详解MySQL慢日志（下） 选项
参数篇：
http://blog.itpub.net/29773961/viewspace-2147352/
〇 long_query_time
场景：
如下图，该图为部分
binlog截取：
9：42：25 后，还有几个6：35：30的event
但是这些event如图中最后一条。
exec_time为11216，但并未被记录到slow log中。
long_query_time 为一个MySQL选项参数。
这个参数不用说了，记录超过执行时间超过该值以上的SQL。
但这个坑在于：是按真正执行的时间（real time），不包括等待锁的时间。
举个简单的例子：
如果long_query_time设置为1秒
一个insert被lock了10秒，执行只耗了0.5秒，那么不会被记录到慢日志。
测试，以下分为三个会话，分别被命名为
lock>，
query>，
slow_log>，下同：
-
lock
>
FLUSH
TABLE
WITH READ LOCK
;
-
Query OK
,
0 rows affected
(
0
.
00 sec
)
-
query
>
SET
profiling
=
1
,
SESSION long_query_time
=
0
.
5
;
-
Query OK
,
0 rows affected
,
1 warning
(
0
.
00 sec
)
-
query
>
INSERT
INTO
t0
SELECT
null
,
create_time
,
uuid
FROM
t1 LIMIT 1
;
-
{
此时hang住
}
-
lock
>
UNLOCK
TABLE
;
-
query
>
{
返回
}
-
Query OK
,
1 row affected
(
9
.
42 sec
)
- Records: 1 Duplicates: 0 Warnings: 0
此处看到，整条SQL花费了9.42秒完成，其中包括长时间的锁等待。
再看一下具体的profile：
-
query
>
SHOW
PROFILES
;
-
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Query_ID
|
Duration
|
Query
|
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-
|
1
|
9
.
41687900
|
INSERT
INTO
t0
SELECT
null
,
create_time
,
uuid
FROM
t1 LIMIT 1
|
-
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-
1 row
in
set
,
1 warning
(
0
.
00 sec
)
-
query
>
SHOW
PROFILE
FOR
QUERY 1
;
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+
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|
Status
|
Duration
|
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-
|
starting
|
0
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000101
|
-
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checking permissions
|
0
.
000009
|
-
|
checking permissions
|
0
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000005
|
-
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Opening tables
|
0
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000016
|
-
|
Waiting
for
global read lock
|
9
.
412835
|
-
|
Opening tables
|
0
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000540
|
-
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init
|
0
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000033
|
-
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System lock
|
0
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000014
|
-
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optimizing
|
0
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000006
|
-
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statistics
|
0
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000020
|
-
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preparing
|
0
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000016
|
-
|
executing
|
0
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000004
|
-
|
Sending data
|
0
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000241
|
-
|
end
|
0
.
000006
|
-
|
query
end
|
0
.
002504
|
-
|
closing tables
|
0
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000012
|
-
|
freeing items
|
0
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000025
|
-
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logging slow query
|
0
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000009
|
-
|
Opening tables
|
0
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000113
|
-
|
System lock
|
0
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000362
|
-
|
cleaning up
|
0
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000011
|
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21 rows
in
set
,
1 warning
(
0
.
00 sec
)
可以看到，等待全局读锁花了9.412835s，总执行时间约为9.42。
再在slow_log表中查一下……什么都没有：
-
slow_log
>
SELECT
start_time
,
query_time
,
lock_time
,
sql_text
FROM
mysql
.
slow_log
;
-
Empty
set
(
0
.
00 sec
)
-
query
>
SELECT
9
.
41687900-9
.
412835
;
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9
.
41687900-9
.
412835
|
-
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0
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00404400
|
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+
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1 row
in
set
(
0
.
00 sec
)
tips：
此时SQL执行时间为0.00404400s，故没有被记录到slow log中。
也可以解释图中，某些event执行了3个小时，但又无法在slow log中查询到。
〇 lock_time与query_time
为slow log中所记录的两个属性：
lock_time：waiting for xxx lock的时间
query_time：real time + lock time的总时间
-
query
>
SET
SESSION long_query_time
=
0
;
-
Query OK
,
0 rows affected
(
0
.
00 sec
)
-
lock
>
LOCK
TABLE
test
.
t0 WRITE
;
-
Query OK
,
0 rows affected
(
0
.
00 sec
)
-
query
>
SELECT
*
FROM
t0 LIMIT 1
;
-
{
此时hang住
}
-
lock
>
LOCK
TABLE
test
.
t0 WRITE
;
-
Query OK
,
0 rows affected
(
0
.
00 sec
)
-
query
>
{
返回
}
-
+
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id
|
create_time
|
uuid
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2
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2017-11-14 15:13:33
|
994e4592-93b4-11e7-bff9-525400b3819a
|
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+
-
1 row
in
set
(
28
.
89 sec
)
-
slow_log
>
SELECT
start_time
,
query_time
,
lock_time
,
sql_text
FROM
mysql
.
slow_log
;
-
+
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+
-
|
start_time
|
query_time
|
lock_time
|
sql_text
|
-
+
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-
|
2017-11-15 17:13:12
.
252836
|
00:00:28
.
894675
|
00:00:28
.
894516
|
SELECT
*
FROM
t0 LIMIT 1
|
-
+
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+
可以看到做一个简单查询，query_time也很长：
实际上query_time记录的是lock_time + real time。
query_time ≥ lock_time
〇 start_time
为slow log中所记录的属性：
start_time：看字面意思很容易会被误认为“sql开始的时间”…
但实际上记录的是sql结束的时间。
测试一下：
-
query
>
SET
SESSION long_query_time
=
0
;
-
Query OK
,
0 rows affected
(
0
.
00 sec
)
-
query
>
SELECT
sysdate
(
)
,
sleep
(
8
)
,
sysdate
(
)
;
-
+
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–
–
+
-
|
sysdate
(
)
|
sleep
(
8
)
|
sysdate
(
)
|
-
+
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-
|
2017-11-15 17:05:15
|
0
|
2017-11-15 17:05:23
|
-
+
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+
-
1 row
in
set
(
8
.
00 sec
)
可以看到，该sql开始时间是17:05:15，结束时间是17:05:23
那么记录在slow log中，实际上是：
-
slow_log
>
SELECT
start_time
,
query_time
,
lock_time
,
sql_text
FROM
mysql
.
slow_log
;
-
+
–
–
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+
-
|
start_time
|
query_time
|
lock_time
|
sql_text
|
-
+
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-
|
2017-11-15 17:05:23
.
633771
|
00:00:08
.
000359
|
00:00:00
.
000000
|
SELECT
sysdate
(
)
,
sleep
(
8
)
,
sysdate
(
)
|
-
+
–
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–
+
-
1 row
in
set
(
0
.
00 sec
)
可以看到，start_time实际上是sql执行完成的时间。
真正的开始时间计算的方法也很简单：
start_time – query_time 即为sql真正开始的时间。
tips：
一般OLTP场景下，大部分query_time都会很短。
但在某些糟糕的场景下，如某一条OLAP语句执行时间很长，如30分钟。
如果需要确认在某个时段的sql，在查询slow log时指定错误的start_time，可能就无法找到合适的sql了。
作者微信公众号（持续更新）
