On Sat, 23 Mar 2019 at 05:40, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> BTW, another thing we could possibly do to answer this objection is to
> give the ordered-Append node an artificially pessimistic startup cost,
> such as the sum or the max of its children's startup costs. That's
> pretty ugly and unprincipled, but maybe it's better than not having the
> ability to generate the plan shape at all?
I admit to having thought of that while trying to get to sleep last
night, but I was too scared to even suggest it. It's pretty much how
MergeAppend would cost it anyway. I agree it's not pretty to lie
about the startup cost, but it does kinda seem silly to fall back on a
more expensive MergeAppend when we know fine well Append is cheaper.
Probably the danger would be that someone pulls it out thinking its a
bug. So we'd need to clearly comment why we're doing it.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
