Reference ARRAY_AGG – length in WHERE clause

aggregatepostgresql

My question is similar to possible reference jsonb_array_length in a where clause, however I cannot use ORDER BY.

I have following tables:

CREATE TABLE IF NOT EXISTS games
(
  id SERIAL PRIMARY KEY,
  ...
)

CREATE TABLE IF NOT EXISTS users
(
  id SERIAL PRIMARY KEY,
  username CHARACTER VARYING(100) NOT NULL UNIQUE,
  ...
)

CREATE TABLE IF NOT EXISTS game_users
(
    game_id INTEGER REFERENCES games (id) NOT NULL,
    user_id INTEGER REFERENCES users (id) NOT NULL,
    joined_at TIMESTAMPTZ NOT NULL,
    UNIQUE (game_id, user_id)
);

I want to query games that have less than 4 participants.

SELECT gm.*,
  array_agg(usr.id) AS users_id,
  array_agg(gmUsr.joined_at) AS users_joined,
  array_agg(usr.username) AS users_username
FROM games AS gm
  LEFT JOIN game_users AS gmUsr ON gmUsr.game_id = gm.id
  LEFT JOIN users AS usr ON gmUsr.user_id = usr.id
WHERE cardinality(array_length(users_id)) < 4
GROUP BY gm.id

I already saw in an answer that the WHERE clause is executed before the SELECT one. So my query will not work.
However I want to keep GROUP BY user id because I want aggregated data as:

type GamesJoinedUsers = {
  id: number,
  users_id: number[],
  users_joined: Date[],
  users_username: string[],
}

How to query such data at least somehow?

Is it possible to do it in one query without subquery?

Best Answer

It seems to me you just need to replace your WHERE clause with a HAVING clause like this:

HAVING
  COUNT(isr.id) < 4

The HAVING clause is the appropriate place for a filter that needs to apply to an aggregated result. In this case, you want a filter on the number of users per game, which is an aggregate result given your GROUP BY. Therefore, use HAVING.

Related Solutions

Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Postgresql – Aggregate all joined rows with where clause

If I understand you requirements, this consists of selecting all the users that have a project in step 3, and using that list to select all the projects for just those users.

Something like:

SELECT ug.id
     , STRING_AGG(DISTINCT p.step_number, ', ') AS summary
FROM user_groups ug
    INNER JOIN (
            SELECT u.user_group_id
                 , u.user_id
            FROM users u
                INNER JOIN projects p ON u.user_id = p.user_id
            WHERE p.step_number = 3
            GROUP BY u.user_group_id, u.user_id
        ) Step3Projects ON ug.user_group_id = Step3Projects.user_group_id 
    INNER JOIN projects p ON Step3Projects.user_id = p.user_id
GROUP BY ug.id;

Best Answer

Related Solutions

Postgresql – Postgres multiple joins slow query, how to store default child record

pgAdmin timing

Postgresql – Aggregate all joined rows with where clause

Related Question