0   Allow dirty reads, non-repeatable reads, and phantom rows.

1   Prevent dirty reads. Allow non-repeatable reads and phantom rows.

2   Prevent dirty reads and non-repeatable reads. Allow phantom rows.

3   Serializable. Prevent dirty reads, non-repeatable reads, and phantom rows.

Snapshot   Use a snapshot of committed data from the time when the first row is read or updated by the transaction.

Statement-snapshot   For each statement, use a snapshot of committed data from the time when the first row is read from the database. Non-repeatable reads and phantom rows can occur within a transaction, but not within a single statement.

Readonly-statement-snapshot   For read-only statements, use a snapshot of committed data from the time when the first row is read from the database. Non-repeatable reads and phantom rows can occur within a transaction, but not within a single statement. For updatable statements, use the isolation level specified by the updatable_statement_isolation option (can be one of 0 (the default), 1, 2, or 3).