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      1. SQL Server:索引列用在什么地方?

        SQL Server: Index columns used in like?(SQL Server:索引列用在什么地方?)
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                  问题描述

                  索引仅在 LIKE 操作中使用的 varchar 列是个好主意吗?根据我从查询分析中读取的信息,我从以下查询中获得:

                  Is it a good idea to index varchar columns only used in LIKE opertations? From what I can read from query analytics I get from the following query:

                  SELECT * FROM ClientUsers WHERE Email LIKE '%niels@bosmainter%'
                  

                  我得到的估计子树成本"为 0.38,没有任何索引,0.14 有索引.如果已使用索引优化查询,这是用于分析的好指标吗?

                  I get an "Estimated subtree cost" of 0.38 without any index and 0.14 with an index. Is this a good metric to use for anlayzing if a query has been optimized with an index?

                  推荐答案

                  给定数据 'abcdefg'

                  Given the data 'abcdefg'

                  WHERE Column1 LIKE '%cde%'  --can't use an index
                  
                  WHERE Column1 LIKE 'abc%' --can use an index
                  
                  WHERE Column1 Like '%defg' --can't use an index, but see note below
                  

                  注意:如果您有需要 '%defg' 的重要查询,您可以使用持久计算列,您可以在其中 REVERSE() 列,然后对其进行索引.然后您可以查询:

                  Note: If you have important queries that require '%defg', you could use a persistent computed column where you REVERSE() the column and then index it. Your can then query on:

                  WHERE Column1Reverse Like REVERSE('defg')+'%' --can use the persistent computed column's index
                  

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