Q:

What does below parallel processing execution plan mean?

I have a table tbl_a with 200 k records. I wrote a query to bring sum of id (int) column. Now execution plans shows 2 workers launched and below that parallel sequential scan on tbl_a is shown 3 times; with processing 66667 records each time(200k/3=66667). Now my question is since there were two workers so should there not be only two times table seq scan and each time with record 100k (200k/3). I am attaching the image which lists the table detail, query and query execution plan for better understanding.

Can someone tell me why with 2 workers there are 3 loops , each processing 6667 records out of total 200k records?

A:

The two worker processes are additional to the session backend process, which is the “parallel leader”, so there are three processes working on the scan.

