What can be seen in pg_stat_statements? What is stored there regarding queries?

pg_stat_statements is an extension for PostgreSQL that collects statistics on executed SQL queries.

This system table stores aggregated data for each unique query (normalized, with parameters replaced), including:

• Number of calls (calls)
• Total execution time (total_time)
• Average execution time (mean_time)
• Minimum and maximum execution times
• Number of locks
• Number of returned rows
• Number of errors
• Execution plan (in some versions)

This allows analyzing query performance, identifying "heavy" or frequently called queries, and optimizing them.

Example query to view the slowest queries:

SELECT query, calls, total_time, mean_time
FROM pg_stat_statements
ORDER BY total_time DESC
LIMIT 10;

Thus, pg_stat_statements helps DBAs and developers monitor and optimize database load.