Aggregazione con granularità diverse

Migliorare le prestazioni delle query in PostgreSQL

Amy McCarty

Instructor

Granularità dei dati: livello di dettaglio

Rende una riga univoca
1+ colonne

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

Join con granularità diverse

Il record di The Legend of Zelda nella tabella Video_Games collega tutti e 3 i record nella tabella 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

Join con granularità diverse

Output

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

Impostare un cambio di granularità

Game_Platforms

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

Impostare un cambio di granularità

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

Output

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

Rivediamo le CTE

Query autonoma con risultati temporanei
Istruzione WITH

WITH cte AS
  ( SELECT * FROM tableA )

SELECT * FROM cte

Consente di aggregare prima del join

CTE in soccorso

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 in soccorso

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

Output

id	game	no_platforms	last_platform
234	Legend of Zelda	3	Wii

Allineare la granularità dei dati nei join

Niente ripetizioni o duplicati
Solo i risultati minimi necessari
Niente doppio conteggio

Ayo berlatih!

Migliorare le prestazioni delle query in PostgreSQL