I am experimenting with SQL query performance for SELECT queries on large tables and I observed that changing/increasing the degree of parallel hint doesn't give the expected performance improvement.
But you still have addressed the fact that PostgreSQL does not have planner hints.
Are you using some nonstandard extension, or nonstandard fork?
I have executed the SELECT query with 2,4 & 6 parallel degree however every time only 4 workers launched & there was a slight increase in Execution time as well,
Adding an ignored comment to your SQL would not be expected to do anything. So it is not surprising that it does not do anything about the number of workers launched. It is just a comment. A note to the human who is reading the code.
why there is an increase in execution time with parallel degree 6 as compared to 2 or 4?
Those small changes seem to be perfectly compatible with random noise. You would need to repeat them dozens of times in random order, and then do a statistical test to convince me otherwise.