Re: [PERFORM] EXPLAIN ANALYZE on 8.2

Martijn van Oosterhout <kleptog@svana.org> writes:
> On Fri, Dec 15, 2006 at 12:20:46PM +0000, Simon Riggs wrote:
>> Maybe sampling every 10 rows will bring things down to an acceptable
>> level (after the first N). You tried less than 10 didn't you?

> Yeah, it reduced the number of calls as the count got larger. It broke
> somewhere, though I don't quite remember why.

The fundamental problem with it was the assumption that different
executions of a plan node will have the same timing.  That's not true,
in fact not even approximately true.  IIRC the patch did realize
that first-time-through is not a predictor for the rest, but some of
our plan nodes have enormous variance even after the first time.
I think the worst case is batched hash joins.

            regards, tom lane