Hello everyone,
Sorry to bother you but I have a query that is driving me crazy.
I need to have the last valid record at a temporal level according to a
specific parameter.
First some data:
Linux Rocky 8.10 environment, minimal installation (on VM KVM with
Fedora 40).
Postgresql 16.3, installed by official Postgresql guide.
effective_cache_size = '1000 MB';
shared_buffers = '500 MB';
work_mem = '16MB';
The changes are deliberately minimal to be able to all to simulate the
problem.
Table script:
CREATE TABLE test_table
(
pk_id int NOT NULL,
integer_field_1 int ,
integer_field_2 int,
datetime_field_1 timestamp,
primary key (pk_id)
)
-- insert 4M records
insert into test_table(pk_id) select generate_series(1,4000000,1);
-- now set some random data, distribuited between specific ranges (as in
my production table)
update test_table set
datetime_field_1 = timestamp '2000-01-01 00:00:00' + random() *
(timestamp '2024-05-31 23:59:59' - timestamp '2000-01-01 00:00:00'),
integer_field_1 = floor(random() * (6-1+1) + 1)::int,
integer_field_2 = floor(random() * (200000-1+1) + 1)::int;
-- indexes
CREATE INDEX idx_test_table_integer_field_1 ON test_table(integer_field_1);
CREATE INDEX xtest_table_datetime_field_1 ON test_table(datetime_field_1
desc);
CREATE INDEX idx_test_table_integer_field_2 ON test_table(integer_field_2);
--vacuum
vacuum full test_table;
Now the query:
explain (verbose, buffers, analyze)
with last_table_ids as materialized(
select xx from (
select LAST_VALUE(pk_id) over (partition by integer_field_2 order by
datetime_field_1 RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED
FOLLOWING) xx
from test_table
where integer_field_1 = 1
and datetime_field_1 <= CURRENT_TIMESTAMP
) ww group by ww.xx
),
last_row_per_ids as (
select tt.* from last_table_ids lt
inner join test_table tt on (tt.pk_id = lt.xx)
)
select * /* or count(*) */ from last_row_per_ids;
This query, on my PC, takes 46 seconds!!!
I was expecting about 2-3 seconds (according with my other queries in
this table) but it seems that the xtest_table_datetime_field_1 index is
not being used.
Do you think there is a way to optimize the query?
Thanks so much for the support,
Agharta
