Matteo Beccati <php@beccati.com> writes:
> The correct strategy IMHO would
> be applying the order by and limit for each child table (which results
> in an index scan, if possible), appending, then finally sorting a bunch
> of rows, and limiting again.

This would be a win in some cases, and in many others a loss (ie, wasted
sort steps). The hard part is determining when to apply it.

regards, tom lane

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