[原文链接：https://www.dubby.cn/detail.html?id=9112](https://www.dubby.cn/detail.html?id=9112)
>这里说的大key是指包含很多元素的set,sorted set,list和hash。
删除操作，我们一般想到有2种，`del`和`expire`。
# DEL
>**Time complexity: O(N)** where N is the number of keys that will be removed. When a key to remove holds a value other than a string, the individual complexity for this key is O(M) where M is the number of elements in the list, set, sorted set or hash. Removing a single key that holds a string value is O(1).
如果要删除的key是一个集合，包含了很多元素，那么`DEL`时的耗时和元素个数成正比，所以如果直接`DEL`，会很慢。
# EXPIRE
>Note that calling EXPIRE/PEXPIRE with a non-positive timeout or EXPIREAT/PEXPIREAT with a time in the past will result in the key being deleted rather than expired (accordingly, the emitted key event will be del, not expired).
想着expire会不会可以不是直接删除，可惜官网的描述让我心灰意冷，如果expire后指定的timeout不是正数，也就是<=0，那其实就是`DEL`。
# 一点一点删
我们知道Redis的工作线程是单线程的，如果一个command堵塞了，那所有请求都会超时，这时候，一些骚操作也许可以帮助你。
其实如果想删除key，可以分解成2个目的，1：不想让其他人访问到这个key，2：释放空间。
那其实我们可以分解成两步，先用`RENAME`把原先的key rename成另一个key，比如：
```
RENAME userInfo:123 "deleteKey:userInfo:123"
```
然后可以慢慢去删"deleteKey:userInfo:123"，如果是set，那么可以用`SREM`慢慢删，最后再用`DEL`彻底删掉。
>这里可以搞个task去`SCAN deleteKey:*`，然后慢慢删除。
# UNLINK
Redis 4.0.0提供了一个更加方便的命令
>Available since 4.0.0.
>**Time complexity: O(1)** for each key removed regardless of its size. Then the command does O(N) work in a different thread in order to reclaim memory, where N is the number of allocations the deleted objects where composed of.
`UNLINK`其实是直接返回，然后在后台线程慢慢删除。
如果你的Redis版本>=4.0.0，那么强烈建议使用`UNLINK`来删除。
# 删除耗时测试结果
>单位：微秒
|Set个数|DEL|EXPIRE|UNLINK|
|--|--|--|--|
|1|90|97|75|
|10|79|67|100|
|100|51|49|47|
|1000|303|296|49|
|10000|2773|2592|52|
|100000|31210|33157|51|
|1000000|549388|501536|62|
```
package main
import (
"github.com/go-redis/redis"
"fmt"
"time"
)
func main() {
client := redis.NewClient(&redis.Options{
Addr: "localhost:6379",
Password: "",
DB: 0,
ReadTimeout: 1000 * 1000 * 1000 * 60 * 60 * 24,
WriteTimeout: 1000 * 1000 * 1000 * 60 * 60 * 24,
})
maxLength := int64(10000 * 100)
for n := int64(1); n <= maxLength; n *= 10 {
fmt.Println("Set个数", n)
TestDelBigSet(client, n)
TestExpireBigSet(client, n)
TestUnlinkBigSet(client, n)
fmt.Println()
}
}
func TestDelBigSet(client *redis.Client, count int64) {
redisKey := fmt.Sprintf("%s%d", "del:", time.Now().Nanosecond())
for n := int64(0); n < count; n++ {
err := client.SAdd(redisKey, fmt.Sprintf("%d", n)).Err()
if err != nil {
panic(err)
}
}
startTime := CurrentTimestampInMicroSecond()
client.Del(redisKey)
endTime := CurrentTimestampInMicroSecond()
fmt.Println("Del", endTime-startTime)
}
func TestUnlinkBigSet(client *redis.Client, count int64) {
redisKey := fmt.Sprintf("%s%d", "unlink:", time.Now().Nanosecond())
for n := int64(0); n < count; n++ {
err := client.SAdd(redisKey, fmt.Sprintf("%d", n)).Err()
if err != nil {
panic(err)
}
}
startTime := CurrentTimestampInMicroSecond()
client.Unlink(redisKey)
endTime := CurrentTimestampInMicroSecond()
fmt.Println("Unlink", endTime-startTime)
}
func TestExpireBigSet(client *redis.Client, count int64) {
redisKey := fmt.Sprintf("%s%d", "expire:", time.Now().Nanosecond())
for n := int64(0); n < count; n++ {
err := client.SAdd(redisKey, fmt.Sprintf("%d", n)).Err()
if err != nil {
panic(err)
}
}
startTime := CurrentTimestampInMicroSecond()
client.Expire(redisKey, 0)
endTime := CurrentTimestampInMicroSecond()
fmt.Println("Expire", endTime-startTime)
}
func CurrentTimestampInMicroSecond() int64 {
return time.Now().UnixNano() / 1000
}
```
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