Postgres Sql新增字段时候判断不存在才新增，存在则不新

DO $$
    BEGIN
        BEGIN
            ALTER TABLE recharge_history ADD COLUMN bank_ls VARCHAR(8);
        EXCEPTION
            WHEN duplicate_column THEN RAISE NOTICE 'column bank_ls already exists in recharge_history.';
        END;
    END;
$$;
COMMENT ON COLUMN "public"."recharge_history"."bank_ls" IS '银行流水号';