mjhjmtu

Given a table created as so...

CREATE TABLE measurement (
 city_id int not null,
 logdate date not null,
 peaktemp int,
 unitsales int
) PARTITION BY RANGE (logdate);

How do I identify which column it's been partitioned on? - in this case 'logdate' solely by querying the postgres catalog.

I've looked in the obvious places in the catalog (pg_class, pg_index) but nothing springs out.

(Using version 10.5)

I'm not a PostgreSQL pro, but digging a bit I've founded a solution that perhaps can help you. (Works only for version 10 or above)

First I've slightly modified you table by adding two columns to the partition definition (just to show you the final result):

CREATE TABLE measurement (
 city_id int not null,
 logdate date not null,
 peaktemp int,
 unitsales int
) PARTITION BY RANGE (city_id,logdate);

This is my solution:

select 
 par.relnamespace::regnamespace::text as schema, 
 par.relname as table_name, 
 partnatts as num_columns,
 column_index,
 col.column_name
from 
 (select
 partrelid,
 partnatts,
 case partstrat 
 when 'l' then 'list' 
 when 'r' then 'range' end as partition_strategy,
 unnest(partattrs) column_index
 from
 pg_partitioned_table) pt 
join 
 pg_class par 
on 
 par.oid = pt.partrelid
join
 information_schema.columns col
on 
 col.table_schema = par.relnamespace::regnamespace::text
 and col.table_name = par.relname
 and ordinal_position = pt.column_index;

schema | table_name | num_columns | column_index | column_name
:-------------------------- | :---------- | ----------: | -----------: | :----------
fiddle_ctcqwfrzpcyngmgnqkdy | measurement | 2 | 1 | city_id 
fiddle_ctcqwfrzpcyngmgnqkdy | measurement | 2 | 2 | logdate 

db<>fiddle here

pg_partitioned_table

The catalog pg_partitioned_table stores information about how tables
are partitioned.

Unnesting partattrs you can get column index of each row involved in the partition. Then you can join information_schema.columns just to retrieve the name of every column.

+-----------+------------+---------------------+-------------------------------------------------------------|
| partattrs | int2vector | pg_attribute.attnum | This is an array of partnatts values that indicate |
| | | | which table columns are part of the partition key. |
| | | | For example, a value of 1 3 would mean that the first |
| | | | and the third table columns make up the partition key. |
| | | | A zero in this array indicates that the corresponding |
| | | | partition key column is an expression, rather than a simple |
| | | | column reference. |
+-----------+------------+---------------------+-------------------------------------------------------------|

The answer by McNets already helps, but here is a query that produces slightly prettier output:

select c.relnamespace::regnamespace::text as schema,
 c.relname as table_name, 
 pg_get_partkeydef(c.oid) as partition_key
from pg_class c
where c.relkind = 'p';

Below is output that the above query produces:

 schema │ table_name │ partition_key 
────────┼─────────────┼──────────────────────────
 public │ measurement │ RANGE (city_id, logdate)
(1 row)