EXISTSCount how many of the given keys exist.
| Syntax | EXISTS key [key ...] |
| Group | generic |
| Arity | -2 |
| Since | 1.0.0 |
| Flags | readonly |
| Complexity | O(N) keys, one O(1) probe each |
| Redis compatibility | differs: the multi-key form is gathered per shard, so the count is not a point-in-time snapshot across shards |