Question

我在where子句中查询了哪些列是主键的一部分，并且在所有列上都有外键索引。

EXPLAIN SELECT   aggEI.c_id AS companyId, aggEI.ei_uid AS uuid
FROM AGG_EI AS aggEI 
WHERE  aggEI.c_id in (8) and aggEI.tg_id IN (1,2,3,4,5,6,7)
       AND aggEI.dt_id = 20130506

我还在（c_id，tg_id，dt_id上定义了多个columnn索引，但EXPLAIN显示它正在c_id上使用外键索引

 1 SIMPLE   aggEI   ref PRIMARY,datedm_id_UNIQUE,agg_ei_comdm_fk_idx,agg_ei_datedm_fk_idx,agg_ei_topgrp_fk_idx,comp_uuid    agg_ei_comdm_fk_idx 8   const   65986   Using where; Using index

agg_ei_comdm_fk_idx是c_id上的外键索引，comp_uuid是（c_id，tg_id，dt_id）上的多列索引

有人可以解释为什么会这样发生吗

编辑：创建表格

          'CREATE TABLE `AGG_EI` (
              `dt_id` int(11) NOT NULL,
              `c_id` bigint(20) NOT NULL,
              `tg_id` bigint(20) NOT NULL,
              `ei_uid` varchar(150) NOT NULL
              `ei_name` varchar(150) NOT NULL,
               `rating` double NOT NULL,
              `cnt` double NOT NULL
               PRIMARY KEY (`dt_id`,`c_id`,`tg_id`,`ei_uid`),
                UNIQUE KEY `datedm_id_UNIQUE` (`dt_id`,`c_id`,`ei_uid`,`tg_id`),
               KEY `agg_ei_comdm_fk_idx` (`c_id`),
               KEY `agg_ei_datedm_fk_idx` (`dt_id`),
               KEY `agg_ei_topgrp_fk_idx` (`tg_id`),
               KEY `comp_uuid` (`c_id`,`tg_id`,`dt_id`),
               CONSTRAINT `agg_ei_comdm_fk` FOREIGN KEY (`c_id`) REFERENCES `COMPDM`     (`c_id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
                CONSTRAINT `agg_ei_datedm_fk` FOREIGN KEY (`dt_id`) REFERENCES `DATEDM` (`dt_id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
                CONSTRAINT `agg_ei_topgrp_fk` FOREIGN KEY (`tg_id`) REFERENCES `TOPGRP` (`tg_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
             ) ENGINE=InnoDB DEFAULT CHARSET=utf8'

Answer 1

MySQL正在使用它认为会发挥最佳性能的计划来执行查询。更有可能的是，它确定将c_id限制为单个值会将结果集减少到足够少的行，这会导致处理另一个索引中额外列的麻烦并不值得。 MySQL最好的猜测是，使用c_id限制行更快，然后只过滤内存中的那些行。仅仅因为你有一个索引并不意味着这是最快的计划。

Answer 2

您可以强制mysql使用如下索引：

EXPLAIN 
SELECT   
    aggEI.c_id AS companyId, 
    aggEI.ei_uid AS uuid
FROM 
    AGG_EI AS aggEI FORCE INDEX(`comp_uuid`)
WHERE  
    aggEI.c_id in (8) 
    AND aggEI.tg_id IN (1,2,3,4,5,6,7)
    AND aggEI.dt_id = 20130506

MySQL没有使用多列索引

2 个答案: