auto_explain sample rate

Hi all

It's sometimes desirable to collect auto_explain data with ANALYZE in order
to track down hard-to-reproduce issues, but the performance impacts can be
pretty hefty on the DB.

I'm inclined to add a sample rate to auto_explain so that it can trigger
only on x percent of queries, and also add a sample test hook that can be
used to target statements of interest more narrowly (using a C hook
function).

Sound like a reasonable approach?

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services