Master the PostgreSQL Row Level Security vocabulary — enabling RLS on tables, USING and WITH CHECK clauses in policies, current_user for dynamic filtering, and the BYPASSRLS privilege.
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How do you enable Row Level Security (RLS) on a PostgreSQL table?
ALTER TABLE ... ENABLE ROW LEVEL SECURITY activates RLS on a table. Once enabled, any role that does not own the table or have BYPASSRLS privilege will see only rows permitted by the applicable policies. Without any policies, all rows are hidden by default.
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What does the USING clause in a CREATE POLICY statement control?
The USING clause filters rows for read operations (SELECT) and for the target rows of UPDATE and DELETE. For example USING (user_id = current_user_id()) ensures each user only sees their own rows. It is evaluated for every row before it is returned or modified.
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What is the purpose of the WITH CHECK clause in a Row Level Security policy?
WITH CHECK governs write operations. While USING filters which existing rows can be touched, WITH CHECK ensures the new/updated row itself satisfies the predicate. Without it, a user could UPDATE a row into a state they would not be able to see, which is usually undesirable.
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What does current_user return inside a Row Level Security policy expression?
current_user in a policy expression returns the PostgreSQL role name of the executing session. Policies like USING (owner = current_user) dynamically restrict each role to their own rows. Application patterns often use SET app.current_user_id and read it with current_setting() for application-level user IDs.
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What does the BYPASSRLS role attribute do?
BYPASSRLS is a role attribute (set with ALTER ROLE name BYPASSRLS) that exempts the role from all RLS policies. Table owners also bypass RLS by default. This is useful for admin or migration roles, but should be granted sparingly since it effectively disables the security boundary for those roles.