LINQ，join的时候，会把所有字段都select出来，然后再join的吗？

[zlhxx90]

我的环境是EF6,VS2017,Mysql,winform
我最近使用join进行三表连接的时候，发现生成的SQL语句中，是首先将join的表的所有字段select后，再套用一个select检索需要的字段。为此，我还测试了再join前先用select选择需要的字段，然后再join，也没有起作用。
比如

                Dim rsSql = From coden In db.m_codens
                            Where coden.companyid = 11 And
                                coden.codeid = 99 And
                                coden.code_n = 1 And
                                coden.code_s = "1"
                            Join user In db.m_user On coden.data1 Equals CStr(user.userid) And
                                user.companyid Equals coden.companyid
                            Join log In db.d_log_operation On user.userid Equals log.userid
                            Where log.pcno = 1
                            Order By log.srdate, log.srtime
                            Select user.userid, user.usename, log.srdate, log.srtime, log.inputtext

生成的SQL语句是这样的：

SELECT
`Project1`.`C1`, 
`Project1`.`userid`, 
`Project1`.`username`, 
`Project1`.`srdate`, 
`Project1`.`srtime`, 
`Project1`.`inputtext`
FROM (SELECT
`Filter1`.`userid`, 
`Filter1`.`username`, 
`Extent3`.`srdate`, 
`Extent3`.`srtime`, 
`Extent3`.`inputtext`, 
1 AS `C1`
FROM (SELECT
`Extent1`.`id`, 
`Extent1`.`companyid `, 
`Extent1`.`codeid`, 
`Extent1`.`code_n`, 
`Extent1`.`code_s`, 
`Extent1`.`data1`, 
`Extent1`.`data2`, 
`Extent1`.`data3`, 
`Extent1`.`data4`, 
`Extent1`.`revison`, 
`Extent1`.`timestamp`, 
`Extent2`.`id` AS `ID1`, 
`Extent2`.`companyid ` AS `companyid 1`, 
`Extent2`.`userid`, 
`Extent2`.`username`, 
`Extent2`.`passwd`, 
`Extent2`.`superkbn`, 
`Extent2`.`revison` AS `REVISON1`, 
`Extent2`.`timestamp` AS `TIMESTAMP1`
FROM `m_codens` AS `Extent1` INNER JOIN `m_user` AS `Extent2` ON ((`Extent1`.`data1` = (`Extent2`.`userid`)) OR ((`Extent1`.`data1` IS  NULL) AND (`Extent2`.`userid` IS  NULL))) AND (`Extent1`.`companyid ` = `Extent2`.`companyid `)
 WHERE ((99 = `Extent1`.`codeid`) AND (1 = `Extent1`.`code_n`)) AND (@gp1 = `Extent1`.`code_s`)) AS `Filter1` INNER JOIN `d_log_operation` AS `Extent3` ON (`Filter1`.`userid` = `Extent3`.`userid`) OR ((`Filter1`.`userid` IS  NULL) AND (`Extent3`.`userid` IS  NULL))
 WHERE (`Filter1`.`companyid ` = @p__linq__0) AND (1 = `Extent3`.`regno`)) AS `Project1`
 ORDER BY 
`Project1`.`srdate` ASC, 
`Project1`.`srtime` ASC

SQL语句中，把m_codens和m_user的字段都select出来了，这显然没有必要。。
另外，最外层的select似乎也没有必要？已经在第二个select中，将数据都检索出来了。

[q107770540]

You dont need to care about the sql script

[正怒月神]

本来ef的join编译到sql就是这样。 这也是被很多诟病的效率问题。嵌套select