Re: Same condition in the CTE and in the subsequent JOIN using it

On Wed, 25 Apr 2018 17:45:39 +0200
Alexander Farber <alexander.farber@gmail.com> wrote:

>         WITH cte AS (
>                 SELECT
>                         DATE_TRUNC('day', m.played) AS day,
>                         m.mid,
>                         EXTRACT(EPOCH FROM m.played - LAG(m.played) OVER
> (PARTITION BY m.gid ORDER BY m.played))::int/60 AS diff
>                 FROM    words_moves m
>                 JOIN    words_games g ON (m.gid = g.gid)
>                 JOIN    words_social s ON (s.uid IN (g.player1, g.player2))
>                 WHERE   s.social = in_social                           --
> CAN THIS BE REFERRED TO FROM BELOW?
>                 AND     s.sid = in_sid
>                 AND     m.played > CURRENT_TIMESTAMP - interval '1 month'
>         )
>         SELECT
>                 TO_CHAR(c.day, 'DD.MM.YYYY'),
>                 ROUND(AVG(c.diff)),
>                 ROUND(AVG(m.score), 1)
>         FROM    words_moves m
>         JOIN    cte c using(mid)
>         JOIN    words_social s USING(uid)
>         WHERE   s.social = in_social
>         AND     s.sid = in_sid
>         AND     m.action = 'play'
>         GROUP BY c.day
>         ORDER BY c.day;
>
> $func$ LANGUAGE sql STABLE;
>
> By looking at the above source code, do you think, that the condition being
> used twice (the s.social = in_social AND s.sid = in_sid) is "too much" and
> can be optimized? :-)

I would say so, because as you've already applied the filter in the CTE it won't have any effect.

But anyway, since you are not using any column from words_social in your main query, you can do away with it entirely
andjust remove  
>         JOIN    words_social s USING(uid)
>         WHERE   s.social = in_social
>         AND     s.sid = in_sid


--
                    Bien à vous, Vincent Veyron

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