active_sql_transaction SET TRANSACTION ISOLATION LEVEL must be called before any query — 25001

Symptoms

SET TRANSACTION ISOLATION LEVEL was issued after the transaction had already executed a query. The isolation level can only be set at the very start of a transaction, so PostgreSQL raises SQLSTATE 25001 (active_sql_transaction).

• Happens when the SET arrives after the first statement.
• Common in pooled connections that already ran a query.
• The isolation level must be the first action in the transaction.

What the server log shows

ERROR:  SET TRANSACTION ISOLATION LEVEL must be called before any query

Why PostgreSQL raises this — what the manual says

As the SET TRANSACTION reference (Notes) explains:

SET TRANSACTION must be issued before the transaction’s first query or data-modification statement; once a statement has run, the isolation level and access mode for that transaction are fixed and can no longer be changed.

The isolation level fixes the transaction snapshot behaviour from its first statement onward. Changing it after work has begun would be ambiguous, so PostgreSQL rejects a late SET with 25001.

Common causes

• Setting the isolation level after running a query in the same transaction.
• A pooler handing back a connection that already issued statements.
• ORM code that issues SELECTs before configuring isolation.

How to fix it

1. Set the isolation level as the first statement after BEGIN.
2. Use BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE; to set it atomically.
3. Set default_transaction_isolation if every transaction needs the same level.

Related & next steps

• SQLSTATE 25001 — Active SQL Transaction (full reference)
• Message: VACUUM cannot run inside a transaction block
• Glossary: Isolation level

Reference: PostgreSQL 18 — SET TRANSACTION.