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If I don't use Nolock on a select statement...

Posted on 2013-06-25
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Can I lock up data?
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Question by:patriotpacer
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dsacker earned 500 total points
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Potentially, yes. Within a pristine design, usually no.

For instance, if you have a query without the NOLOCK that does lengthy table scans, against fields that are not indexed, etc., you could lock the table. If others are trying to update that table, they could be in a wait state until your query finishes.

If you have good indexing, and tailor your queries accordingly, even though some deadlocking may still occur, the query is usually very quick and will use indexing efficiently. Very little deadlocking will occur.

In general, WITH (NOLOCK) will prevent locking the table, if you don't mind having the potential of "dirty" reads. I know programming shops where it is still an offense subject to termination if queries do not include the WITH (NOLOCK).
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