Thread: Correct way of using complex expressions as partitioning key

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)

test=# explain analyze select * from test where v1 = 100 and v2 = 100;
                                               QUERY PLAN
--------------------------------------------------------------------------------------------------------
 Append  (cost=0.00..52.11 rows=2 width=48) (actual time=0.011..0.017 rows=1 loops=1)
   ->  Seq Scan on test_1  (cost=0.00..26.05 rows=1 width=48) (actual time=0.011..0.011 rows=1 loops=1)
         Filter: ((v1 = 100) AND (v2 = 100))
   ->  Seq Scan on test_2  (cost=0.00..26.05 rows=1 width=48) (actual time=0.004..0.004 rows=0 loops=1)
         Filter: ((v1 = 100) AND (v2 = 100))
         Rows Removed by Filter: 1
 Planning Time: 0.457 ms
 Execution Time: 0.036 ms
(8 rows)

test=# explain analyze select * from test where ((v1 + v2) % 10) = 0 and v1 = 100 and v2 = 100;
                                              QUERY PLAN
-------------------------------------------------------------------------------------------------------
 Seq Scan on test_1 test  (cost=0.00..34.08 rows=1 width=48) (actual time=0.010..0.011 rows=1 loops=1)
   Filter: ((v1 = 100) AND (v2 = 100) AND (((v1 + v2) % '10'::bigint) = 0))
 Planning Time: 0.131 ms
 Execution Time: 0.031 ms
(4 rows)

On Fri, 2023-10-27 at 07:27 +0300, Alexander Rumyantsev wrote:
> 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=# explain analyze select * from test where v1 = 100 and v2 = 100;
> [no partition pruning]
>
> test=# explain analyze select * from test where ((v1 + v2) % 10) = 0 and v1 = 100 and v2 = 100;
> [partition pruning]

Yes, you only get partition pruning if the WHERE clause contains a comparison with
the partitioning key.  There is no way around that.

Yours,
Laurenz Albe

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