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Don Burleson Blog 







Oracle SQL Tuning tricks

Oracle Database Tips by Donald BurlesonJuly 25, 2015

Oracle SQL tuning experts use an endless number of techniques, but there are some common tricks that can be used to optimize all SQL.  Here is a short list of common SQL tuning tips and tricks.  For a more complete list, and details on these SQL tuning tricks, see my bestselling book "Oracle Silver Bullets":

  • Tune the workload first - Always tune your workload as a whole, optimizing the optimizer parameters (optimizer_mode, optimizer_index_cost_adj, db_file_multiblock_read_count) before tuning individual SQL statements. 



  • Avoid re-parsing of SQL statements - The library cache is intended to make SQL re-entrant, and you can improve response time by cutting-down on redundant SQL parsing.  Be sure to use cursor_sharing when appropriate.

  • Use materialized views - Materialized views can pre-summarize aggregations and pre-join tables, making SQL run super fast in systems with low volume update activity.

  • Never assume that CBO statistics are correct - Using the GIGO principle (garbage-in, garbage-out), don't hesitate to re-analyze tables and indexes with dbms_stats.

  • Use histograms for tuning - Many common SQL problems (e.g. sub-optimal table join order) are caused by poor cardinality estimates.  Apply histograms providently (only when required) to help the optimizer estimate the size of intermediate rowset operations.

  • Use function-based indexes - In almost all cases, the use of a built-in function like to_char, decode, substr, etc. in an SQL query may cause a full-table scan of the target table. To avoid this problem, many Oracle DBAs will create corresponding indexes that make use of function-based indexes. If a corresponding function-based index matches the built-in function of the query, Oracle will be able to service the query with an index range scan thereby avoiding a potentially expensive full-table scan.

  • Decompose complex SQL - You can use the with clause and global temporary tables to flatten-out complex subqueries and make execution times faster.

  • Avoid subqueries - May types of subqueries (exists, in, not in) can be re-written  as a standard join with faster performance.

  • Watch out for counterintuitive tips - tricks such as using where rownum=1 can be dangerous.

  • Use views sparingly - Views were designed to assist end-users, and views make complex queries "appear" as-if they were a discrete table.  Hence, running production queries against views can cause a host of optimization problems. 

  • Watch out for the having clause - You can often decompose a complex query using the with clause to avoid the use of the expensive having clause.

  • Use union all when possible - The union clause removes duplicates, and it must perform an expensive sort to remove duplicate rows.  Instead, the union all clause is faster because it does not sort to remove duplicate rows.

  • Always reference an indexed column - SQL with a where clause that does not reference any indexed columns can result in an unnecessary large-table full-table scan.

  • Avoid using BIF's in where clause predicates - Don't invalidate columns by changing the left-hand side of a where clause predicate (where substr(last_name,1,3) = 'Jon';  where trunc(my_date) = trunc(sysdate)).

  • Test with the RULE hint - The RULE hint is fantastic for testing whether a sub-optimal SQL query is failing because of a missing index, or bad CBO statistics.  In many cases, the RULE hints simplicity can help tune SQL statements faster.


Again, these are just a few of the common SQL tuning tricks, and there are many, many more.

Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.



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