Re: Selecting records with highest timestamp - for a join

[Alexander Farber <alexander.farber@xxxxxxxxx>]

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