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

> On 25 Apr 2018, at 17:45, Alexander Farber <alexander(dot)farber(at)gmail(dot)com> wrote:

(…)

> And here is the function source code:
>
> CREATE OR REPLACE FUNCTION words_stat_scores(
> in_social integer,
> in_sid text
> ) RETURNS TABLE (
> out_day text,
> out_diff numeric,
> out_score numeric
> ) AS
> $func$
> 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? :-)

Actually, no. The conditions are part of different joins.

Within the CTE, you have a join that boils down to:

> FROM words_games g ON (m.gid = g.gid)
> JOIN words_social s ON (s.uid IN (g.player1, g.player2) AND s.social = in_social AND s.sid = in_sid)

In your outer query, you have:

> FROM words_moves m
> JOIN words_social s ON (s.uid = m.uid AND s.social = in_social AND s.sid = in_sid)

The joins are on different fields, in different tables even, so you can't just leave the conditions out because they filter different rows.

What you _can_ do is move the words_social JOIN and it's conditions into a new CTE and join with that instead. Something like so:

WITH words_in_social AS (
SELECT sid, uid
FROM words_social
WHERE social = in_social
AND sid = in_sid
),
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_in_social s ON (s.uid IN (g.player1, g.player2))
WHERE m.played > CURRENT_TIMESTAMP - interval '1
)
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_in_social s USING(uid)
WHERE m.action = 'play'
GROUP BY c.day
ORDER BY c.day;

That may look cleaner, but your original query probably performs better, since CTE's also act as an optimisation fence.

BTW, I suggest to use a better name for your CTE than cte; I'd rather use a name that clarifies its purpose.

> Thank you for any hints, I apologize if my question is too specific and difficult to answer...
>
> Regards
> Alex

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.