什么是慢查询
和mysql的慢SQL日志分析一样，redis也有类似的功能，来帮助定位一些慢查询操作。
Redis slowlog是Redis用来记录查询执行时间的日志系统。
查询执行时间指的是不包括像客户端响应(talking)、发送回复等IO操作，而单单是执行一个查询命令所耗费的时间。
另外，slow log保存在内存里面，读写速度非常快，因此你可以放心地使用它，不必担心因为开启slow log而损害Redis的速度。
慢查询参数
首先来关注下慢日志分析对应的两个参数：
1、slowlog-log-slower-than：预设阀值，即记录超过多少时间的记录，默认为10000微秒，即10毫秒。
2、slowlog-max-len：记录慢查询的条数，默认为128条，当超过设置的条数时最早进入队列的将被移除。线上建议增大数值，如：1000，这样可减少队列移除的频率。
- 127.0.0.1:6379> config get slowlog-log-slower-than
- 1) "slowlog-log-slower-than"
- 2) "10000"
- 127.0.0.1:6379> config get slowlog-max-len
- 1) "slowlog-max-len"
- 2) "128"
可以用config set对这两个参数进行调整，或者在配置文件中设置。
- ################################## SLOW LOG ###################################
- # The Redis Slow Log is a system to log queries that exceeded a specified
- # execution time. The execution time does not include the I/O operations
- # like talking with the client, sending the reply and so forth,
- # but just the time needed to actually execute the command (this is the only
- # stage of command execution where the thread is blocked and can not serve
- # other requests in the meantime).
- #
- # You can configure the slow log with two parameters: one tells Redis
- # what is the execution time, in microseconds, to exceed in order for the
- # command to get logged, and the other parameter is the length of the
- # slow log. When a new command is logged the oldest one is removed from the
- # queue of logged commands.
- # The following time is expressed in microseconds, so 1000000 is equivalent
- # to one second. Note that a negative number disables the slow log, while
- # a value of zero forces the logging of every command.
- slowlog-log-slower-than 10000
- # There is no limit to this length. Just be aware that it will consume memory.
- # You can reclaim memory used by the slow log with SLOWLOG RESET.
- slowlog-max-len 128
慢查询命令
语法：slowlog subcommand [argument]
如，进行查询慢查询、获取慢查询记录的数量、重置慢查询日志等操作：
- 192.168.10.38:9001> slowlog get
- (empty list or set)
- 192.168.10.38:9001> slowlog get 10
- (empty list or set)
- 192.168.10.38:9001> slowlog len
- (integer) 0
- 192.168.10.38:9001> slowlog reset
- OK
