Query Optimization Tips

Before optimize any query clean buffer first after we can see what's exactly going on

  - DBCC dropcleanbuffer
  - DBCC FreeProcCatch

For See Statistsics Also See In Execution Plan

  - Set Statistics IO ON
  - Set Statistics Time ON

For Optimization We are Concentrate on 3 Most Statistic

1) NO of logical reads
2) CPU Time
3) Query Cost (QC)

Both 1st and 2nd You will Get From Upper Commands
But For QC Use Execution Plan

See and Note All Statistic and Then Apply This Instruction For optimization that Given Below:

1) Limit no of colums return from query
2) Create Primary Key
3) Create index which are using in workhours
4) Limit the no of rows by using top
5) Working with join have index on the column in the join
6) If you use multiple columns in where or order by clause then create compound index
7) If you are trying to fetch unique values use group by instead of distinct
8) Use exists in behalf of in().
9) set nocount on

Query Optimization OR and Union

When we execute query with OR clause,filtering the records from two 
different field. Oracle Database can not use index to filter the that.

1) create Index Idx_empno on emp(empno)

2) create Index Idx_salary on emp(salary)

Select * from emp
where empno = 7319 or salary > 15000

In this query Both the field have an index then also you can see
that it takes to much time because of "or" indexing field not work because of OR

so use Union to overcome this,

Select * from emp where empno = 7319
Union
Select * from emp where salary > 15000