Friday, October 19, 2012

SQL Server: Why We Should Avoid NOLOCK Table Hint in DELETE/UPDATE Queries

Recently, I was asked to review, already written stored procedures for optimization purpose. During this review process I have found that a group of developers is regularly committing a big mistake. This group of developers believes that table hint NOLOCK is used to execute queries quickly, as this hint will avoid placing any lock on target table records and it can you used in any query. Even they have applied this NOLOCK in DML statements.
First thing, NOLOCK hint means, it will not take care of any lock (instead of placing lock). It will return data, that could be dirty (NOT YET COMMITTEED by other transactions). We can use this table hint to get results quickly when we are dead sure that dirty data is TOTALLY bearable.
In DELETE/UPDATE queries it should be totally avoided as it can produce junk results. Let’s prove.
In following example, we need to correct discount column of SalesOrderDetail, but according to discount provided in lookup table of SpecialOffer. Before we execute our update statement (Statement #2 in Transaction# 2), someone has accidently changed SpecialOffer, but good thing is that, he has not committed these changes yet. But as we have placed NOLOCK hint in our Statement #2 in Transaction# 2, it will change data according to dirty data, though, later on transaction#1 is rolledback.

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