Index Usage Monitoring
Learn to monitor index effectiveness using system views and identify unused or underperforming indexes.
Index Usage Monitoring is a free Advanced PostgreSQL: Indexing, Partitioning, Replication lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Advanced PostgreSQL: Indexing, Partitioning, Replication learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Monitor Index Usage?
Indexes are powerful tools for speeding up queries, but they aren't free. They consume disk space and add overhead to data modifications (INSERT, UPDATE, DELETE).
Monitoring index usage helps us understand if our indexes are actually working for us or just taking up space.
Introducing `pg_stat_user_indexes`
PostgreSQL provides several system views to monitor database activity. For index usage, the pg_stat_user_indexes view is your best friend.
This view tracks statistics for indexes on user-defined tables, giving you insights into how often each index is being scanned.
Key Index Usage Metrics
When you query pg_stat_user_indexes, look out for these columns:
idx_scan: The number of times the index has been scanned.idx_tup_read: The number of index entries returned by scans.idx_tup_fetch: The number of live table rows fetched through the index.
These tell you how frequently and effectively an index is being used.
Finding Unused Indexes
The easiest win in index optimization is identifying indexes that are never used. An index with idx_scan = 0 is a strong candidate for removal.
Removing unused indexes can reduce disk space, speed up writes, and simplify database maintenance.
Demo: Querying Unused Indexes
Let's run a query to find all indexes that have never been scanned since the last statistics reset. Try running this example:
SELECT
relname AS table_name,
indexrelname AS index_name,
idx_scan
FROM pg_stat_user_indexes
WHERE idx_scan = 0
ORDER BY table_name, index_name;Beyond Unused: Underperforming Indexes
An index might be used (idx_scan > 0) but still be 'underperforming' if it's not chosen by the query planner when it should be, or if it's leading to many sequential scans on the table itself.
To spot these, we need to compare index usage with overall table access patterns.
Table Scan Insights with `pg_stat_user_tables`
The pg_stat_user_tables view provides statistics at the table level. Key columns here are:
seq_scan: Number of sequential scans initiated on this table.idx_scan: Number of index scans initiated on this table.
A high seq_scan count on a large table often indicates a missing or ineffective index.
Comparing Sequential vs. Index Scans
By comparing seq_scan and idx_scan from pg_stat_user_tables, we can identify tables that are frequently being scanned sequentially, even if indexes exist.
A high ratio of sequential scans to index scans on a table suggests potential indexing issues or queries that aren't utilizing available indexes.
Demo: Scan Ratio Query
This query calculates the percentage of sequential scans for each table. Tables with a high percentage of seq_scan might need attention.
SELECT
relname AS table_name,
seq_scan,
idx_scan,
(seq_scan * 100.0) / (CASE WHEN seq_scan + idx_scan = 0 THEN 1 ELSE seq_scan + idx_scan END) AS seq_scan_percent
FROM pg_stat_user_tables
WHERE seq_scan > 0
ORDER BY seq_scan_percent DESC;Quick Check
You're trying to find indexes that are consuming disk space but are never being used by any query. Which PostgreSQL system view would you primarily consult for this information?
Recap & Next Steps
Great job! In this lesson, you learned how to monitor index effectiveness using PostgreSQL's system views.
pg_stat_user_indexeshelps find unused indexes (idx_scan = 0).pg_stat_user_tablesreveals the balance between sequential and index scans on tables.- By combining these, you can identify indexes that are candidates for removal or further investigation.
In the next lesson, we'll dive into reindexing and maintaining index health!
Frequently asked questions
Is the “Index Usage Monitoring” lesson free?
Yes — the full text of “Index Usage Monitoring” is free to read here on the web, and the Advanced PostgreSQL: Indexing, Partitioning, Replication course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Advanced PostgreSQL: Indexing, Partitioning, Replication course, upgrade to CoddyKit PRO.
What will I learn in “Index Usage Monitoring”?
Learn to monitor index effectiveness using system views and identify unused or underperforming indexes. You practise Advanced PostgreSQL: Indexing, Partitioning, Replication with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Advanced PostgreSQL: Indexing, Partitioning, Replication?
No prior experience is required. Advanced PostgreSQL: Indexing, Partitioning, Replication on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Index Usage Monitoring” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Advanced PostgreSQL: Indexing, Partitioning, Replication lesson?
Yes. Every Advanced PostgreSQL: Indexing, Partitioning, Replication lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.