sys.worker_id()

Return the logical ID of the worker node that runs this function.
sys.worker_id()

To return a meaningful result, always use this function in a query with a table reference, such as a reference to sys.const.

The following repeated queries return a different worker ID, depending on which worker runs the query in each case.
premdb=# select sys.worker_id() from sys.const;
 worker_id 
-----------
         9
(1 row)

premdb=# select sys.worker_id() from sys.const;
 worker_id 
-----------
         3
(1 row)

premdb=# select sys.worker_id() from sys.const;
 worker_id 
-----------
         8
(1 row)
The query counts the number of rows by the worker that processed the row (not the worker where the rows are stored).
premdb=# select sys.worker_id(), count(*) from newmatchstats group by 1 order by 1;
 worker_id | count  
-----------+--------
         0 |  68400
         1 |  34200
         4 |  41580
         5 |  75780
         6 |  34200
         7 |  34200
         8 | 102600
         9 | 102600
        10 | 144180
        12 |  68400
        13 |  68400
(11 rows)