Boy, you picked a *really* bad example ;-)
The problem is that Postgres decided to filter on myfunc() *first*, and
then filter on row_num, resulting in a query time that jumped from
seconds to hours. And there's no way for me to tell Postgres not to do
that!
Can you paste explain analyze and your effective_cache_size, etc. settings.
So, "you still have no problem" is exactly wrong, because Postgres
picked the wrong plan. Postgres decided that applying myfunc() to
10,000,000 rows was a better plan than an index scan of 50,000
row_nums. So I'm screwed.
This seems like a case where PostgreSQL's current optimiser should
easily know what to do if your config settings are correct and you've
been running ANALYZE, so I'd like to see your settings and the explain
analyze plan...
Chris
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TIP 6: explain analyze is your friend
