为何不可使用Redis的KEYS命令

在Redis KEYS命令的文档中,有如下一句话:

consider KEYS as a command that should only be used in production environments with extreme care. It may ruin performance when it is executed against large databases.

简而言之,KEYS命令性能很差,尽量不要在生产环境中使用。

不过官方文档也提到了,可以使用SCAN命令

那么,KEYS命令为什么性能差呢?

server.c中可以看到,KEYS命令被keysCommand函数处理

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void keysCommand(client *c) {
dictIterator *di;
dictEntry *de;
sds pattern = c->argv[1]->ptr;
int plen = sdslen(pattern), allkeys;
unsigned long numkeys = 0;
void *replylen = addDeferredMultiBulkLength(c);

di = dictGetSafeIterator(c->db->dict);
allkeys = (pattern[0] == '*' && pattern[1] == '0');
while((de = dictNext(di)) != NULL) {
sds key = dictGetKey(de);
robj *keyobj;

if (allkeys || stringmatchlen(pattern,plen,key,sdslen(key),0)) {
keyobj = createStringObject(key,sdslen(key));
if (expireIfNeeded(c->db,keyobj) == 0) {
addReplyBulk(c,keyobj);
numkeys++;
}
decrRefCount(keyobj);
}
}
dictReleaseIterator(di);
setDeferredMultiBulkLength(c,replylen,numkeys);
}

db本来就是一个dict,所以直接拿到一个dictIterator di,然后依次遍历,整个KEYS操作的复杂度是O(n),n为redis中key的数量。

这个操作,会阻塞到redis进程的。

可以看到,redis对于KEYS *这种情况做了优化。如果pattern是*,那么直接遍历,而不用判断是否match


Robert Lu

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