The joys of PostgreSQL query optimization!

Firstly, congratulations on identifying that adding a clause was causing the slow plan to be selected. That’s great detective work!

Regarding VACUUM and its impact on query performance, here are some key points to help you understand why it worked in your case:

1. Vacuuming permanently deletes obsolete deleted/updated tuples: When you run VACUUM, PostgreSQL removes any dead tuples from the table that can no longer be used by the planner. This process is called “vacuuming” or “autovacuum”. By removing these dead tuples, the planner has a more accurate view of the available data in the table.
2. Vacuum also updates statistics: After vacuuming, PostgreSQL updates the statistics stored in the table’s index. These statistics are used by the planner to estimate the number of rows that will be affected by a query. When you run VACUUM, these statistics are updated, which can improve the planner’s ability to choose an optimal plan.
3. Vacuum can also affect the cost estimates: The cost estimates used by the planner are based on the statistics stored in the index. By removing dead tuples and updating statistics, VACUUM can help reduce the estimated costs of certain queries.

Now, why did you experience a significant performance improvement after running VACUUM?

In your case, it’s possible that:

• The original plan was using a poor estimate of the row count for one or more tables. By removing dead tuples and updating statistics, VACUUM corrected this estimate, leading to a faster plan.
• The planner was selecting an expensive plan due to the presence of many dead tuples. By removing these tuples, VACUUM helped the planner choose a more efficient plan.

Regarding your question about resource recommendations for learning more about PostgreSQL and vacuuming:

• Official documentation: Start with the official PostgreSQL documentation on Vacuuming and Autovacuum.
• PostgreSQL documentation on query optimization: The PostgreSQL documentation on query optimization is an excellent resource. Check out the section on Planning and Execution.
• Blogs and tutorials:
  • PostgreSQL Tutorial by W3Schools: A comprehensive tutorial covering various aspects of PostgreSQL, including query optimization.
  • “Query Optimization in PostgreSQL” by Tutorials Point: A detailed article on query optimization techniques in PostgreSQL.
  • “PostgreSQL Query Planning” by DataCamp: A video course that covers the basics of PostgreSQL query planning and optimization.

As for upvoting the answer, I’m glad you’re interested! If someone else provides a helpful response in the future, feel free to upvote it.

Last modified on 2024-03-11