Re: Optimising a two column OR check

>>>>> "MichaelDBA" == MichaelDBA  <MichaelDBA@sqlexec.com> writes:

 MichaelDBA> Yep, you're right, Andrew, adding a couple rows made it do
 MichaelDBA> the index only scan.  I reckon I got misled by turning off
 MichaelDBA> sequential scans, thinking that actual rows were not
 MichaelDBA> important anymore.  Overly simplistic reasonings can get
 MichaelDBA> one into trouble, lol.

We do some odd stuff with the statistics estimates for completely empty
tables because (a) it's not common in practice for a table to be always
empty (i.e. the emptiness is usually transient) and (b) if you take the
emptiness of a table at face value, you end up generating insanely bad
plans for certain FK check queries that may not get replanned quickly
enough to mitigate the performance impact.

--
Andrew (irc:RhodiumToad)