EXISTSCount how many of the given keys exist.
| 语法 | EXISTS key [key ...] |
| 分组 | generic |
| 参数个数 | -2 |
| 起始版本 | 1.0.0 |
| 标志 | readonly |
| 复杂度 | O(N) keys, one O(1) probe each |
| Redis 兼容性 | differs: the multi-key form is gathered per shard, so the count is not a point-in-time snapshot across shards |