Sybase 12.4.2 - Manual (Page 476)

Improving your queries
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Deciding to denormalize
When deciding whether to denormalize, you need to analyze the data access
requirements of the applications in your environment and their actual
performance characteristics. Some of the issues to examine when considering
denormalization include:
• What are the critical queries, and what is the expected response time?
• What tables or columns do they use? How many rows per access?
• What is the usual sort order?
• What are concurrency expectations?
• How big are the most frequently accessed tables?
• Do any processes compute summaries?
• Should you create join indexes to gain performance?
Improving your queries
This section discusses several ways to improve queries for better performance,
including:
• Tips on how to structure your queries to avoid operations that may be time
consuming
• Suggestions for using the query plans Adaptive Server IQ provides
• Options you can set to modify query processing
Tips for structuring queries
Here are some hints for better query structure:
• In some cases, command statements that include subqueries can also be
formulated as joins and may run faster.
• If you group on multiple columns in a GROUP BY clause, list the columns
by descending order by number of unique values. This will give you the
best query performance.