Is there a command in Redis for HASH data structure similar to MGET? - redis

Question

I need to get in one call all data fields for a set of known REDIS hash keys. I've used MGET for string keys such as :
MGET key [key ...]
Available since 1.0.0.
Time complexity: O(N) where N is the number of keys to retrieve.
Returns the values of all specified keys. For every key that does not hold a string value or does not exist, the special value nil is returned. Because of this, the operation never fails.
HMGET only brings all fields for one key. I need many keys all fields by key.

Answer 1

There is no command like that, redis hashes work within the hash, so HMGET work inside one hash and give all the fields in that hash. There is no way to access all the fields in multiple hashes at ones.
However you can user several HMGET on each hash and get all the fields. you can pipeline these commands to execute in a one go.
Option 1
Ex. implementation in pseudo code
Pipeline p
List<String> = p.hgetall('key1', fields...);
List<String> = p.hgetall('key2', fields...);
List<String> = p.hgetall('key3', fields...);
p.exec();
Option 2
Other option is to write a LUA script and call that using EVAL
local array = {}
local keys = redis.call('KEYS', '<your pattern>')
for _,key in ipairs(keys) do
local val = redis.call('HGETALL', key)
array[#array + 1] = val
end
return array
Call the lua sctipt
redis-cli EVAL "$(cat test.lua)" 0
1) 1) "field1"
2) "val"
2) 1) "field1"
2) "val"
3) "field2"
4) "val2"

Answer 2

As noted in another answer, there's no built in way but there more workarounds besides a transaction.
Option 1: use a Lua script (i.e. EVAL "..." 3 myhash1 myhash2 myhash3 myfield)
local r = {}
while (#KEYS > 0) do
local k = table.remove(KEYS,1)
r[#r+1] = redis.call('HGET', k, ARGV[1])
end
return r
Option 2: write a Redis module
Out of scope as a an answer :)