Cookbook recipe

What a dead tuple is and why VACUUM exists

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

What This Is

A production experience capsule based on real incident patterns.

Why It Matters

Builds production judgment under pressure.

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Estimated investigation4 min

Scenario

You delete a million rows but the table does not shrink. Those rows became dead tuples, and only VACUUM makes their space reusable. Diagnose it Count dead tuples on a table: SELECT relname, n_dead_tup FROM pg_stat_user_tables…

Investigation Path

You delete a million rows but the table does not shrink. Those rows became dead tuples, and only VACUUM makes their space reusable.

Diagnose it

Count dead tuples on a table:

SELECT relname, n_dead_tup FROM pg_stat_user_tables WHERE relname='orders';

Why it happens

Because of MVCC, DELETE and UPDATE do not erase data immediately — they mark the old version dead. The space is not reusable until VACUUM confirms no transaction can still see it.

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Concepts on this page

Transaction Autovacuum Dead tuple VACUUM Tuple
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