EXPLAIN SELECT 将类型显示为 ALL (SQL)答案

【问题标题】：EXPLAIN SELECT Shows Type as ALL (SQL)EXPLAIN SELECT 将类型显示为 ALL (SQL)
【发布时间】：2013-07-22 18:27:00
【问题描述】：

我正在对这里的一个相当复杂的 SQL 查询的性能进行故障排除。当我运行以下命令时，结果概述如下...

EXPLAIN SELECT
    Import_Values.id,
    Import_Values.part_id,
    Import_Values.qty,
    Import_Values.note,
    Parts.partterminologyname,
    GROUP_CONCAT(BaseVehicle.YearID, ' ', Make.MakeName, ' ', Model.modelname, ' ', SubModel.SubModelName SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineDesignation.EngineDesignationName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineVIN.EngineVINName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.Liter) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.CC) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.CID) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.Cylinders) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.BlockType) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.EngBoreIn) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.EngBoreMetric) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.EngStrokeIn) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(EngineBase.EngStrokeMetric) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(FuelDeliveryType.FuelDeliveryTypeName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(FuelDeliverySubType.FuelDeliverySubTypeName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(FuelSystemControlType.FuelSystemControlTypeName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(FuelSystemDesign.FuelSystemDesignName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(Aspiration.AspirationName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(CylinderHeadType.CylinderHeadTypeName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(FuelType.FuelTypeName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(IgnitionSystemType.IgnitionSystemTypeName) SEPARATOR ', '),
    GROUP_CONCAT(DISTINCT(Mfr.MfrName) SEPARATOR ', ')
    FROM 
    Import_Values
    INNER JOIN BaseVehicle 
        ON Import_Values.base_vehicle_id=BaseVehicle.BaseVehicleID
    INNER JOIN Parts 
        ON Import_Values.part_type_id=Parts.PartTerminologyID
    INNER JOIN Make
        ON BaseVehicle.MakeID=Make.MakeID
    INNER JOIN Model
        ON BaseVehicle.ModelID=Model.ModelID
    INNER JOIN Vehicle
        ON Import_Values.base_vehicle_id=Vehicle.BaseVehicleID
    INNER JOIN SubModel
        ON Vehicle.SubModelID=SubModel.SubModelID
    INNER JOIN VehicleToEngineConfig
        ON Vehicle.VehicleID=VehicleToEngineConfig.VehicleID
    INNER JOIN EngineConfig
        ON VehicleToEngineConfig.EngineConfigID=EngineConfig.EngineConfigID
    INNER JOIN EngineDesignation
        ON EngineConfig.EngineDesignationID=EngineDesignation.EngineDesignationID
    INNER JOIN EngineVIN
        ON EngineConfig.EngineVINID=EngineVIN.EngineVINID
    INNER JOIN EngineBase
        ON EngineConfig.EngineBaseID=EngineBase.EngineBaseID
    INNER JOIN FuelDeliveryConfig
        ON EngineConfig.FuelDeliveryConfigID=FuelDeliveryConfig.FuelDeliveryConfigID
    INNER JOIN FuelDeliveryType
        ON FuelDeliveryConfig.FuelDeliveryTypeID=FuelDeliveryType.FuelDeliveryTypeID
    INNER JOIN FuelDeliverySubType
        ON FuelDeliveryConfig.FuelDeliverySubTypeID=FuelDeliverySubType.FuelDeliverySubTypeID
    INNER JOIN FuelSystemControlType
        ON FuelDeliveryConfig.FuelSystemControlTypeID=FuelSystemControlType.FuelSystemControlTypeID
    INNER JOIN FuelSystemDesign
        ON FuelDeliveryConfig.FuelSystemDesignID=FuelSystemDesign.FuelSystemDesignID
    INNER JOIN Aspiration
        ON EngineConfig.AspirationID=Aspiration.AspirationID
    INNER JOIN CylinderHeadType
        ON EngineConfig.CylinderHeadTypeID=CylinderHeadType.CylinderHeadTypeID
    INNER JOIN FuelType
        ON EngineConfig.FuelTypeID=FuelType.FuelTypeID
    INNER JOIN IgnitionSystemType
        ON EngineConfig.IgnitionSystemTypeID=IgnitionSystemType.IgnitionSystemTypeID
    INNER JOIN Mfr
        ON EngineConfig.EngineMfrID=Mfr.MfrID
    GROUP BY part_id

结果表明Import_Values 表有147377 行，类型为'ALL'，FuelDeliveryType 有3 行，类型为'ALL'。根据我的阅读，“ALL”类型会导致性能不佳。

我添加了一个索引，但它仍然显示“全部”——还有什么我必须做的吗？

谢谢！

【问题讨论】：

标签： mysql sql select query-optimization

【解决方案1】：

好吧，您肯定知道索引的必要性，但关键是要知道索引是否真的完全优化了您的查询。

如果列的类型和大小相同（即 CHAR(10)、VARCHAR 等），MySQL 会以最大效率对列使用索引。您可以更改表 ALTER TABLE，以确保这些类型在您加入的列中保持一致。

MySQL 所做的另一件事是假设列中的值是均匀分布的，但情况可能并非如此。如果您在有问题的表上运行 ANALYZE TABLE，您可能会发现它返回的行数甚至与实际表中的实际行数都不接近。要解决此问题，您可以使用不同类型的 JOIN，甚至可以在 FROM 子句中重新排序表。

如果在这些测试之后 ALL 类型仍未消失，则可能是查询已达到预期的优化程度。

【讨论】：