The DECLARE TRANSACTION defaults are the same as the defaults for
    the SET TRANSACTION statement. See the SET_TRANSACTION statement
    for complete information.

    In general, you should not rely on default transaction
    characteristics. Use explicit DECLARE TRANSACTION statements,
    specifying read/write, read-only, or batch-update options;
    a list of tables in the RESERVING clause; and a share mode
    and lock type for each table. The more specific you are in a
    DECLARE TRANSACTION statement, the more efficient your database
    operations will be.

    When a transaction starts using characteristics specified in a
    DECLARE TRANSACTION statement, any transaction characteristics
    unspecified in the DECLARE TRANSACTION statement take the SQL
    defaults. This is true even if the characteristics unspecified in
    DECLARE TRANSACTION were specified in an earlier SET or DECLARE
    TRANSACTION statement.