Re: Selecting records with highest timestamp - for a join

Please let me rephrase my question so that it is better understandable -

In PostgreSQL 9.5.3 I keep player infos from various social networks:

# TABLE words_social;
sid | social | female | given | family | photo | place | stamp |
uid
-------+--------+--------+---------+--------+-------+-------+------------+-----
aaaaa | 1 | 0 | Abcde1 | | | | 1470237061 |
1
aaaaa | 2 | 0 | Abcde2 | | | | 1477053188 |
1
aaaaa | 3 | 0 | Abcde3 | | | | 1477053330 |
1
kkkkk | 3 | 0 | Klmnop3 | | | | 1477053810 |
2
kkkkk | 4 | 0 | Klmnop4 | | | | 1477053857 |
2
ggggg | 2 | 0 | Ghijk2 | | | | 1477053456 |
3
ggggg | 3 | 0 | Ghijk3 | | | | 1477053645 |
3
ggggg | 4 | 0 | Ghijk4 | | | | 1477053670 |
3
xxxxx | 4 | 0 | Xyzok | | | | 1470237393 |
4
(9 rows)

The 1,2,3,4 in column "social" means "Facebook", "Twitter", etc.

For a player I can always select her "most recent" info by:

# select * from words_social s1 WHERE stamp = (SELECT max(stamp) FROM
words_social s2 WHERE s1.uid = s2.uid);
sid | social | female | given | family | photo | place | stamp |
uid
-------+--------+--------+---------+--------+-------+-------+------------+-----
aaaaa | 3 | 0 | Abcde3 | | | | 1477053330 |
1
kkkkk | 4 | 0 | Klmnop4 | | | | 1477053857 |
2
ggggg | 4 | 0 | Ghijk4 | | | | 1477053670 |
3
xxxxx | 4 | 0 | Xyzok | | | | 1470237393 |
4
(4 rows)

Then there is another table storing current games (I have omitted some
columns with game data below):

# select gid, created, finished, player1, player2 from words_games;
gid | created | finished | player1 | player2
-----+-------------------------------+----------+---------+---------
1 | 2016-10-21 14:51:12.624507+02 | | 4 | 1
2 | 2016-10-21 14:51:22.631507+02 | | 3 |
(2 rows)

Whenever a user (for example with uid=1) connects to the server, I send her
the games she is taking part in:

# select gid, created, finished, player1, player2 from words_games where
player1=1
union select gid, created, finished, player2, player1 from words_games
where player2=1;
gid | created | finished | player1 | player2
-----+-------------------------------+----------+---------+---------
1 | 2016-10-21 14:51:12.624507+02 | | 4 | 1
(1 row)

My problem: to the above UNION SELECT statement I need to add user infos
from words_social table.

(So that I can display user photos and names above the game board)

So I try this with CTE:

# with user_infos AS (select * from words_social s1 WHERE stamp = (SELECT
max(stamp) FROM words_social s2 WHERE s1.uid = s2.uid))
select g.gid, g.created, g.finished, g.player1, g.player2,
i.given from words_games g join user_infos i on (g.player1=i.uid) where
g.player1=1
union select g.gid, g.created, g.finished, g.player2, g.player1, i.given
from words_games g join user_infos i on (g.player2=i.uid) where g.player2=1;
gid | created | finished | player1 | player2 | given
-----+-------------------------------+----------+---------+---------+--------
1 | 2016-10-21 14:51:12.624507+02 | | 1 | 4 | Abcde3
(1 row)

This works well (I have advanced since me first asking few days ago), but I
still have the following problem -

I am worried that the CTE-table user_infos will get very large, once my
game has many players.

How to rewrite my query, so that I fetch games and users (player1, player2)
for a certain user id (uid) - without making huge intermediate tables?

Thank you
Alex