How to search in redis for hash keys?

Question

I'm using hash keys to store user details like:

 hmset user:1 user_name lee  age 21
 hmset user:2 user_name david  age 25
 hmset user:3 user_name chris  age 25

I need to search for users having age = 25, name = lee. How to do a search for a specified value in a given field?

Mejia

A recently added page in Redis' documentation provides more information about Secondary indexing with Redis and covers this case under the Non range indexes section. – Saddlery

Answer 1

You cannot. Redis is a key-value store, not a relational database.

In order to search for a specific data, you need to build an access path to this data. For instance, to get the users having age = 25, you need to build an index to map the age values to users. It can be done with a set. This is the same for the name.

Once you have sets for age and name, you can search users by intersecting the sets. For example:

# Add 3 users
hmset user:1 user_name lee age 21
hmset user:2 user_name david age 25
hmset user:3 user_name chris age 25

# Maintain age index
sadd age:21 1
sadd age:25 2 3

# Maintain name index
sadd name:lee 1
sadd name:david 2
sadd name:chris 3

# Get the ID of users having age = 25 and name = lee
sinter age:25 name:lee
  -> will return an empty set

Sissie

I'm facing same design problem, but my question is after the command "sinter age:25 name:lee", the result should be list of id. So I need to query second time to get the real user data. Is this the only way to do? – Vibrio

If you want to perform a single roundtrip, a server-side Lua script will do what you want (i.e. sinter, and then one hgetall per selected item). – Sissie

It doesn't makes sense to maintain the HASH itself coz you're already storing the reverse index for all the HASH fields. – Majolica

So really the only advantage of using hashes in Redis rather than just saving marshalled objects (like JSON) as strings is that you can fetch single fields? – Cutshall

Now you can! This answer only applies to early versions of Redis. – Tori

Answer 2

Acually You can do it with put values in key

HMSET lee:25 user_name lee age 25
HMSET massi:43 user_name massi age 43
HMSET lee:24 user_name lee age 24
HMSET lee:28 user_name lee age 28 city Berlin

Now you can find them with "keys" command

127.0.0.1:6379> keys *:25
1) "lee:25"
127.0.0.1:6379> keys lee*
1) "lee:25"
2) "lee:24"
3) "lee:28"
127.0.0.1:6379> keys massi:43
1) "massi:43"

Finally find specific hashes

127.0.0.1:6379> HGETALL lee:24
1) "user_name"
2) "lee"
3) "age"
4) "24"

All that I said is that you can do it in two steps put any number of values that you need in key then find them. But consider that it's not a good idea to put all values inside key Just put ones that you need to do filtering with. Cheers :)

Benco

While this may work, I really would recommend against this for building anything serious. In the Redis docs, there's a specific warning against using the KEYS command in a production environment as it can degrade performance in a large database: redis.io/commands/keys. – Harbert

Answer 3

SCAN is the recommended way to search keys in Redis v6.0 and later.

 127.0.0.1:6379>  SCAN 0 TYPE hash COUNT 1000000

COUNT needs to be passed as last parameter, otherwise there will be a limit of 10 records returned by default.

Tori