SQL Server SELECT statements causing blocking

sql-server performance blocking
Neil Barnwell picture Neil Barnwell · Jun 19, 2009 · Viewed 75.7k times · Source

We're using a SQL Server 2005 database (no row versioning) with a huge select statement, and we're seeing it block other statements from running (seen using sp_who2). I didn't realise SELECT statements could cause blocking - is there anything I can do to mitigate this?

Answer

Remus Rusanu picture Remus Rusanu · Jun 19, 2009

SELECT can block updates. A properly designed data model and query will only cause minimal blocking and not be an issue. The 'usual' WITH NOLOCK hint is almost always the wrong answer. The proper answer is to tune your query so it does not scan huge tables.

If the query is untunable then you should first consider SNAPSHOT ISOLATION level, second you should consider using DATABASE SNAPSHOTS and last option should be DIRTY READS (and is better to change the isolation level rather than using the NOLOCK HINT). Note that dirty reads, as the name clearly states, will return inconsistent data (eg. your total sheet may be unbalanced).

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