When I measured the execution time of a certain query with parallel query enabled and disabled, I found that the execution time was slower when parallel query was enabled.
To improve the performance of the parallel query, I considered adjusting the execution plan and attempted to switch from GroupAggregate to HashAggregate. However, I noticed that there was no GUC parameter to disable GroupAggregate.
Therefore, I propose adding a new GUC parameter: enable_groupagg.
Below are the results of a performance test where I disabled GroupAggregate using enable_groupagg. In this case, the planner chose HashAggregate instead, which improved performance by about 35 times.
# Query Execution Results (Average of 3 measurements) - With parallel query: 39546 seconds - With parallel query and enable_groupagg turned off: 1115 seconds
# Query and Data Used (attached to this email) - Query: test_query.sql - Data: create_table.sql
# The steps to run the test are as follows. For example, on psql:
1. Create tables: \i create_table.sql
2. Execute a query: \i test_query.sql
3. Execute a query using the new GUC parameter: set enable_groupagg to off; \i test_query.sql
As a benefit to users, while there has previously been a GUC parameter to control HashAggregate, there was no corresponding way to control GroupAggregate. This patch addresses that, giving users more flexibility in tuning execution plans.
I've attached a WIP patch that adds this GUC parameter. I would appreciate any feedback, especially regarding how many test cases I should create.
To create new test cases for enable_groupagg, I looked into existing test cases that use enable_hashagg and found that it is used in many places (62 places). Should I add a test case for enable_groupagg in the same place as enable_hashagg? I think that adding a new feature requires a minimum number of test cases, so I would appreciate your advice.
Additionally, based on the execution plan, I suspect the slowdown in the parallel query might be caused by misestimates related to Sort or Gather Merge. While resolving those misestimates would ideally improve the root issue, I'd like to keep the focus of this thread on adding the GUC parameter. Then, I plan to report or address the estimation problem in a separate thread.
