Allowing DESC for a PRIMARY KEY column

Mitar <mmitar@gmail.com> writes:

And I would like to be able to specify PRIMARY KEY (id, revision DESC)
because the most common query I am making is:
SELECT data FROM values WHERE id=123 ORDER BY revision DESC LIMIT 1

Did you experiment with whether that actually needs a special index?
I get

regression=# create table t(id int, revision int, primary key(id, revision));
CREATE TABLE
regression=# explain select * from t WHERE id=123 ORDER BY revision DESC LIMIT 1;
QUERY PLAN
--------------------------------------------------------------------------------------
Limit (cost=0.15..3.45 rows=1 width=8)
-> Index Only Scan Backward using t_pkey on t (cost=0.15..36.35 rows=11 width=8)
Index Cond: (id = 123)
(3 rows)

My understanding, based on [2], is that the primary key index cannot
help here, unless it is defined with DESC on revision. But this does
not seem to be possible. Would you entertain a patch adding this
feature? It seems pretty straightforward?

You would need a lot stronger case than "I didn't bother checking
whether I really need this".

regards, tom lane

Hi!

On Fri, Mar 29, 2024 at 9:41 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

You would need a lot stronger case than "I didn't bother checking
whether I really need this".

Thanks! I have tested it this way (based on your example):

create table t (id int not null, revision int not null);
create unique index on t (id, revision desc);
explain select * from t where id=123 order by revision desc limit 1;
QUERY PLAN
----------------------------------------------------------------------------------------
Limit (cost=0.15..3.45 rows=1 width=8)
-> Index Only Scan using t_id_revision_idx on t (cost=0.15..36.35
rows=11 width=8)
Index Cond: (id = 123)
(3 rows)

It is very similar, with only the direction difference. Based on [1]https://www.postgresql.org/docs/16/indexes-ordering.html I
was under the impression that "Index Only Scan Backward" is much
slower than "Index Only Scan", but based on your answer it seems I
misunderstood and backwards scanning is comparable with forward
scanning? Especially this section:

"Consider a two-column index on (x, y): this can satisfy ORDER BY x, y
if we scan forward, or ORDER BY x DESC, y DESC if we scan backward.
But it might be that the application frequently needs to use ORDER BY
x ASC, y DESC. There is no way to get that ordering from a plain
index, but it is possible if the index is defined as (x ASC, y DESC)
or (x DESC, y ASC)."

I am curious, what is then an example where the quote from [1]https://www.postgresql.org/docs/16/indexes-ordering.html
applies? Really just if I would be doing ORDER BY id, revision DESC on
the whole table? Because one future query I am working on is where I
select all rows but for only the latest (highest) revision. Curious if
that will have an effect there.

Mitar

[1]: https://www.postgresql.org/docs/16/indexes-ordering.html

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