Tuesday, 7 June 2016

Tuning SQL Statement #4



Using Functions in QueriesWhen 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';




Sequences
A sequence is often used to create unique integer identifiers as primary keys for tables. A sequence is a distinct database object and is accessed as sequence.NEXTVAL and sequence.CURRVAL.
Use sequences for unique integer identifiers; they are centralized, more controllable, more easily maintained, and perform better than other methods of counting.

Do’s  
INSERT INTO supplier (supplier_id, name, ticker)
      VALUES (supplier_seq.NEXTVAL,'A new supplier', 'TICK');    


 Don’ts
INSERT INTO supplier (supplier_id, name, ticker)
      VALUES ((SELECT MAX(supplier_id)+1
      FROM supplier), 'A new supplier', 'TICK');


    

Equi, Anti, and Range
Using an equals sign (equi) is the fastest comparison condition if a unique index exists. Any type of anti comparison such as != or NOT is looking for what is not in a table and thus must read the entire table; sometimes full index scans can be used.

Do’s   
SELECT account_name, trans_date, amount
FROM transaction
WHERE amount > 0;    SELECT account_name, trans_date, amount
FROM transaction
WHERE amount != 0;
 

Don’ts
SELECT account_name, trans_date, amount
FROM transaction
WHERE amount > 0;    SELECT account_name, trans_date, amount
FROM transaction
WHERE amount  NOT = 0;



LIKE Pattern MatchingIn general, since LIKE will match patterns which are in no way related to indexes, LIKE will usually read an entire table.






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