On Sat, 23 Mar 2019 at 05:40, Tom Lane <t...@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
