Re: Incorrect behavior with CE and ORDER BY

Alvaro Herrera <alvherre@commandprompt.com> writes:
> Is this possible?  It would be very fast.

It's possible but not exactly simple.  As an example, your proposed
plan:

> Limit (50)
>   Sort (key: pse_lastlogin)
>     Result
>        Append
>           Limit (50)
>          SeqScan tbl_profile_search
>       Limit (50)
>          Indexscan tbl_profile_search_interest_1
>       Limit (50)
>          IndexScan on the index mentioned above

is wrong because there's no guarantee that the first 50 elements of a
seqscan will be anything special.  You could imagine dealing with that
by sorting the seqscan results and limiting to 50, or by not
sorting/limiting that data at all but letting the upper sort see all the
seqscan entries.  Offhand I think either of those could win depending on
how many elements the seqscan will yield.  Also, it might be interesting
to consider inventing a "merge" plan node type that takes N
already-sorted inputs and produces a sorted output stream.  Then we'd
need to trade off this approach versus doing the top-level sort, which
could cope with some of its inputs not being pre-sorted.

This seems to have some aspects in common with the recent discussion
about how to optimize min/max aggregates across an appendrel set.
        regards, tom lane