How to count the number of keys matching a pattern?

From here:

eval "return #redis.pcall('keys', 'abc:*')" 0

It's not O(1), but at least the count is done on the server side.

From the command line, redis-cli --scan --pattern 'abc:*' | wc -l

DISCLAIMER I hope this old answer haven't damaged any production systems, with millions of keys. If you still want to still count the matching keys of redis in production for some reason, better use scan with a match pattern.

If you simply search with KEYS, with your redis client, you will get a number list of all you matching keys, right?

e.g.

KEYS abc:*

will give you

1) abc:random-text-1
2) abc:random-text-2

or you can run the following:

./redis-cli KEYS "abc:*" | wc -l

and you will get 2 as an output.

By considering the performance, I would not recommend you use KEYS

Warning: 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. This command is intended for debugging and special operations, such as changing your keyspace layout. Don't use KEYS in your regular application code. If you're looking for a way to find keys in a subset of your keyspace, consider using sets.

I would suggest you considering scan, if your redis version > 2.8.0. But it rely on which data type you are going to use.

Here is an simple example from redis doc:

redis 127.0.0.1:6379> sadd myset 1 2 3 foo foobar feelsgood
(integer) 6
redis 127.0.0.1:6379> sscan myset 0 match f*
1) "0"
2) 1) "foo"
   2) "feelsgood"
   3) "foobar"