Agregarea la granularități diferite

Îmbunătățirea performanței interogărilor în PostgreSQL

Amy McCarty

Instructor

Granularitatea datelor - nivelul de detaliu

Face un rând unic
1+ coloane

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

Joinul la granularități diferite

Înregistrarea Legend of Zelda din tabela Video_Games este legată de toate cele 3 înregistrări din tabela 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

Joinul la granularități diferite

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

Pregătirea unei schimbări de granularitate

Game_Platforms

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

Pregătirea unei schimbări de granularitate

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

CTE-uri revizuite

Interogare independentă cu set de rezultate temporare
Instrucțiunea WITH

WITH cte AS
  ( SELECT * FROM tableA )

SELECT * FROM cte

Permite agregarea înainte de join

CTE-urile vin în ajutor

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-urile vin în ajutor

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

Potrivirea granularității la join

Fără repetări sau duplicate
Rezultate minime necesare
Fără numărare dublă

Să exersăm!

Îmbunătățirea performanței interogărilor în PostgreSQL