Cases should be avoided –1. DO NOT use FUNCTIONS (,TO_NUMBER ,Decode,NVL, TO_CHAR) unnecessarily when a simple join would work
2. DO NOT use ‘SELECT *’ explicitly. Make sure that you select only required columns from the tables.
3. DO NOT use HAVING clause where a WHERE clause would do.
4. DO NOT use ORDER BY clause in a query unless necessarily by logic.
5. DO NOT use UNION and DISTINCT together in a single query
6. DO NOT use CURSOR LOOPS unnecessarily when a simple/single DML/SELECT can serve the purpose
7. DO NOT use DUAL table unnecessarily in programs and queries.
Cases should be followed –
1. Use UNION ALL as far as possible instead of UNION.
2. Use single/direct DML’s/SELECT’s instead of LOOP’s.
3. Use BULK COLLECT & FORALL for looping when the loop is going to get executed multiple time (take > 100 iterations as a base).
4. Use MATERIALIZED VIEWS or GLOBAL TEMPORARY TABLE for queries fetching data across dblinks
5. Use EXISTS in place of IN in the queries.
6. Care should be taken to use the LIMIT clause while using BULK operations in order to limit memory utilization. LIMIT should be set between 500-1000 and reduced or decreased as per performance behavior of the program
References – http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14261/tuning.htm#i48876
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/design.htm#i29012