Tuning SQL Statement #2

Tuning SQL Statement #1.......
Tuning SQL Statement #3.......


•    The UNION statement performs the equivalent of a SELECT DISTINCT on the final result set. In other words, the UNION clause forces all rows returned by each portion of the UNION to be sorted and merged and duplicate to be filtered before the first row is returned.  A UNION ALL simply returns all rows including duplicates and does not have to perform any sort, merge or filter.  If your tables are mutually exclusive (include no duplicate records), or you don't care if duplicates are returned, the UNION ALL is much more efficient as it does not perform the SELECT DISTINCT function, which saves a lot of unnecessary resources from being used.

Do's-
SELECT acct_num, balance_amt FROM debit_transactions  WHERE tran_date = '1-JAN-06' 
UNION ALL
SELECT acct_num, balance_amt  FROM credit_transactions  WHERE tran_date = '1-JAN-06' ;  

Don'ts-
SELECT acct_num, balance_amt  FROM debit_transactions  WHERE tran_date = '1-JAN-06'
UNION
SELECT acct_num, balance_amt  FROM credit_transactions  WHERE tran_date = '1-JAN-06' ;

•    Try to avoid using any type of data conversion function in any part of an SQL statement which could potentially match an index, especially if you are trying to assist performance by matching appropriate indexes

Do's-
SELECT ...  FROM customer  
WHERE cust_id = TO_NUMBER('11');  

SELECT ...  FROM customer
WHERE cust_type = 1;    

Don'ts-
SELECT ...  FROM customer
WHERE cust_id = '11';

SELECT ...  FROM emp
WHERE TO_NUMBER (cust_type) = 1;


•    When using functions in SQL statements it is best to keep the functions away from any columns involving index matching unless function-based indexes are available and can be created.

Do's-
SELECT account_name, trans_date, amount  FROM transaction
WHERE account_name LIKE 'CAPITAL%';  

Don'ts-
SELECT account_name, trans_date, amount  FROM transaction
WHERE SUBSTR(account_name,1,7) = 'CAPITAL';


•    If your SELECT statement contains a HAVING clause, write your query so that the WHERE clause does most of the work (removing undesired rows) instead of the HAVING clause do the work of removing undesired rows. Using the WHERE clause appropriately can eliminate unnecessary rows before they get to the GROUP BY and HAVING clause, saving some unnecessary work, and boosting performance

Do's-
SELECT country, AVG (population)  FROM survey
WHERE country != 'INDIA'
AND country != 'CHINA';
GROUP BY country;  

Don'ts-
SELECT country, AVG (population)  FROM survey
GROUP BY country
HAVING country != ‘INDIA'
AND country != 'CHINA’;


•    The ORDER BY clause is always executed after the WHERE clause. Try not to override the WHERE clause with the ORDER BY clause because the Optimizer may choose a less efficient method of execution based on the ORDER BY clause. Try to put your where columns in order by clause if possible.

Do's-
SELECT * FROM ordersline
WHERE order_id < 10
ORDER BY order_id, seq#;  

Don'ts-
SELECT * FROM ordersline
WHERE order_id < 10
ORDER BY seq#;

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