Top 10 SQL server Performance tuning tips - 2

Friday, 4 September 2009

Today's performance tuning tip.

Use of UNION/UNION ALL

UNION move the results to the tempdb, sort it, identify unique entries and retrieve the results.

UNION ALL will not do the sort, and will not identify unique entries. The results will not be unique. This can be used when the results from two queries are distinctly different

UNION will have adverse impact on the number of Reads and Writes and hence the performance.