I stumbled across this question on SO:
https://stackoverflow.com/questions/56517852
Disregarding the part about Postgres 9.3, the example for Postgres 11 looks a
bit confusing.
There is a script to setup test data in that question:
start of script
create table foo (
foo_id integer not null,
foo_name varchar(10),
constraint foo_pkey primary key (foo_id)
);
insert into foo
(foo_id, foo_name)
values
(1, 'eeny'),
(2, 'meeny'),
(3, 'miny'),
(4, 'moe'),
(5, 'tiger'),
(6, 'toe');
create table foo_bar_baz (
foo_id integer not null,
bar_id integer not null,
bazinteger not null,
constraint foo_bar_baz_pkey primary key (foo_id, bar_id, baz),
constraint foo_bar_baz_fkey1 foreign key (foo_id)
references foo (foo_id)
) partition by range (foo_id)
;
create table if not exists foo_bar_baz_0 partition of foo_bar_baz for
values from (0) to (1);
create table if not exists foo_bar_baz_1 partition of foo_bar_baz for
values from (1) to (2);
create table if not exists foo_bar_baz_2 partition of foo_bar_baz for
values from (2) to (3);
create table if not exists foo_bar_baz_3 partition of foo_bar_baz for
values from (3) to (4);
create table if not exists foo_bar_baz_4 partition of foo_bar_baz for
values from (4) to (5);
create table if not exists foo_bar_baz_5 partition of foo_bar_baz for
values from (5) to (6);
with foos_and_bars as (
select ((random() * 4) + 1)::int as foo_id, bar_id::int
from generate_series(0, 1499) as t(bar_id)
), bazzes as (
select baz::int
from generate_series(1, 1500) as t(baz)
)
insert into foo_bar_baz (foo_id, bar_id, baz)
select foo_id, bar_id, baz
from bazzes as bz
join foos_and_bars as fab on mod(bz.baz, fab.foo_id) = 0;
end of script
I see the some strange behaviour similar to to what is reported in the comments
to that question:
When I run the test query immediately after populating the tables with the
sample data:
explain analyze
select count(*)
from foo_bar_baz as fbb
join foo on fbb.foo_id = foo.foo_id
where foo.foo_name = 'eeny'
I do see an "Index Only Scan (never executed)" in the plan for the
irrelevant partitions:
https://explain.depesz.com/s/AqlE
However once I run "analyze foo_bar_baz" (or "vacuum analyze"), Postgres
chooses to do a "Parallel Seq Scan" for each partition:
https://explain.depesz.com/s/WwxE
Why does updating the statistics mess up (runtime) partition pruning?
I played around with random_page_cost and that didn't change anything.
I tried to create extended statistics on "foo(id, name)" so that the planner
would no, that there is only one name per id. No change.
I saw the above behaviour when running this on Windows 10 (my Laptop) or CentOS
7 (a test environment on a VM)
On the CentOS server default_statistics_target is set to 100, on my laptop it
is set to 1000
In both cases the Postgres version was 11.4
Any ideas?
Thomas
