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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