Re: PATCH: Using BRIN indexes for sorted output

On Fri, 7 Jul 2023 at 20:26, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
> Hi,
>
> I finally had time to look at this patch again. There's a bit of bitrot,
> so here's a rebased version (no other changes).

Thanks!

> On 2/27/23 16:40, Matthias van de Meent wrote:
> > Some initial comments on 0004:
> >
> >> +/*
> >> + * brin_minmax_ranges
> >> + * Load the BRIN ranges and sort them.
> >> + */
> >> +Datum
> >> +brin_minmax_ranges(PG_FUNCTION_ARGS)
> >> +{
> >
> > Like in 0001, this seems to focus on only single columns. Can't we put
> > the scan and sort infrastructure in brin, and put the weight- and
> > compare-operators in the opclasses? I.e.
> > brin_minmax_minorder(PG_FUNCTION_ARGS=brintuple) -> range.min and
> > brin_minmax_maxorder(PG_FUNCTION_ARGS=brintuple) -> range.max, and a
> > brin_minmax_compare(order, order) -> int? I'm thinking of something
> > similar to GIST's distance operators, which would make implementing
> > ordering by e.g. `pointcolumn <-> (1, 2)::point` implementable in the
> > brin infrastructure.
> >
>
> However, it's not quite clear to me is what you mean by the weight- and
> compare-operators? That is, what are
>
> - brin_minmax_minorder(PG_FUNCTION_ARGS=brintuple) -> range.min
> - brin_minmax_maxorder(PG_FUNCTION_ARGS=brintuple) -> range.max
> - brin_minmax_compare(order, order) -> int
>
> supposed to do? Or what does "PG_FUNCTION_ARGS=brintuple" mean?

_minorder/_maxorder is for extracting the minimum/maximum relative
order of each range, used for ASC/DESC sorting of operator results
(e.g. to support ORDER BY <->(box_column, '(1,2)'::point) DESC).
PG_FUNCTION_ARGS is mentioned because of PG calling conventions;
though I did forget to describe the second operator argument for the
distance function. We might also want to use only one such "order
extraction function" with DESC/ASC indicated by an argument.

> In principle we just need a procedure that tells us min/max for a given
> page range - I guess that's what the minorder/maxorder functions do? But
> why would we need the compare one? We're comparing by the known data
> type, so we can just delegate the comparison to that, no?

Is there a comparison function for any custom orderable type that we
can just use? GIST distance ordering uses floats, and I don't quite
like that from a user perspective, as it makes ordering operations
imprecise. I'd rather allow (but discourage) any type with its own
compare function.

> Also, the existence of these opclass procedures should be enough to
> identify the opclasses which can support this.

Agreed.

> >> +/*
> >> + * XXX Does it make sense (is it possible) to have a sort by more than one
> >> + * column, using a BRIN index?
> >> + */
> >
> > Yes, even if only one prefix column is included in the BRIN index
> > (e.g. `company` in `ORDER BY company, date`, the tuplesort with table
> > tuples can add additional sorting without first reading the whole
> > table, potentially (likely) reducing the total resource usage of the
> > query. That utilizes the same idea as incremental sorts, but with the
> > caveat that the input sort order is approximately likely but not at
> > all guaranteed. So, even if the range sort is on a single index
> > column, we can still do the table's tuplesort on all ORDER BY
> > attributes, as long as a prefix of ORDER BY columns are included in
> > the BRIN index.
> >
>
> That's now quite what I meant, though. When I mentioned sorting by more
> than one column, I meant using a multi-column BRIN index on those
> columns. Something like this:
>
> CREATE TABLE t (a int, b int);
> INSERT INTO t ...
> CREATE INDEX ON t USING brin (a,b);
>
> SELECT * FROM t ORDER BY a, b;
>
> Now, what I think you described is using BRIN to sort by "a", and then
> do incremental sort for "b". What I had in mind is whether it's possible
> to use BRIN to sort by "b" too.
>
> I was suspecting it might be made to work, but now that I think about it
> again it probably can't - BRIN pretty much sorts the columns separately,
> it's not like having an ordering by (a,b) - first by "a", then "b".

I imagine it would indeed be limited to an extremely small subset of
cases, and probably not worth the effort to implement in an initial
version.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)