transaction-isolation-levels.md

title	Transaction Isolation Levels | Microsoft Docs
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ms.date	03/14/2017
ms.prod	sql-non-specified
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ms.technology	database-engine
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ms.topic	language-reference
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helpviewer_keywords	locking [SQL Server], hints isolation levels [SQL Server], metadata access hints [SQL Server], locking
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Transaction Isolation Levels

[!INCLUDEtsql-appliesto-ss2012-xxxx-xxxx-xxx_md]

[!INCLUDEssNoVersion] does not guarantee that lock hints will be honored in queries that access metadata through catalog views, compatibility views, information schema views, metadata-emitting built-in functions.

Internally, the [!INCLUDEssDEnoversion] only honors the READ COMMITTED isolation level for metadata access. If a transaction has an isolation level that is, for example, SERIALIZABLE and within the transaction, an attempt is made to access metadata by using catalog views or metadata-emitting built-in functions, those queries will run until they are completed as READ COMMITTED. However, under snapshot isolation, access to metadata might fail because of concurrent DDL operations. This is because metadata is not versioned. Therefore, accessing the following under snapshot isolation might fail:

• Catalog views

• Compatibility views

• Information Schema Views

• Metadata-emitting built-in functions

• sp_help group of stored procedures

• [!INCLUDEssNoVersion] Native Client catalog procedures

• Dynamic management views and functions

For more information about isolation levels, see SET TRANSACTION ISOLATION LEVEL (Transact-SQL).

The following table provides a summary of metadata access under various isolation levels.

Isolation level	Supported	Honored
READ UNCOMMITTED	No	Not guaranteed
READ COMMITTED	Yes	Yes
REPEATABLE READ	No	No
SNAPSHOT ISOLATION	No	No
SERIALIZABLE	No	No