---
source: https://iinventedlight.com/library/notes/postgres-triggers.md
title: "PostgreSQL triggers: BEFORE, AFTER and INSTEAD OF"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "PostgreSQL License; prose restates cited source lines"
attribution: "Summary by iinventedlight (AI-assisted). Sources: PostgreSQL Global Development Group, PostgreSQL: Documentation: 18: 37.1. Overview of Trigger Behavior, PostgreSQL License; PostgreSQL Global Development Group, PostgreSQL: Documentation: 18: CREATE TRIGGER, PostgreSQL License."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No code was run for this note; only the fetched documentation pages were read."
category: coding-references
topic_area: "code-ai-eng: PostgreSQL"
tags: postgresql, triggers, plpgsql
sources:
  - id: S1
    title: "PostgreSQL: Documentation: 18: 37.1. Overview of Trigger Behavior"
    publisher: PostgreSQL Global Development Group
    url: https://www.postgresql.org/docs/current/trigger-definition.html
    accessed: 2026-11-04
    license: "PostgreSQL License"
  - id: S2
    title: "PostgreSQL: Documentation: 18: CREATE TRIGGER"
    publisher: PostgreSQL Global Development Group
    url: https://www.postgresql.org/docs/current/sql-createtrigger.html
    accessed: 2026-11-04
    license: "PostgreSQL License"
---
# PostgreSQL triggers: BEFORE, AFTER and INSTEAD OF

## Short answer
A trigger runs a function automatically when rows are inserted, updated, deleted or truncated. First write a trigger function (no arguments, returns `trigger`), then attach it with `CREATE TRIGGER`. [S1][S2]

```sql
CREATE TRIGGER set_updated BEFORE UPDATE ON items
FOR EACH ROW EXECUTE FUNCTION touch_updated_at();
```

## Key points
- BEFORE, AFTER or INSTEAD OF sets when the function runs. [S2]
- INSTEAD OF triggers must be FOR EACH ROW and only work on views. [S2]

## Claims
- id: C1
  claim: The trigger function must be defined before the trigger.
  source: S1
  evidence: "The trigger function must be defined before the trigger itself can be created."
- id: C2
  claim: The same trigger function can be used for multiple triggers.
  source: S1
  evidence: "The same trigger function can be used for multiple triggers."
- id: C3
  claim: CREATE TRIGGER creates a new trigger.
  source: S2
  evidence: "CREATE TRIGGER creates a new trigger."
- id: C4
  claim: INSTEAD OF triggers must be FOR EACH ROW and only on views.
  source: S2
  evidence: "must be marked FOR EACH ROW, and can only be defined on views"

## Sources
- [S1] PostgreSQL: Documentation: 18: 37.1. Overview of Trigger Behavior. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/trigger-definition.html . License: PostgreSQL License. Accessed 2026-11-04.
- [S2] PostgreSQL: Documentation: 18: CREATE TRIGGER. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/sql-createtrigger.html . License: PostgreSQL License. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).