mysql 查询结构，UNION ALL 真的是这样做的正确方法吗？

Question

我遇到一个问题，mysql 查询卡在“正在发送数据”状态，有时会持续 12-20 分钟以上，然后我看到其他查询堆积在它后面，直到我用完连接和站点关闭。看起来它们最终确实完成了，如果我坐着观察它们足够长的时间，它们就会去“删除临时表”，然后它们就会清除。

现在我没有编写这段代码，但我的任务是修复它。我为我们公司做 IT 和服务器管理工​​作，但我将网站的所有实际编码留给了我的开发人员。我对 mysql 几乎一无所知，但我的网络人员正在使用的 SELECT 查询对我来说看起来很可疑。如果我没看错，他会说“在每个表中查找 product_id xxxx 并给我这些结果，丢弃其余的”他声称因为表已编入索引，所以以这种方式调出数据不是问题。

如果我从 mysql 控制台运行查询，返回结果需要 3 到 20 秒，当 PHP 实际调用它时显然更长。请注意，它不限于任何一个产品 ID。我尝试调用的产品似乎没有太大区别。

这是我在mysql.log中看到的

    47384 Connect   ecom_a@localhost on 
    47384 Init DB   ecom_Products
    47384 Query SELECT table_name FROM INFORMATION_SCHEMA.TABLES WHERE table_schema = 'ecom_Products'
    47384 Query 
SELECT * FROM ((SELECT * FROM `AC_Electric_Motor_Run_Capacitors`) 
UNION ALL (SELECT * FROM `AC_Electric_Motor_Start_Capacitors`) 
UNION ALL (SELECT * FROM `AC_Filters`) 
UNION ALL (SELECT * FROM `AC_Gear_Motors`) 
UNION ALL (SELECT * FROM `AC_Line_&_Load_Reactors`) 
UNION ALL (SELECT * FROM `AC_Voltage_Regulators`) 
UNION ALL (SELECT * FROM `Auxiliary_Contact_Blocks`) UNION ALL (SELECT * FROM `Bleed_Down_Resistors`) UNION ALL (SELECT * FROM `Coils`) UNION ALL (SELECT * FROM `Contactors`) UNION ALL (SELECT * FROM `Crimpers`) UNION ALL (SELECT * FROM `DC_Gear_Motors`) UNION ALL (SELECT * FROM `Diesel_Engines`) UNION ALL (SELECT * FROM `Disconnects`) UNION ALL (SELECT * FROM `Electric_Motor_Slide_Bases`) UNION ALL (SELECT * FROM `Electric_Motors`) UNION ALL (SELECT * FROM `Electric_Powered_Water_Pumps`) UNION ALL (SELECT * FROM `Electrical_Enclosures`) UNION ALL (SELECT * FROM `Engine_Powered_Water_Pumps`) UNION ALL (SELECT * FROM `External_Control_Interfaces`) UNION ALL (SELECT * FROM `Float_Switches`) UNION ALL (SELECT * FROM `Foot_Switches`) UNION ALL (SELECT * FROM `Generator_Heads`) UNION ALL (SELECT * FROM `Horizontal_Shaft_Gas_Engines`) UNION ALL (SELECT * FROM `Insulating_Resins`) UNION ALL (SELECT * FROM `Limit_Switches`) UNION ALL (SELECT * FROM `Magnet_Wire`) UNION ALL (SELECT * FROM `Manual_Transfer_Switches`) UNION ALL (SELECT * FROM `Mechanical_Interlock_Blocks`) UNION ALL (SELECT * FROM `Medium_Voltage_Transformers`) UNION ALL (SELECT * FROM `Motor_Starters`) UNION ALL (SELECT * FROM `Multi-fuel_Engines`) UNION ALL (SELECT * FROM `Overload_Relays`) UNION ALL (SELECT * FROM `PTO_Generator_3-Point_Hitch_Mounts`) UNION ALL (SELECT * FROM `PTO_Generator_Drive_Adapters`) UNION ALL (SELECT * FROM `PTO_Generator_Drive_Shafts`) UNION ALL (SELECT * FROM `PTO_Generator_Trailers`) UNION ALL (SELECT * FROM `PTO_Generators`) UNION ALL (SELECT * FROM `Packaged_Standby_Generators`) UNION ALL (SELECT * FROM `Portable_Generator_Covers`) UNION ALL (SELECT * FROM `Portable_Generator_Lifting_Provisions`) UNION ALL (SELECT * FROM `Portable_Generator_Wheel_Kits`) UNION ALL (SELECT * FROM `Portable_Generators`) UNION ALL (SELECT * FROM `Resilient_Vibration_Isolators`) UNION ALL (SELECT * FROM `Resistance_Wire`) UNION ALL (SELECT * FROM `Rotary_Frequency_Converters`) UNION ALL (SELECT * FROM `Rotary_Phase_Converters`) UNION ALL (SELECT * FROM `SO_Cable`) UNION ALL (SELECT * FROM `Single_Phase_Motors_with_Base_Mount`) UNION ALL (SELECT * FROM `Single_Phase_Motors_with_Face_&_Base_Mount`) UNION ALL (SELECT * FROM `Single_Phase_Motors_with_Face_Mount`) UNION ALL (SELECT * FROM `Soft_Starters`) UNION ALL (SELECT * FROM `Special_Metal_Wire`) UNION ALL (SELECT * FROM `Static_Frequency_Converters`) UNION ALL (SELECT * FROM `Static_Phase_Converters`) UNION ALL (SELECT * FROM `Surge_Suppressors`) UNION ALL (SELECT * FROM `Three_Phase_Motors_with_Base_Mount`) UNION ALL (SELECT * FROM `Three_Phase_Motors_with_Face_&_Base_Mount`) UNION ALL (SELECT * FROM `Three_Phase_Motors_with_Face_Mount`) UNION ALL (SELECT * FROM `Transformers_-_General`) UNION ALL (SELECT * FROM `Variable_Frequency_Drives`) UNION ALL (SELECT * FROM `Variable_Transformers`) UNION ALL (SELECT * FROM `Vehicle_&_Equipment_Batteries`) UNION ALL (SELECT * FROM `Vertical_Shaft_Gas_Engines`) UNION ALL (SELECT * FROM `Welding_Cable`) UNION ALL (SELECT * FROM `_Default`)) 
AS t WHERE product_ID = 'LF0009' LIMIT 1    

Answer 1

假设确实需要查找每张表，如果每个子查询都应用WHERE子句会更好。就目前而言，此查询将在所有表上构建一个大型结果集，然后再应用 where 然后限制结果集。也许 MySQL 可以对此进行优化，也许不能，但您可以尝试:

....
UNION ALL (SELECT * FROM `AC_Filters` WHERE product_ID = 'LF0009')
UNION ALL (SELECT * FROM `AC_Line_&_Load_Reactors` WHERE product_ID = 'LF0009')
....

我怀疑这个查询是在 PHP 中以编程方式生成的，考虑到运行的第一个查询，所以进行更改应该不难。

更广泛地说，从查询的外观来看，您似乎每个产品类型都有一个表，而不是例如一个名为“产品”的表，其中包含类型的 ID 列，以及列出唯一产品的表。