How to feed Set operations represented as symbols to CLI/pyredis?

[Tharmalingam Viswanathan]

Hi,

Perhaps this is a simple question for the experts, but I am struggling to solve this for a while now.

Let's say I have these sets stored in redis:  S1, S2, and S3.

Below is what I want to get evaluated within redis (without having to taking the route that involves pulling independent sets into numpy and operate on them):

(S1 + S2) - S3

i.e., Intersection of S1 and S2, and remove elements from S3.

So essentially I want to get a solution to get a much more complex operations like that evaluated (involves intersection and NOR stuff like that).

I would greatly appreciate if you could throw some lights on the above.

Thanks,

Tharma

[Michel Martens]

You can use set operations and store intermediate values, for example:

SUNIONSTORE temp S1 S2
SDIFF temp S3

Then you can delete the `temp` key. If that works for you, an
optimization would be to wrap those commands in a MULTI/EXEC block:

MULTI
SUNIONSTORE temp S1 S2
SDIFF temp S3
DEL temp
EXEC

At some point I had to do something similar but without knowing in
advance the operations I needed to perform. I wrote a Lua script
called Stal (https://github.com/soveran/stal) that receives an
s-expression in the form of a JSON array and returns the result. For
example, let's say you want to express the above operation as an
s-expression:

(SDIFF (SUNION S1 S2) S3)

Translated to a JSON array, it would be like this:

["SDIFF", ["SUNION", "S1", "S2"], "S3"]

And that's what you can ask Stal to solve:

$ redis-cli --eval stal.lua , '["SDIFF", ["SUNION", "S1", "S2"], "S3"]'

Stal will take care of creating and removing any temporary keys.

There are two wrappers available, one for Ruby and one for Crystal,
that allow you interact with Stal. Writing a wrapper for Python should
be trivial if you think this solves your problem.

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[Tharmalingam Viswanathan]

Thank you so much, Michel.

I found your stal.lua script solution is what I was looking for. 

Here is how it finally ended up like :

from redis_lua import load_script
import redis

# Loads the 'create_foo.lua' in the 'lua' directory.
script = load_script(name='stal', path='./')

redis_client = redis.StrictRedis(host='xx..xx..xx..xx', port=6379, db=0)

redis_client.sadd  ('A', 1 ,2, 3)
redis_client.sadd  ('B', 2 ,3, 4)
redis_client.sadd  ('C', 3 ,4, 5)


# Run the script with the specified arguments.
foo = script.get_runner(client=redis_client)(
    expr = '["SDIFF", ["SUNION", "A", "C"], "B"]'
)

print (foo)

Thank you!!! Really appreciated.

Tharma

[Michel Martens]

Glad it worked!