That entered the record into the SalesHistory table SELECT COUNT(*) FROM SalesHistory WITH(NOLOCK) Returning the number of records in the SalesHistory Query window, run the following script, which uses the NOLOCK table hint in That was inserted into the table still has locks issued against it. The transaction is still open, which means that the record In the script below, I begin a transaction and insert a record The following example shows how NOLOCK works and how dirty (I will take a closer look at this side effect in part two of this “phantom” data, or data rows that are available in one database transaction The trick is being able to know when youĪs a side note, NOLOCK queries also run the risk of reading Transaction rolls back its work, the data read from the connection using NOLOCK A “dirty” read is one in which theĭata being read is involved in a transaction from another connection. Uncommitted data could potentially be read. Statement does not issue any locks against the tables being read, some “dirty,” Increases concurrency and performance because the database engine does not have The database engine from issuing locks against the tables in your queries this The benefit of the statement is that it allows you to keep Issued against the table that would prohibit other transactions from modifying NOLOCK indicates that no shared locks are This table hint, also known as READUNCOMMITTED, isĪpplicable to SELECT statements only. Script in Listing A to create the SalesHistory table and populate the table with data. I’ll set up a table to use for our example queries. This article focuses on two table hints: NOLOCK and However, there will be times when it isĪdvantageous to manually make hints to how locks are issued on your tables in your SQL statements. Typically, the SQL Server defaults work fineįor what you are trying to accomplish. Mind the different types of locks that will be issued, and the different levels of isolation your When designing database applications, you should keep in TechRepublic’s SQL Server newsletter, delivered each Tuesday, contains hands-on tips that will help you become more adept with this powerful relational database management system.
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