My query was fine last week and now it is slow. Why?

The likely cause of this is because the execution plan has changed. Generate a current explain plan of the offending query and compare it to

a previous one that was taken when the query was performing well.

Usually the previous plan is not available.

Some factors that can cause a plan to change…

• Hardware traffic managers redirect user requests to a farm of servers based on server availability, IP address, or port number. All traffic is routed
  
  to the load balancer, then requests are fanned out to servers based on
  
  the balancing…

• Set a goal: Before you begin tuning your J2EE application's performance, set a goal. Often this goal addresses the maximum concurrent
  
  users the application will support for a given limit on
  
  response times. But the goal can also focus on other variables—for
  
  example, the response times should not increase
  
  more than 10 percent during the peak hour of user…

This is a common problem in the software industry, especially in new technology areas. There is generally no easy solution in this situation. One approach is:

• Hire good people
• Management should 'ruthlessly prioritize' quality issues and maintain focus on the customer
• Everyone in the organization should be clear on what 'quality' means to the customer