Re: Correct way of using complex expressions as partitioning key

[Thomas Kellerer <shammat@xxxxxxx>]

Alexander Rumyantsev schrieb am 27.10.2023 um 06:27:
> Hello!
>
> Is there some correct way to use complex expressions as a key for partitioned table?
> Inserting works as expected, but select runs over all partitions until use complete partition key expression as predicate
>
> test=# create table test (
>         id text,
>         v1 bigint,
>         v2 bigint
> )
> partition by range (((v1 + v2) % 10));
> CREATE TABLE
>
> test=# create table test_1 partition of test for values from (0) to (1);
> CREATE TABLE
>
> test=# create table test_2 partition of test for values from (1) to (2);
> CREATE TABLE
>
> test=# insert into test values (1, 100, 101);
> INSERT 0 1
>
> test=# insert into test values (1, 100, 100);
> INSERT 0 1
>
> test=# select * from test_1;
>  id | v1  | v2
> ----+-----+-----
>  1  | 100 | 100
> (1 row)
>
> test=# select * from test_2;
>  id | v1  | v2
> ----+-----+-----
>  1  | 100 | 101
> (1 row)

It seems you are trying to simulate hash partitioning using that expression.
Why not use hash partitioning directly then?

create table test (
  id text,
  v1 bigint,
  v2 bigint
)
partition by hash (v1, v2);

create table test_1
  partition of test
  for values with (modulus 2, remainder 0);
create table test_2
  partition of test
  for values with (modulus 2, remainder 1);

Then the select will result in:

QUERY PLAN
-----------------------------------------------------------------------------------------------------
Seq Scan on test_1 test  (cost=0.00..17.49 rows=1 width=48) (actual time=0.015..0.016 rows=1 loops=1)
  Filter: ((v1 = 100) AND (v2 = 100))
Planning Time: 0.159 ms
Execution Time: 0.037 ms


Demo: https://dbfiddle.uk/wuopLYeQ