Simon Riggs <simon@2ndquadrant.com> writes: > I agree that the issue of mixing sorts at various points will make nonsense > of the startup cost/total cost results.
Right.
> I don't see LIMIT costing being broken as a reason to restrict this > optimization. I would ask that we allow improvements to the important use > case of ORDER BY/LIMIT, then spend time on making LIMIT work correctly.
There's not time to reinvent LIMIT costing for v12. I'd be happy to see some work done on that in the future, and when it does get done, I'd be happy to see Append planning extended to allow this case. I just don't think it's wise to ship one without the other.
I was hoping to motivate you to look at this personally, and soon. LIMIT is so broken that any improvements count as bug fixes in my book.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services