在 select 子句中嵌套 select 需要很长时间才能完成
Nesting select in select clause take forever to complete
Table markclr
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
name varchar(255) NOT NULL,
color varchar(255) NOT NULL,
Table dvcgrp
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
name varchar(255) NOT NULL,
markclrid int(10) UNSIGNED NOT NULL,
Table dvc
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
name varchar(255) NOT NULL,
shrtname varchar(255) NOT NULL,
dvceui varchar(255) NOT NULL,
dvcgrpid int(10) UNSIGNED NOT NULL,
Table vhsrdt
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
dvcid int(10) UNSIGNED NOT NULL,
dtrcvtm timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
dtraw varchar(255) NOT NULL,
dtdvcnm varchar(255) NOT NULL,
dtgtwid varchar(255) NOT NULL,
dtport int NOT NULL,
dtdr int NOT NULL,
dtadr varchar(255) NOT NULL,
dtriptype varchar(255),
dlatitudedeg decimal(10,8),
dlongitudedeg decimal(11,8),
dvextgood int(10) UNSIGNED NOT NULL,
dgpscurrent int(10) UNSIGNED NOT NULL,
dignition int(10) UNSIGNED NOT NULL,
ddigin1 int(10) UNSIGNED NOT NULL,
ddigin2 int(10) UNSIGNED NOT NULL,
ddigout int(10) UNSIGNED NOT NULL,
dheadingdeg varchar(255),
dspeedkmph varchar(255),
dbatv varchar(255),
请帮我解释一下我的查询发生了什么：
select
dvc1.id,
dvc1.name as dvcname,
dvc1.dvceui as dvcdvceui,
dvc1.shrtname as dvcshrtname,
markclr.color as markclrcolor,
(
select
tbl1.dtrcvtm
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dtrcvtm,
(
select
tbl1.dtgtwid
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dtgtwid,
(
select
tbl1.dtriptype
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dtriptype,
(
select
tbl1.dlatitudedeg
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dlatitudedeg,
(
select
tbl1.dlongitudedeg
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dlongitudedeg,
(
select
tbl1.dspeedkmph
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dspeedkmph,
(
select
tbl1.dheadingdeg
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dheadingdeg
from dvc as dvc1
left join dvcgrp on dvcgrp.id = dvc1.dvcgrpid
left join markclr on markclr.id = dvcgrp.markclrid
table vhsrdt 填充了大约 90K 数据。
如果我使用 XAMPP 模拟数据库和 运行 在我的开发环境中查询，它会在不到 1 秒的时间内成功获取。
但是，如果我 运行 使用带有 MySQL 5.7 的 linux 服务器对生产进行查询，它永远不会完成。
我也试过删除数据库，重新构造所有结构和数据，还是无法完成查询。
感谢您的帮助。
EXPLAIN的结果：
id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 | PRIMARY | dvc1 | | ALL | | | | | 50 | 100 |
1 | PRIMARY | dvcgrp | | eq_ref | PRIMARY | PRIMARY | 4 | klksensor.dvc1.dvcgrpid | 1 | 100 |
1 | PRIMARY | markclr | | ALL | PRIMARY | | | | 5 | 100 | Using where; Using join buffer (Block Nested Loop)
14 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
15 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
12 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
13 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
10 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
11 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
8 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
9 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
6 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
7 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
4 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
5 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
2 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where; Using index
3 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
从您的 EXPLAIN 语句中，您可以看到很多 Dependent Subquery 是 运行；尽管在您的情况下，它们都是相似的，只是 SELECT 差异字段不同。不幸的是，MySQL 在 SELECT 子句中使用子查询时不允许返回元组。
您可以通过在 FROM 子句中使用单个 "run" 计算所有 MAX() 值，将所有这些子查询缩减为单个子查询。请尝试以下查询：
select
dvc1.id,
dvc1.name as dvcname,
dvc1.dvceui as dvcdvceui,
dvc1.shrtname as dvcshrtname,
markclr.color as markclrcolor,
vhsrdt.dtrcvtm,
vhsrdt.dtgtwid,
vhsrdt.dtriptype,
vhsrdt.dlatitudedeg,
vhsrdt.dlongitudedeg,
vhsrdt.dspeedkmph,
vhsrdt.dheadingdeg
from dvc as dvc1
left join dvcgrp on dvcgrp.id = dvc1.dvcgrpid
left join markclr on markclr.id = dvcgrp.markclrid
left join vhsrdt on vhsrdt.dvcid = dvc1.id
left join (select max(dtrcvtm) as max_dtrcvtm, dvcid
from vhsrdt
group by dvcid
) as dt on dt.dvcid = vhsrdt.dvcid
and dt.max_dtrcvtm = vhsrdt.dtrcvtm
无论我们是否需要，上述方法都限制了计算所有 dvcid 的 max() 值。此外，它会导致 temp table 创建（如果小则在内存中，否则在磁盘上）。值得庆幸的是，MySQL 8.0.14 引入了 LATERAL Derived Table；所以如果你可以升级到最新的 MySQL 版本，更多 performant 查询将是：
select
dvc1.id,
dvc1.name as dvcname,
dvc1.dvceui as dvcdvceui,
dvc1.shrtname as dvcshrtname,
markclr.color as markclrcolor,
v1.dtrcvtm,
v1.dtgtwid,
v1.dtriptype,
v1.dlatitudedeg,
v1.dlongitudedeg,
v1.dspeedkmph,
v1.dheadingdeg
from dvc as dvc1
left join dvcgrp on dvcgrp.id = dvc1.dvcgrpid
left join markclr on markclr.id = dvcgrp.markclrid
left join vhsrdt v1 on v1.dvcid = dvc1.id
left join lateral (select max(v2.dtrcvtm) as max_dtrcvtm
from vhsrdt v2
where v2.dvcid = v1.dvcid
) as dt on dt.max_dtrcvtm = v1.dtrcvtm
Table markclr
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
name varchar(255) NOT NULL,
color varchar(255) NOT NULL,
Table dvcgrp
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
name varchar(255) NOT NULL,
markclrid int(10) UNSIGNED NOT NULL,
Table dvc
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
name varchar(255) NOT NULL,
shrtname varchar(255) NOT NULL,
dvceui varchar(255) NOT NULL,
dvcgrpid int(10) UNSIGNED NOT NULL,
Table vhsrdt
=============
id int(10) UNSIGNED AUTO_INCREMENT NOT NULL,
dvcid int(10) UNSIGNED NOT NULL,
dtrcvtm timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
dtraw varchar(255) NOT NULL,
dtdvcnm varchar(255) NOT NULL,
dtgtwid varchar(255) NOT NULL,
dtport int NOT NULL,
dtdr int NOT NULL,
dtadr varchar(255) NOT NULL,
dtriptype varchar(255),
dlatitudedeg decimal(10,8),
dlongitudedeg decimal(11,8),
dvextgood int(10) UNSIGNED NOT NULL,
dgpscurrent int(10) UNSIGNED NOT NULL,
dignition int(10) UNSIGNED NOT NULL,
ddigin1 int(10) UNSIGNED NOT NULL,
ddigin2 int(10) UNSIGNED NOT NULL,
ddigout int(10) UNSIGNED NOT NULL,
dheadingdeg varchar(255),
dspeedkmph varchar(255),
dbatv varchar(255),
请帮我解释一下我的查询发生了什么：
select
dvc1.id,
dvc1.name as dvcname,
dvc1.dvceui as dvcdvceui,
dvc1.shrtname as dvcshrtname,
markclr.color as markclrcolor,
(
select
tbl1.dtrcvtm
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dtrcvtm,
(
select
tbl1.dtgtwid
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dtgtwid,
(
select
tbl1.dtriptype
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dtriptype,
(
select
tbl1.dlatitudedeg
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dlatitudedeg,
(
select
tbl1.dlongitudedeg
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dlongitudedeg,
(
select
tbl1.dspeedkmph
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dspeedkmph,
(
select
tbl1.dheadingdeg
from vhsrdt as tbl1
where
tbl1.dvcid = dvc1.id and
tbl1.dtrcvtm = (
select
max(tbl2.dtrcvtm) as dtrcvtm
from vhsrdt as tbl2
where tbl2.dvcid = tbl1.dvcid
)
) as dheadingdeg
from dvc as dvc1
left join dvcgrp on dvcgrp.id = dvc1.dvcgrpid
left join markclr on markclr.id = dvcgrp.markclrid
table vhsrdt 填充了大约 90K 数据。
如果我使用 XAMPP 模拟数据库和 运行 在我的开发环境中查询，它会在不到 1 秒的时间内成功获取。
但是，如果我 运行 使用带有 MySQL 5.7 的 linux 服务器对生产进行查询，它永远不会完成。
我也试过删除数据库，重新构造所有结构和数据，还是无法完成查询。
感谢您的帮助。
EXPLAIN的结果：
id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 | PRIMARY | dvc1 | | ALL | | | | | 50 | 100 |
1 | PRIMARY | dvcgrp | | eq_ref | PRIMARY | PRIMARY | 4 | klksensor.dvc1.dvcgrpid | 1 | 100 |
1 | PRIMARY | markclr | | ALL | PRIMARY | | | | 5 | 100 | Using where; Using join buffer (Block Nested Loop)
14 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
15 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
12 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
13 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
10 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
11 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
8 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
9 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
6 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
7 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
4 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where
5 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
2 | DEPENDENT SUBQUERY | tbl1 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.dvc1.id | 1627 | 100 | Using where; Using index
3 | DEPENDENT SUBQUERY | tbl2 | | ref | vhsrdt_dvcid_index,vhsrdt_dvcid_dtrcvtm_dtport_index,vhsrdt_dvcid_dtgtwid_index,vhsrdt_dvcid_dtriptype_index,vhsrdt_dvcid_dlatitudedeg_index,vhsrdt_dvcid_dlongitudedeg_index,vhsrdt_dvcid_dlatitudedeg_dlongitudedeg_index | vhsrdt_dvcid_dtrcvtm_dtport_index | 4 | klksensor.tbl1.dvcid | 1627 | 100 | Using index
从您的 EXPLAIN 语句中，您可以看到很多 Dependent Subquery 是 运行；尽管在您的情况下，它们都是相似的，只是 SELECT 差异字段不同。不幸的是，MySQL 在 SELECT 子句中使用子查询时不允许返回元组。
您可以通过在 FROM 子句中使用单个 "run" 计算所有 MAX() 值，将所有这些子查询缩减为单个子查询。请尝试以下查询：
select
dvc1.id,
dvc1.name as dvcname,
dvc1.dvceui as dvcdvceui,
dvc1.shrtname as dvcshrtname,
markclr.color as markclrcolor,
vhsrdt.dtrcvtm,
vhsrdt.dtgtwid,
vhsrdt.dtriptype,
vhsrdt.dlatitudedeg,
vhsrdt.dlongitudedeg,
vhsrdt.dspeedkmph,
vhsrdt.dheadingdeg
from dvc as dvc1
left join dvcgrp on dvcgrp.id = dvc1.dvcgrpid
left join markclr on markclr.id = dvcgrp.markclrid
left join vhsrdt on vhsrdt.dvcid = dvc1.id
left join (select max(dtrcvtm) as max_dtrcvtm, dvcid
from vhsrdt
group by dvcid
) as dt on dt.dvcid = vhsrdt.dvcid
and dt.max_dtrcvtm = vhsrdt.dtrcvtm
无论我们是否需要，上述方法都限制了计算所有 dvcid 的 max() 值。此外，它会导致 temp table 创建（如果小则在内存中，否则在磁盘上）。值得庆幸的是，MySQL 8.0.14 引入了 LATERAL Derived Table；所以如果你可以升级到最新的 MySQL 版本，更多 performant 查询将是：
select
dvc1.id,
dvc1.name as dvcname,
dvc1.dvceui as dvcdvceui,
dvc1.shrtname as dvcshrtname,
markclr.color as markclrcolor,
v1.dtrcvtm,
v1.dtgtwid,
v1.dtriptype,
v1.dlatitudedeg,
v1.dlongitudedeg,
v1.dspeedkmph,
v1.dheadingdeg
from dvc as dvc1
left join dvcgrp on dvcgrp.id = dvc1.dvcgrpid
left join markclr on markclr.id = dvcgrp.markclrid
left join vhsrdt v1 on v1.dvcid = dvc1.id
left join lateral (select max(v2.dtrcvtm) as max_dtrcvtm
from vhsrdt v2
where v2.dvcid = v1.dvcid
) as dt on dt.max_dtrcvtm = v1.dtrcvtm