Re: BRIN indexes and ORDER BY

Darren,

* Darren Lafreniere (dlafreniere(at)onezero(dot)com) wrote:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > > 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.
[...]
> A b-tree index would certainly be faster for ordering. But in scenarios
> where you have huge datasets that can't afford the space or update time
> required for b-tree, could such a BRIN-accelerated ordering algorithm at
> least be faster than ordering with no index?

For at least some of the common BRIN use-cases, where the rows are
inserted in-order and never/very-rarely modified or deleted, this
approach would work very well.

Certainly, using this would be much cheaper than a seqscan/top-N sort,
for small values of 'N', relative to the number of rows in the table,
in those cases.

In general, I like the idea of supporting this as BRIN indexes strike me
as very good for very large tables which have highly clumped data in
them and being able to do a top-N query on those can be very useful at
times.

Thanks!

Stephen