Partial Results


Sometimes you do not need the full result of an SQL query but only the first few rows—e.g., to show only the ten most recent messages. In this case, it is also common to allow users to browse through older messages—either using traditional paging navigation or the more modern “infinite scrolling” variant. The related SQL queries used for this function can, however, cause serious performance problems if all messages must be sorted in order to find the most recent ones. A pipelined order by is therefore a very powerful means of optimization for such queries.

Using a pipelined order by is not only about saving the effort to sort the result, it is more about delivering the first results without reading and sorting all rows. That means the pipelined order by has very low startup costs. It is therefore possible to abort the execution after fetching a few rows without discarding the efforts to prepare the final result.

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This chapter demonstrates how to use a pipelined order by to efficiently retrieve partial results. Although the syntax of these queries varies from database to database, they still execute the queries in a very similar way. Once again, this illustrates that they all put their pants on one leg at a time.

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About the Author

As an author, trainer, and coach Markus Winand specializes in helping developers cope with SQL performance issues. He also published the book SQL Performance Explained and tweets his best performance tips via @SQLPerfTips.http://winand.at/

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