PostgreSQL 12.4 query planner ignores sub-partition constraint, resulting in table scan

Question

I have a table

T (A int, B int, C long, D varchar)

partitioned by each A and sub-partitioned by each B (i.e. list partitions with a single value each). A has cardinality of <10 and B has cardinality of <100. T has about 6 billion rows.

When I run the query

select distinct B from T where A = 1;

it prunes the top-level partitions (those where A != 1) but performs a table scan on all sub-partitions to find distinct values of B. I thought it would know, based on the partition design, that it would only have to check the partition constraint to determine the possible values of B given A, but alas, that is not the case.

There are no indexes on A or B, but there is a primary key on (C,D) at each partition, which seems immaterial, but figured I should mention it. I also have a BRIN index on C. Any idea why the Postgres query planner is not consulting the sub-partition constraints to avoid the table scan?

Answer 1

The reason is that nobody implemented such an optimization in the query planner. I cannot say that that surprises me, since it is a rather unusual query. Every such optimization built into the optimizer would mean that each query on a partitioned table that has a DISTINCT would need some extra query planning time, while only few queries would profit. Apart from the expense of writing and maintaining the code, that would be a net loss for most users.

Maybe you could use a metadata query:

CREATE TABLE list (id bigint NOT NULL, p integer NOT NULL) PARTITION BY LIST (p);
CREATE TABLE list_42 PARTITION OF list FOR VALUES IN (42);
CREATE TABLE list_101 PARTITION OF list FOR VALUES IN (101);

SELECT regexp_replace(
          pg_get_expr(
             p.relpartbound,
             p.oid
          ),
          '^FOR VALUES IN \((.*)\)$',
          '\1'
       )::integer
FROM pg_class AS p
   JOIN pg_inherits AS i ON p.oid = i.inhrelid
WHERE i.inhparent = 'list'::regclass;

 regexp_replace 
----------------
             42
            101
(2 rows)