使用不同数据粒度进行聚合

提升 PostgreSQL 查询性能

Amy McCarty

Instructor

数据粒度：细节层级

使行唯一
1 个或多个列

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

连接不同粒度

Video_Games 表中《塞尔达传说》记录关联到 Games_Platforms 表中的全部 3 条记录。

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

连接不同粒度

输出

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

设置粒度变更

Game_Platforms

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

设置粒度变更

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

输出

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

重温 CTE

独立查询，带临时结果集
WITH 语句

WITH cte AS
  ( SELECT * FROM tableA )

SELECT * FROM cte

允许在连接前先聚合

CTE 来解围

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 来解围

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

输出

id	game	no_platforms	last_platform
234	Legend of Zelda	3	Wii

连接时匹配数据粒度

无重复或副本
仅保留必要结果
避免重复计数

¡Vamos a practicar!

提升 PostgreSQL 查询性能