Re: How to optimize SELECT query with multiple CASE statements? - Mailing list pgsql-general

On 10/31/2016 05:53 AM, Alexander Farber wrote:
> Good afternoon,
>
> is it please posible to optimize the following SQL query with numerous
> CASE statements (on same condition!) without switching to PL/pgSQL?

Offered with the following caveats, one I do not fully understand what
is going on below and two it may not optimize anything but might help
with the first caveat:

If you used plpgsql you could use IF conditions to organize the
assignment of values for player1 and player2 and eliminate some of the
duplication of effort shown in the CASE statements.

>
>     SELECT
>         g.gid,
>         EXTRACT(EPOCH FROM g.created)::int,
>         EXTRACT(EPOCH FROM g.finished)::int,
>         g.letters,
>         g.values,
>         g.bid,
>         m.tiles,
>         m.score,
>         /* HOW TO OPTIMIZE THE FOLLOWING CASE STATEMENTS? */
>         CASE WHEN g.player1 = in_uid THEN g.player1 ELSE g.player2 END,
>         CASE WHEN g.player1 = in_uid THEN g.player2 ELSE g.player1 END,
>         EXTRACT(EPOCH FROM CASE WHEN g.player1 = in_uid THEN g.played1
> ELSE g.played2 END)::int,
>         EXTRACT(EPOCH FROM CASE WHEN g.player1 = in_uid THEN g.played2
> ELSE g.played1 END)::int,
>         CASE WHEN g.player1 = in_uid THEN g.score1 ELSE g.score2 END,
>         CASE WHEN g.player1 = in_uid THEN g.score2 ELSE g.score1 END,
>         ARRAY_TO_STRING(CASE WHEN g.player1 = in_uid THEN g.hand1 ELSE
> g.hand2 END, ''),
>         REGEXP_REPLACE(ARRAY_TO_STRING(CASE WHEN g.player1 = in_uid THEN
> g.hand2 ELSE g.hand1 END, ''), '.', '?', 'g'),
>         CASE WHEN g.player1 = in_uid THEN s1.female ELSE s2.female END,
>         CASE WHEN g.player1 = in_uid THEN s2.female ELSE s1.female END,
>         CASE WHEN g.player1 = in_uid THEN s1.given ELSE s2.given END,
>         CASE WHEN g.player1 = in_uid THEN s2.given ELSE s1.given END,
>         CASE WHEN g.player1 = in_uid THEN s1.photo ELSE s2.photo END,
>         CASE WHEN g.player1 = in_uid THEN s2.photo ELSE s1.photo END,
>         CASE WHEN g.player1 = in_uid THEN s1.place ELSE s2.place END,
>         CASE WHEN g.player1 = in_uid THEN s2.place ELSE s1.place END
>     FROM words_games g
>         LEFT JOIN words_moves m ON m.gid = g.gid
>                 -- find move record with the most recent timestamp
>         AND NOT EXISTS (SELECT 1
>             FROM words_moves m2 WHERE m2.gid = m.gid
>             AND m2.played > m.played)
>     LEFT JOIN words_social s1 ON s1.uid = g.player1
>                 -- find social record with the most recent timestamp
>         AND NOT EXISTS (SELECT 1
>             FROM words_social s WHERE s1.uid = s.uid
>             AND s.stamp > s1.stamp)
>     LEFT JOIN words_social s2 ON s2.uid = g.player2
>                 -- find social record with the most recent timestamp
>         AND NOT EXISTS (SELECT 1
>             FROM words_social s WHERE s2.uid = s.uid
>             AND s.stamp > s2.stamp)
>     WHERE in_uid IN (g.player1, g.player2)
>     AND (g.finished IS NULL OR g.finished > CURRENT_TIMESTAMP - INTERVAL
> '1 day');
>
> It is a two-player, PostgreSQL-based game and in the statement above I
> am using the CASE-statements to ensure that always player1, given1,
> score1 columns are returned for the player in question.
>
> Here is a bit more context:
> http://stackoverflow.com/questions/40342426/how-to-optimize-select-query-with-multiple-case-statements
>
> Thank you
> Alex


--
Adrian Klaver
adrian.klaver@aklaver.com