What is the default transaction mode in Postgres?

In PostgreSQL, transactions by default operate in the "Read Committed" mode. This means that each command within a transaction only sees data that was committed at the time the command was executed. In other words, a transaction does not see uncommitted changes made by other transactions.

Example:

BEGIN;
-- Read Committed mode by default
SELECT * FROM users WHERE id = 1;
COMMIT;

To change the isolation level, you can use the command:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

or at the start of a transaction:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;