这段SQL怎么优化，需要查10分钟！

Question

SELECT distinct c.id,
                        c.authentication_name,
                        c.customer_no,
                        c.reg_status,
                        c.legal_person_name,
                        c.estiblish_time,
                        c.credit_code,
                        c.reg_capital,
                        c.province_code,
                        c.province,
                        c.city_code,
                        c.city,
                        c.area_code,
                        c.area,
                        c.address,
                        c.register_time,
                        c.uniformcode,
                        c.invitationcode,
                        c.belong_person,
                        c.status,
                        c.door_head_name,
                        c.clinic_type,
                        c.doctor_quantity,
                        c.nurse_quantity,
                        c.dental_chair_quantity,
                        c.passenger_flow,
                        c.business_scope,
                        c.public_holiday,
                        c.checkout_type,
                        c.first_order_time,
                        c.last_order_time,
                        c.last_follow_time,
                        c.mm_sale_amount,
                        c.suya_order_quantity,
                        c.tianyancha_update_time,
                        c.customer_type,
                        c.create_by,
                        c.create_time,
                        c.update_by,
                        c.update_time,
                        c.del_flag,
                        cc.name      as primaryContact,
                        cc.phone     as primaryPhone,
                        su.user_id   as belongPersonId,
                        su.nick_name as belongPerson
        from customer c
                 left join (select id,
                                   customer_id,
                                   name,
                                   phone,
                                   is_primary
                            from customer_contacts
                            where is_primary = '1') cc on cc.customer_id = c.id
                 left join sys_user su on c.invitationcode = su.invitationcode
                 left join backlog_follow bf on c.customer_no = bf.customer_no
        where c.del_flag = '0'
        <if test="invitationcodeList != null and invitationcodeList.size() > 0">
            and c.invitationcode in (<foreach collection="invitationcodeList" item="i" separator=",">
            #{i}
        </foreach>)
        </if>
        <if test="authenticationName != null and authenticationName != ''">
            and c.authentication_name like CONCAT('%', #{authenticationName}, '%')
        </if>
        <if test="regionCodeList != null and regionCodeList.size() > 0">
            and (
                c.province_code in (<foreach collection="regionCodeList" item="i" separator=",">
            #{i}
        </foreach>)
            or c.city in (<foreach collection="regionCodeList" item="i" separator=",">
            #{i}
        </foreach>)
            )
        </if>
        <if test="belongPersonId != null">
            and su.user_id = #{belongPersonId}
        </if>

Answer 1

这段SQL语句中使用了多个左连接会导致查询变慢。可以尝试以下优化方法：

1. 确保表中的列都有适当的索引，以便查询可以更快地执行。可以使用EXPLAIN命令来查看查询的执行计划，并确定是否使用了索引。

2. 尽可能使用简单的查询，避免使用复杂的子查询或连接查询。可以考虑将左连接改为内连接，以减少查询的执行时间。

3. 数据库结构优化：如果表结构不合理，可能会导致查询变慢。可以考虑使用范式化来减少数据冗余，并确保表中的列具有适当的数据类型。

4. 缓存优化：可以使用缓存来减少查询的执行时间。例如，可以使用Memcached或Redis来缓存查询结果，以便下次查询时可以更快地获取数据。

5. 服务器优化：如果服务器配置不足，可能会导致查询变慢。可以考虑升级服务器硬件或增加服务器数量，以提高查询性能。