在SQLSERVER中导致索引查找变成索引扫描的问题分析

SQL Server 中什么情况会导致其执行计划从索引查找（Index Seek）变成索引扫描（Index Scan）呢？ 下面从几个方面结合上下文具体场景做了下测试、总结、归纳。

1：隐式转换会导致执行计划从索引查找（Index Seek）变为索引扫描（Index Scan）

Implicit Conversion will cause index scan instead of index seek. While implicit conversions occur in SQL Server to allow data evaluations against different data types, they can introduce performance problems for specific data type conversions that result in an index scan occurring during the execution.  Good design practices and code reviews can easily prevent implicit conversion issues from ever occurring in your design or workload. 

如下示例，AdventureWorks2014数据库的HumanResources.Employee表，由于NationalIDNumber字段类型为NVARCHAR，下面SQL发生了隐式转换，导致其走索引扫描（Index Scan）

SELECT NationalIDNumber, LoginID 
FROM HumanResources.Employee 
WHERE NationalIDNumber = 112457891  

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