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The performance improvements associated with accessing rows directly using rowids rather than primary key searches will be covered next.Using rowids when updating Rowids are Oracle's internal row-level identifiers and are used during index creation to provide the link between index keys and the rows they point to.Manually updating statistics There are two ways to manually update statistics on a table.One is via the sp_updatestats system stored procedure. While the sp_updatestats procedure is an easier way to program the command, the UPDATE STATISTICS command is much more flexible and much more powerful.The next section will compare the performance of implicit and explicit cursors.This is an excerpt from the bestselling book "Oracle PL/SQL Tuning: Expert Secrets for High Performance Programming" by Dr. During the first pass the rows are updated using the primary key value, whilst the rowid is used in the second pass. The next section will show how the order of logical expressions and branching structures can affect the performance of code.Short-circuit evaluations and ordering logic When conditional logic contains several expressions separated by the AND or OR operators it is sometimes possible to determine the final result without evaluating all the expressions.
The database engine employs statistics when generating execution plans that are used to access data stored within the database.For that reason, having accurate and up-to-date statistics is crucial.In contrast, if you have invalid or out-of-date statistics, it can lead the SQL Server engine to take the wrong path to the data, and taking the wrong path means that an index scan is made when an index seek would have been appropriate or a seek is performed against the wrong index.Once the threshold is reached, SQL Server will automatically update the statistic(s) defined in the table.If you have a table that stays at approximately the same number of rows, but data is added and removed on a regular basis, your statistics will become out of date rather quickly.
By default, statistics are compiled based on a sampling of the data in the table.