How to copy values from one list into another in Redis?

Question

I have a Redis list with some values

LRANGE LIST 0 -1
> 1
> 2
> 3

And I want to RPUSH these values into another list. How can this be done? I've tried to do it with MULTI and EXEC, but with no results.

Answer 1

A server-side Lua script is more convenient that a WATCH/MULTI/EXEC block to implement this kind of operation.

Here is an example of a script with takes two lists (source and destination) as parameters, and two integers defining a range in the source list. It then pushes the corresponding items to the destination list.

> rpush foo 1 2 3 4
(integer) 4
> rpush bar x
(integer) 1
> eval "local res = redis.call( 'lrange', KEYS[1], ARGV[1], ARGV[2] ); return redis.call( 'rpush', KEYS[2], unpack(res) ); "  2  foo bar 0 -1
(integer) 5
> lrange bar 0 -1
1) "x"
2) "1"
3) "2"
4) "3"
5) "4"

Answer 2

if you want to move the key to a new key, you can use RENAME command , the only will change the key name RENAME COMMAND

Answer 3

this approach

> eval "local res = redis.call( 'lrange', KEYS[1], ARGV[1], ARGV[2] ); return redis.call( 'rpush', KEYS[2], unpack(res) ); "  2  foo bar 0 -1

may produce "too many results to unpack" error when list is too long.

here is a script to do this

-- @desc:   copies a list with POP and PUSH
-- @usage:  redis-cli --eval copy_list_with_popnpush.lua <source> <dest>

local s = KEYS[1]
local d = KEYS[2]
local l = redis.call("LLEN", s)
local i = tonumber(l)

while i > 0 do
  local v = redis.call("RPOPLPUSH", s, s)
  redis.call("LPUSH", d, v)
  i = i - 1
end

return l

some other awesome scripts https://gist.github.com/itamarhaber/d30b3c40a72a07f23c70

Answer 4

SUNIONSTORE

SUNIONSTORE new_set old_set

https://redis.io/commands/sunionstore

Answer 5

You can move them from one list to another and LPUSH them using the RPOPLPUSH command multiple times:

RPOPLPUSH old_list new_list
RPOPLPUSH old_list new_list
RPOPLPUSH old_list new_list

Of course you might want to do this in a client program or script, there's no way I can find to move all members of a list to another list.