wide table, many many partitions, poor query performance

[S Bob]

All;

We have a client that is running PostgreSQL 12, they have a table with 
212 columns and 723 partitions


It seems the planning time is consumed by generating 723 sub plans

I suspect it's due to the fact that they are using hash based 
partitioning, example:


CREATE TABLE rental_transaction_hash_p723 PARTITION OF 
rental_transaction FOR VALUES WITH (MODULUS 723, REMAINDER 723);


Based on a strategy like this, queries will ALWAYS scan all partitions 
unless a hash value is specified as part of the query, correct? I 
suspect this is the issue... looking for confirmation, or feedback if 
i'm off base


Thanks in advance