Agregasi dengan granularitas data berbeda

Meningkatkan Performa Kueri di PostgreSQL

Amy McCarty

Instructor

Granularitas data - tingkat detail

Menjadikan baris unik
1+ kolom

Video_Games

id	game	first_yr
012	Grand Theft Auto	1997
234	Legend of Zelda	1986

Game_Platforms

game_id	platform	year
234	FCDS	1986
234	GameCube	2003
234	Wii	2006

Menggabungkan granularitas berbeda

Rekaman The Legend of Zelda dari tabel Video_Games terhubung ke semua 3 rekaman di tabel Games_Platforms.

SELECT g.id, g.game, g.first_yr
  , COUNT(p.platform) AS no_platforms
  , MAX(p.year) AS last_platform_yr
  , p.platform
FROM video_games g
INNER JOIN game_platforms p
  ON g.id = p.game_id
GROUP BY g.id, g.game, g.first_yr
, p.platform

Menggabungkan granularitas berbeda

Keluaran

id	game	first_yr	no_platforms	last_platform_yr	platform
234	Legend of Zelda	1986	3	2006	FCDS
234	Legend of Zelda	1986	3	2006	GameCube
234	Legend of Zelda	1986	3	2006	Wii

Menyiapkan perubahan granularitas

Game_Platforms

game_id	platform	year
234	FCDS	1986
234	GameCube	2003
234	Wii	2006

Menyiapkan perubahan granularitas

SELECT game_id, platform, year
  , COUNT(platform) AS no_platforms
  , MAX(year) AS last_platform_yr
FROM game_platforms
GROUP BY game_id, platform, year

Keluaran

game_id	platform	year	no_platforms	last_platform_yr
234	FCDS	1986	3	2006
234	GameCube	2003	3	2006
234	Wii	2006	3	2006

Meninjau kembali CTE

Kueri mandiri dengan hasil sementara
Pernyataan WITH

WITH cte AS
  ( SELECT * FROM tableA )

SELECT * FROM cte

Memungkinkan agregasi sebelum join

CTE sebagai solusi

WITH platforms_cte AS 
(  SELECT game_id, platform, year
      , COUNT(platform) AS no_platforms
      , MAX(year) AS last_platform_yr
   FROM game_platforms
   GROUP BY game_id, platform, year )

SELECT g.id, g.game, cte.no_platforms
  , cte.platform AS last_platform
FROM video_games g
INNER JOIN platforms_cte cte 
  ON g.id = cte.game_id
  AND cte.last_platform_yr = cte.year

CTE sebagai solusi

WITH platforms_cte AS 
(  SELECT game_id, platform, year
      , COUNT(platform) AS no_platforms
      , MAX(year) AS last_platform_yr
   FROM game_platforms
   GROUP BY game_id, platform, year )

SELECT g.id, g.game, cte.no_platforms
  , cte.platform AS last_platform
FROM video_games g
INNER JOIN platforms_cte cte 
  ON g.id = cte.game_id
  AND cte.last_platform_yr = cte.year

Keluaran

id	game	no_platforms	last_platform
234	Legend of Zelda	3	Wii

Mencocokkan granularitas saat join

Tanpa pengulangan atau duplikasi
Hasil minimum yang diperlukan
Tanpa hitung ganda

Ayo berlatih!

Meningkatkan Performa Kueri di PostgreSQL