Alvaro Herrera <[EMAIL PROTECTED]> writes:
> Is this possible?  It would be very fast.

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

> 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

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
       choose an index scan if your joining column's datatypes do not

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