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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