CODEWITHSUNDEEP
sqlperformancepostgresqlwindow-functionspostgresql-performance
Will
Will

Reputation: 958

SQL Window Functions - SELECT DISTINCT ORDER BY LIMIT

I have these 3 tables in my PostgreSQL database:

Basically each artist has multiple albums, and each album has multiple songs.

My query correctly returns 25 distinct artist.id's who have a song whose title begins with "The " ordered by the year of the album:

SELECT id
FROM (

    -- Remove the duplicate artists
    SELECT DISTINCT ON (a.id) id, row
    FROM (

        -- Get all matching artists 
        -- This is slow because there is no limit
        SELECT
            artist.id,
            row_number() OVER(ORDER BY album.year DESC) as row
        FROM artist
        LEFT JOIN album ON album.artist_id = artist.id
        LEFT JOIN song ON song.album_id = album.id
        WHERE song.title ilike 'The %'
        ORDER BY album.year DESC

    ) as a

) as b
ORDER BY row
LIMIT 25

However it is slow and inefficient because the innermost query has no LIMIT so it will search the entire table for all possible matches. Ideally it would stop searching when the 25 distinct artist.id's have been found.

Can this query be rewritten or optimized to execute faster?

I think window functions could speed things up here, but I haven't been able to figure out something that works.

Thanks!

Upvotes: 4

Views: 1191

Answers (3)

PoliDev
PoliDev

Reputation: 1468

Try this one, 

Select id, year
from (
    SELECT DISTINCT ON (artist.id) artist.id, album.year
    FROM artist
    rightJOIN album ON album.artist_id = artist.id
    left JOIN song ON song.album_id = album.id
    WHERE song.title ilike 'The %'
    ORDER BY artist.id, album.year DESC
) s
order by year desc
LIMIT 25

Upvotes: 0

Clodoaldo Neto
Clodoaldo Neto

Reputation: 125444

select id, year
from (
    SELECT DISTINCT ON (artist.id) artist.id, album.year
    FROM artist
    inner JOIN album ON album.artist_id = artist.id
    inner JOIN song ON song.album_id = album.id
    WHERE song.title ilike 'The %'
    ORDER BY artist.id, album.year DESC
) s
order by year desc
LIMIT 25

An index on song.title can help when the ilike expression does start with a %

Upvotes: 2

Akash
Akash

Reputation: 5012

Try this, should work faster than your current query

SELECT
  artist.id,                        
  MAX( album.year ) as latest_album_date
FROM 
  artist
  JOIN album ON album.artist_id = artist.id  -- JOIN, not LEFT JOIN
  JOIN song ON song.album_id = album.id      -- Since you have song.title in WHERE clause, it makes no sense to make these as a LEFT JOIN
WHERE 
  song.title ilike 'The %'
GROUP BY
  artist.id
ORDER BY
  latest_album_date DESC
limit 25;

SQLFIDDLE

Upvotes: 0

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