The SET USER system privilege allows a user to assume the exact roles and system privileges of (impersonate) another user. Once begun, impersonation of another user remains in effect until it is terminated or the current session ends.
Each grantee (user doing the impersonating) must be an existing user or role.
Each target user (user being impersonated) must be an existing user with a login password.
The grantor (user granting the system privilege) must have administrative rights to the SET USER system privilege.
Each grantee must have been granted at least all roles and system privileges with same or higher administrative options as the target user.