Re: Hash partitioning, what function is used to compute the hash?

On Mon, 2020-05-11 at 04:33 +0000, Dennis Ryan wrote:
> Regarding hash partitioning, what is the function/algorithm that is used to compute the hash for the partition key?  I need to write a query like
> 
> “SELECT unknown_partition_hash_function(id) AS hash_value, COUNT(id) AS number_of_records
> FROM existing_table
> GROUP BY 1”

To find the function that PostgreSQL uses to hash a data type,
try something like

SELECT amp.amproc,
       amp.amproclefttype::regtype
FROM pg_amproc AS amp
   JOIN pg_opfamily AS opf ON amp.amprocfamily = opf.oid
   JOIN pg_am ON opf.opfmethod = pg_am.oid
WHERE pg_am.amname = 'hash'
  AND amp.amprocnum = 1;

Yours,
Laurenz Albe
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