在SQL Server WHERE子句条件中优化CASE WHEN语句

Question

我正在重写我的sql以降低执行成本,并想知道是否有一种有效的方法来编写WHERE条件中使用的以下CASE WHEN语句:

SELECT l.*,tg.*
FROM  RefTable tg, 
      InputTbl l
WHERE tg.areascheme = l.areascheme
  AND tg.countrycode = l.strareabriefnamel1  
  AND ( CASE WHEN l.strareabriefnamel2 IS NULL THEN '' ELSE tg.areacode END ) = COALESCE( l.strareabriefnamel2,'' )
  AND ( CASE WHEN l.strareabriefnamel3 IS NULL THEN '' ELSE tg.subareaname END ) = COALESCE( l.strareabriefnamel3,'' )
  AND ( CASE WHEN l.strareabriefnamel4 IS NULL THEN '' ELSE tg.postalname END ) = COALESCE( l.strareabriefnamel4,'' )
  option( MAXDOP 0 ); 

执行计划: -

更多细节 :-

InputTable(466K记录)总共有四个参与JOIN逻辑的字段,总共有16个可能的(NULL,NOT NULL)组合.

L1,  L2,  L3,  L4
NULL,NULL,NULL,NULL
NULL,NULL,NULL,NOT NULL
NULL,NULL,NOT NULL, NULL
NULL,NULL,NOT NULL,NOT NULL
NULL,NOT NULL,NULL,NULL
NULL,NOT NULL,NULL, NOT NULL
NULL,NOT NULL, NOT NULL,NULL
NULL,NOT NULL,NOT NULL,NOT NULL
NOT NULL,NULL,NULL,NULL
NOT NULL,NULL,NULL,NOT NULL
NOT NULL,NULL,NOT NULL,NULL
NOT NULL,NULL,NOT NULL,NOT NULL
NOT NULL,NOT NULL,NULL,NULL
NOT NULL,NOT NULL,NULL,NOT NULL
NOT NULL,NOT NULL,NOT NULL,NULL
NOT NULL,NOT NULL,NOT NULL,NOT NULL

将与InputTable一起参与JOIN逻辑的RefTable(45k记录)根据上述标准生成结果集,产生约3.51亿行.

我的输入数据目前仅满足两种情况.

输入表: -

NULL,NULL,NULL,NULL - 225776 rows
NOT NULL, NOT NULL, NULL, NULL - 240360 rows

任何输入将不胜感激.谢谢.

Answer 1

简单规则: 不要在FROM子句中使用逗号. 始终使用明确,正确的JOIN语法.

这可能不会改变查询的性能,但它是一种更典型的编写方式.我很确定这个意图是:

SELECT l.*, tg.*
FROM RefTable tg JOIN
     InputTbl l
     ON tg.areascheme = l.areascheme AND tg.countrycode = l.strareabriefnamel1  
WHERE (l.strareabriefnamel2 IS NULL OR tg.areacode = l.strareabriefnamel2) AND
      (l.strareabriefnamel3 IS NULL OR tg.subareaname  = l.strareabriefnamel3) AND
      (l.strareabriefnamel4 IS NULL OR tg.postalname = l.strareabriefnamel4)
  option( MAXDOP 0 ); 

优化此查询的起点是索引.我建议:RefTable(areascheme, countrycode)和InputTbl(areascheme, strareabriefnamel1).