Note: the original email from David went to my spam folder, and it also didn't show up on the archives (I assume caught by a spam filter there also?)
Thanks for taking this on!
As far as you can tell, is the default correct at 100?
I'm not sure what a good way of measuring it would be (that is, what all the possible cases are). I did try very simple SELECT * FROM t WHERE i IN (...) style queries with increasing size and was able to see increased planning time, but nothing staggering (going from 1000 to 2000 increased from ~1.5ms to 2.5ms planning time, in an admittedly very unscientific test.)
What are some issues that might arise if it's set too low/too high?
Too low would result in queries being planned unsatisfactorily (i.e., scalar array ops switching from partial index scans to seq scans), and setting it too high could significantly increase planning time.
