Re: Request for review of new redis-fdw module

[Leon Dang <ldang@xxxxxxxxxxxxxx>]

George Silva wrote on 01/28/2015 11:49 AM:

Great. Congratulations.

How big is the latency in the FDW? This opens up new possibilities using redis. Very cool.

# explain analyze insert into rstr values ('k4434', '3234234');
                                         QUERY PLAN                                        
--------------------------------------------------------------------------------------------
 Insert on rstr  (cost=0.00..0.01 rows=1 width=0) (actual time=0.288..0.288 rows=0 loops=1)
   ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.001..0.002 rows=1 loops=1)
 Planning time: 0.092 ms
 Execution time: 0.582 ms

# explain analyze select * from rstr where key = 'k4434';
                                             QUERY PLAN                                             
-----------------------------------------------------------------------------------------------------
 Foreign Scan on rstr  (cost=10.00..11.00 rows=1 width=68) (actual time=0.541..0.595 rows=1 loops=1)
 Planning time: 0.382 ms
 Execution time: 0.642 ms



I did do a benchmark in Golang to see the difference between redis-fdw, temp-table, and table:

SELECT:
  Redis FDW:   240663 ns/op
 TEMP TABLE: 1130329 ns/op
 TABLE:       764774 ns/op

INSERT:
 Redis FDW:   187788 ns/op
 TEMP TABLE:  106843 ns/op
 TABLE:      3093156 ns/op


redis-fdw is currently unoptimized (no table option caching, etc) so there is room for improvement. But so far so good.

Leon

> I've implemented a completely new Redis FDW module which has little to do with
> github.com/pg-redis-fdw/redis_fdw; although I did take some inspiration from in
> on how the tables were to be designed but most I got from looking at the
> oracle-fdw.
>
> My redis-fdw implementation supports read and write to the Redis backend, so
> you can do insert, update, and delete. e.g. you can define a hash table as:

is it possible to write multiple row's into the redis? something like

insert into foreign_redis_table select * from big_table

Thanks for pointing this out. I had a small bug which didn't let it succeed, but now it's been fixed and committed; I've also added a bulkdata.sql test script in the code to show an example.

Anyway, thx, compiled and installed (just for fun, i'm not familiar with
redis, and i'm not a coder)

Redis is great for session management as it allows you to set an expiry for each key. So by using redis_fdw, you don't need to do multiple queries to determine of the session is still valid. e.g.:

-- use a string (key-value) dataset for user sessions
CREATE FOREIGN TABLE rsessions(
    sessid TEXT,
    value  TEXT,
    expiry INT
) SERVER localredis
  OPTIONS (tabletype 'string');
ALTER FOREIGN TABLE rsessions ALTER COLUMN sessid OPTIONS (ADD param 'key');

-- a user table in postgres, can contain a whole bunch of other fields.
CREATE TEMP TABLE users (
    userid   INT,
    username TEXT,
    sessid   TEXT
);


--
-- get user's details at the same time as determining if they're session is still valid
--
WITH u AS (SELECT * FROM users WHERE username = 'foo')
SELECT u.*, r.value, r.expiry
FROM rsessions r, u
WHERE r.sessid = (SELECT u.sessid FROM u);



If the user's session is still valid then a row will be returned (Redis automatically destroys the key on expiry).


--
-- to reset the expiry timeout for the user
--
UPDATE rsessions SET expiry = 40 WHERE sessid = $1;



Leon