Postgres Dead Tuple Percentage (Table Bloat)

Introduction

Table bloat in PostgreSQL refers to the excessive use of disk space within database tables and indexes, typically caused by inefficient storage due to deleted or updated rows. This happens when rows are updated or deleted but the space previously occupied by these rows isn't immediately reclaimed. As a result, the table grows in size, even though the number of rows may not have increased significantly, leading to wasted storage and potential performance degradation.

Causes of Table Bloat, AKA Dead Tuples

1. Row Updates: PostgreSQL uses a Multi-Version Concurrency Control (MVCC) system, where each update to a row creates a new version of that row instead of modifying it in place. The old versions of the row are marked as obsolete but are not immediately removed. Over time, if there are many updates to rows, this leads to bloat.

2. Row Deletions: When a row is deleted, it is not immediately removed from the table; instead, it is marked as deleted. The space taken by these "dead" rows is not reclaimed until a VACUUM operation is run, which can leave behind unused space.

3. Lack of Vacuuming: PostgreSQL requires periodic vacuuming (either manually with VACUUM or automatically with autovacuum) to reclaim space taken by deleted or updated rows. If vacuuming isn't frequent enough, bloat can accumulate.

Issues Because of Table Bloat

• Increased disk usage without a corresponding increase in the number of rows.
• Slow query performance due to inefficient access patterns caused by the bloated tables and indexes.
• Large table or index size relative to the amount of data it stores.

DPA Alert to Detect Table Bloat

The attached SQL statement can be used in a Custom SQL Multiple Numeric alert to detect this issue.

Note 1: the dblink extension needs to be added to each database where you want this alert to detect the problem. Creating the extension is a manual step you must do before the alert will be meaningful.

Note 2: adjust the SQL statement WHERE clause as needed and documented within the attachment

Here is an example of the custom alert definition: