Understanding Your PostgreSQL Index Cache Hit Ratio: Optimizing Performance

In PostgreSQL, keeping frequently accessed data readily available is crucial for optimal query performance. Indexes act as shortcuts, enabling the database to locate specific data within tables faster. The PostgreSQL cache plays a vital role in this process, storing frequently used index pages for quicker retrieval. The cache hit ratio is a key metric that tells you how effectively your cache is serving up these indexed pages.

Here we dive into a PostgreSQL code snippet specifically designed to calculate the index cache hit ratio and explores its significance for database administrators and developers.

Sample Code from Command Line

1SELECT
2  SUM(idx_blks_read) AS idx_blks_read,
3  SUM(idx_blks_hit)  AS idx_blks_hit,
4           SUM(idx_blks_hit) /
5  GREATEST(SUM(idx_blks_hit) + SUM(idx_blks_read), 1)::float
6              AS ratio
7FROM
8  pg_statio_user_indexes;

Notes PostgreSQL Indexes Cache-hit Ratio. Should be closer to 1, eg. 0.99. Tested on PostgreSQL 8.4, 9.x, 10.x, 11.x, 12.x, 13.0.

Code Breakdown:

• pg_statio_user_indexes: This built-in PostgreSQL view provides statistics on how indexes are being used.
• idx_blks_read: This column represents the total number of index blocks that were read from disk.
• idx_blks_hit: This column represents the total number of index blocks that were found in the cache and retrieved without disk access.
• SUM(): This function aggregates the values in the specified columns.
• GREATEST(): This function ensures we don't have a division by zero error, returning the larger value between the sum of idx_blks_hit and idx_blks_read, and 1.
• ::float: This casts the result to a floating-point number for a more precise ratio.
• AS: This renames the calculated ratio for better readability.

Key Points:

• The resulting ratio represents the index cache hit ratio, ideally aiming closer to 1 (or 0.99).
• A higher ratio indicates that most index accesses are served from the cache, resulting in faster queries.
• A lower ratio suggests that the cache isn't efficiently storing frequently used index pages, potentially leading to slower performance.

Insights and Explanations:

• A good cache hit ratio signifies that PostgreSQL is effectively utilizing its cache to minimize disk access for frequently used indexes.
• If the ratio is low, it might be due to:
  • Insufficient shared buffers allocated in your PostgreSQL configuration, limiting the cache size.
  • Inappropriately designed or unused indexes that clutter the cache with irrelevant data.
• By analyzing the cache hit ratio, you can gain valuable insights into your database's performance and identify areas for improvement.

Conclusion:

By monitoring and understanding the PostgreSQL index cache hit ratio, you can optimize your database configuration and ensure efficient query performance for your applications.

References:

Understanding Postgres Performance: https://www.craigkerstiens.com/2013/01/10/more-on-postgres-performance/

Postgresql - How to improve low cache hit ratio: https://stackoverflow.com/questions/16806064/low-postgres-cache-hit-ratio-data-size-or-something-else

High level performance analysis | Tutorials: https://stackoverflow.com/questions/71052660/postgresql-how-to-improve-low-cache-hit-ratio

Source:

https://github.com/HariSekhon/SQL-scripts/blob/master/postgres_indexes_cache_hit_ratio.sql