Hi,

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.

I don't actually know how many smaller separate sorts compare to a single big sort, but I guess the difference wouldn't be so big if the LIMIT is low. Add to this that you don't need to append the whole rowsets, but just smaller ones.


Best regards
--
Matteo Beccati
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