Re: BRIN indexes and ORDER BY

Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> Darren Lafreniere wrote:
>> We found a pgsql-hackers thread from about a year ago about optimizing
>> ORDER BY for BRIN indexes. Tom Lane suggested that he was working on it:
>> https://www.postgresql.org/message-id/11881.1443393360%40sss.pgh.pa.us

> Tom said he was working on some infrastructure planner changes
> ("upper-planner path-ification"), not that he was working on improving
> usage of BRIN indexes. As far as I know, nobody has worked on that.

Alvaro's reading is correct; I wasn't planning to work on any such thing,
and still am not.

Looking again at the original thread:

> Gavin Wahl wrote:
>> It seems trivial to accelerate a MAX or MIN query with a BRIN index. You
>> just find the page range with the largest/smallest value, and then only
>> scan that one. Would that be hard to implement? I'm interested in working
>> on it if someone can give me some pointers.

I think this proposal is fairly broken anyway. The page range with the
largest max-value may once have contained the largest live row, but
there's no guarantee that it still does. It might even be completely
empty. You could imagine an algorithm like this:

1. Find page-range with largest max. Scan it to identify live row with
largest value. If *no* live values, find page-range with next largest
max, repeat until no page ranges remain (whereupon return NULL).

2. For each remaining page-range whose indexed max exceeds the value
currently in hand, scan that page-range to see if any value exceeds
the one in hand, replacing the value if so.

This'd probably allow you to omit scanning some of the page-ranges
in the table, but in a lot of cases you'd end up scanning many of them;
and you'd need a lot of working state to remember which ranges you'd
already looked at. It'd certainly always be a lot more expensive than
answering the same question with a btree index, because in no case do
you get to avoid scanning the entire contents of the index.

regards, tom lane