DB2的coalesce和inner join性能

Question

我有一个性能很差的查询（我继承了）并且我不太清楚如何优化它...据我所知，它将第二列的值设置为第一列的值PLUS另一个表中找到关系的值。

update table1 set
col2 = col1 || coalesce (( 
    select table2.the_column_wanted from table2 where table2.fk = table1.pk and
    table2.flag = 'Y'))

where flag = 'Y' and pk in ( select distinct fk from table2 );

Answer 1

确切的速度问题取决于您的表格的特征，但一般情况下我会针对此类问题调查MERGE。像这样：

MERGE INTO table1 USING table2
   ON table1.pk = table2.fk and 
      table1.flag = 'Y' and
      table2.flag = 'Y'
   WHEN MATCHED THEN UPDATE SET
      table1.col2 = table1.col1 || table2.the_column_wanted;

写的查询非常值得怀疑。你应该好好看看你“继承”的所有代码。