DDL Locking: Why ALTER TABLE Can Freeze Your App
Most ALTER TABLE commands grab the strongest possible lock, the kind that blocks even a plain SELECT.
While it's held, nobody can read or write the table at all. On a busy table, a schema tweak you expected to be instant can freeze the whole application.
Pro members see the rest of this lesson
- A 4-step state diagram: Open → Request → Pile up → Drain
- The full mechanism: what PostgreSQL does internally, in source terms
- 2 SQL queries you can run, each labelled by how it was verified
- 1 transcript captured on PostgreSQL 17.10
- The closing insight, the mistake it prevents, and what it changes in your work
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Check it against the source2 citations in postgres/postgres · file, symbol and line verified on REL_17_STABLE
Primary symbol: AlterTableInternal · line 4437
Primary symbol: LockAcquire · line 756
Anchored to postgres/postgres on REL_17_STABLE and cross-checked against the manual for PostgreSQL 15–18. Every query here was run and its output captured on a throwaway PostgreSQL 17.10 lab; corrections are noted inline. ALTER TABLE (locking) in the official docs →
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