Rule Definition
In SQL the UNION operator combines the results of two SQL queries into a single table of all matching rows. The two queries must have matching fields and data types in order to join them. Any duplicate records are automatically removed unless UNION ALL is used. The problem is that MOST of the time it is unecessary to remove duplicate as there are none and doing the search for duplicate rows can be very costly
Remediation
Try to reduce the use of UNION. Use instead UNION ALL.
Reference
Standard suggestions based on CAST Quality Model
Related Technologies
Technical Criterion
Efficiency - SQL and Data Handling Performance
About CAST Appmarq
CAST Appmarq is by far the biggest repository of data about real IT systems. It's built on thousands of analyzed applications, made of 35 different technologies, by over 300 business organizations across major verticals. It provides IT Leaders with factual key analytics to let them know if their applications are on track.