jobapply wrote:
> The 2 queries are almost same, but ORDER BY x||t is FASTER than ORDER BY x..
>
> How can that be possible?
>
> Btw: x and x||t are same ordered
>
> phoeniks=> explain analyze SELECT * FROM test WHERE i<20 ORDER BY x || t;
> QUERY PLAN
I also thought of another possibility. Are there a lot of similar
entries in X? Meaning that the same value is repeated over and over? It
is possible that the sort code has a weakness when sorting equal values.
For instance, if it was doing a Hash aggregation, you would have the
same hash repeated. (It isn't I'm just mentioning a case where it might
affect something).
If it is creating a tree representation, it might cause some sort of
pathological worst-case behavior, where all entries keep adding to the
same side of the tree, rather than being more balanced.
I don't know the internals of postgresql sorting, but just some ideas.
John
=:->
