Re: [GENERAL] Comibining UPDATE ... SET ... FROM (SELECT ...) with a JOIN

[Alexander Farber]

Thank you Brian and others, but -

On Fri, Sep 9, 2016 at 5:22 PM, Brian Dunavant  wrote:

> I'm making the assumption that you only have one ip/user in words_users.
>
> with lockrow as (
>SELECT g.gid, u.ip
>FROM   words_games g join words_users u
> ON (g.player1 = u.uid)
>   WHERE  g.finished IS NULL
> ANDg.player1 <> in_uid
> ANDg.played1 IS NOT NULL
> ANDg.player2 IS NULL
>LIMIT  1
>FORUPDATE SKIP LOCKED
> ), do_the_update as (
>UPDATE words_games g1
>SETplayer2 = in_uid
>FROM lockrow g2
>WHERE g1.gid = g2.gid
>RETURNING g1.gid, g1.player2
> )
> select m.gid into out_gid, u.ip into out_uip
> from do_the_update m
>   join lockrow u on (gid)
> ;
>
> The general idea being lock the row in the first CTE, update it in the
> second, returning your values, and then query against those in the
> final select to get the ip.  If it didn't update anything, you'll get
> no results.
>

unfortunately, the above query does not seem to ensure, that players with
same ip can not join the same game, which is actually my question...

But thanks for showing the CTE for UPDATE ... RETURNING - that is probably
the way to go for me

Regards
Alex

[GENERAL] Comibining UPDATE ... SET ... FROM (SELECT ...) with a JOIN

[Alexander Farber]

Good afternoon,

I have a question please.

In one table I store user ids and their IP addresses -

CREATE TABLE words_users (
uid SERIAL PRIMARY KEY,
ip inet NOT NULL
);

And in another table I keep 2-player games and timestamps of last moves
(NULL if a player hasn't played yet):

CREATE TABLE words_games (
gid SERIAL PRIMARY KEY,
finished timestamptz,

player1 integer REFERENCES words_users(uid) ON DELETE
CASCADE NOT NULL,
player2 integer REFERENCES words_users(uid) ON DELETE
CASCADE,

played1 timestamptz,
played2 timestamptz
);

When a user wants to start a new game, I first check if there is maybe a
new game already available - with just 1 player while the other "seat" is
vacant:

UPDATE words_games g1
SETplayer2 = in_uid
FROM (
SELECT gid
FROM   words_games
WHERE  finished IS NULL
ANDplayer1 <> in_uid
ANDplayed1 IS NOT NULL
ANDplayer2 IS NULL
LIMIT  1
FORUPDATE SKIP LOCKED
) g2
WHERE g1.gid = g2.gid
RETURNING g1.gid
INTO  out_gid;

This code works great, but now I am trying to add an (obviously not solving
all cheating/proxy/etc. problems) check, that the IP addresses of both
users must be different.

Fetching "ip" in the internal SELECT statement is trivial with:

UPDATE words_games g1
SETplayer2 = in_uid
FROM (
SELECT g.gid, u.ip
FROM   words_games g, words_users u
WHERE  g.finished IS NULL
ANDg.player1 <> in_uid
ANDg.played1 IS NOT NULL
ANDg.player2 IS NULL
ON (g.player1 = u.uid)
LIMIT  1
FORUPDATE SKIP LOCKED
) g2
WHERE g1.gid = g2.gid
RETURNING g1.gid
INTO  out_gid;

But how to fetch the "ip" column in the surrounding UPDATE statement?

Thank you
Alex