What is the automatic vacuum of the table pg_catalog.pg_statistic?

The automatic vacuum process for the pg_catalog.pg_statistic table in PostgreSQL is a crucial maintenance operation. This table contains statistical data about the contents of database tables, which is essential for the query planner to create efficient execution plans.

Here's a breakdown of why and how automatic vacuuming of pg_catalog.pg_statistic works:

1. Purpose of pg_catalog.pg_statistic:

- This system catalog table stores statistical data such as the distribution of data in columns, most common values, and null value frequencies. - The PostgreSQL query planner uses this information to make informed decisions about how to execute a query. - If the statistics are outdated or incorrect, the query planner can generate inefficient plans, leading to poor performance.

2. Need for Automatic Vacuum:

- The pg_catalog.pg_statistic table needs to be updated when the underlying tables' data changes significantly. - PostgreSQL's autovacuum daemon automatically analyzes and updates this table when needed. - Without this process, the statistics would become stale over time, leading to less effective query plans.

3. Automatic Vacuum Process:

- The autovacuum daemon monitors the number of insertions, updates, and deletions within tables. - When the changes on a table exceed a certain threshold, it triggers an ANALYZE operation on that table. - During the ANALYZE, the system computes and updates the statistics, which then reflects on pg_catalog.pg_statistic.

4. How it Affects Performance:

- Keeping the statistics in pg_catalog.pg_statistic up-to-date allows the query planner to select the optimal query plan, which results in faster query execution. - Without proper vacuuming and analysis, queries may execute much slower because the query planner is making decisions based on outdated statistics.

5. Configuration:

- You can configure the behavior of the autovacuum process, including the thresholds that trigger analyze and vacuum operations, using configuration parameters. - Important configuration settings include `autovacuum_analyze_threshold`, `autovacuum_analyze_scale_factor`, and `autovacuum_vacuum_threshold`.

6. Manual Analysis:

- You can also manually force an ANALYZE operation with the SQL command `ANALYZE table_name`, in cases where you are aware of the data changes and want to update the statistics immediately.

In summary, the automatic vacuum of the pg_catalog.pg_statistic table is an automatic, behind-the-scenes, process that maintains the performance of your PostgreSQL database by ensuring the query planner uses accurate statistical data when generating query plans. It's one of the reasons why PostgreSQL can perform optimally without constant manual intervention.

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