Subject: Re: Allow ordered partition scans in more cases

From: Ronan Dunklau <[email-masked]>
Date: Wed, 22 Feb 2023 14:10:48 +0100

Thank you for improving this optimization !

Le mardi 21 février 2023, 04:14:02 CET David Rowley a écrit :
> I still need to look to see if there's some small amount of data that
> can be loaded into the table to help coax the planner into producing
> the ordered scan for this one.  It works fine as-is for ORDER BY a,b
> and ORDER BY a; so I've put tests in for that.

I haven't looked too deeply into it, but it seems reasonable that the whole 
sort would cost cheaper than individual sorts on partitions + incremental 
sorts, except when the the whole sort would spill to disk much more than the 
incremental ones. I find it quite difficult to reason about what that threshold 
should be, but I managed to find a case which could fit in a test:

create table range_parted (a int, b int, c int) partition by range(a, b);
create table range_parted1 partition of range_parted for values from (0,0) to 
(10,10);
create table range_parted2 partition of range_parted for values from (10,10) 
to (20,20);
insert into range_parted(a, b, c) select i, j, k from generate_series(1, 19) 
i, generate_series(1, 19) j, generate_series(1, 5) k;
analyze range_parted;
set random_page_cost = 10;
set work_mem = '64kB';
explain (costs off) select * from range_parted order by a,b,c;

It's quite convoluted, because it needs the following:
 - estimate the individual partition sorts to fit into work_mem (even if that's 
not the case here at runtime)
 - estimate the whole table sort to not fit into work_mem
 - the difference between the two should be big enough to compensate the 
incremental sort penalty (hence raising random_page_cost).

This is completely tangential to the subject at hand, but maybe we have 
improvements to do with the way we estimate what type of sort will be 
performed ? It seems to underestimate the memory amount needed. I'm not sure 
it makes a real difference in real use cases though. 

Regards,

--
Ronan Dunklau