EXPLAIN ANALYZE results in a database table.  Without that, there is no
possibility of statistical-scale analysis.

--
Josh Berkus
Aglio Database Solutions
San Francisco