Can PostgreSQL's ON CONFLICT Clause Handle Multiple Unique-Indexed Columns as Conflict Targets?

PostgreSQL's ON CONFLICT Clause: Managing Conflicts Across Multiple Unique Columns

PostgreSQL's powerful ON CONFLICT clause simplifies handling duplicate row insertions, allowing for updates instead of errors. A key question arises: can this clause effectively manage conflicts involving multiple uniquely indexed columns?

The need to update additional columns when a conflict occurs across multiple unique columns necessitates a solution beyond single-column conflict targets. The solution lies in defining a composite unique index spanning the relevant columns.

Here's how it works: Suppose we have a table with unique constraints on both id and name:

<code class="language-sql">CREATE TABLE test (id INT, name TEXT, UNIQUE (id), UNIQUE (name));</code>

Note that creating separate unique indexes on id and name is functionally equivalent to creating a single unique constraint UNIQUE (id, name). To handle conflicts based on either id or name (or both), we use a composite index in the ON CONFLICT clause:

<code class="language-sql">INSERT INTO test (id, name) ON CONFLICT (id, name) DO UPDATE SET ...;</code>

This approach enables the database to efficiently identify conflicts based on the combined values of id and name, triggering the DO UPDATE action as needed.

Crucially, the ON CONFLICT clause relies on the existence of a unique constraint (or index) covering all specified conflict target columns. Attempting to use ON CONFLICT without such a constraint will result in a database error.

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