Why SQL Statements regressed

• Bad execution plans 
• Full tables scans 
• Cartesian join (details)
• Poorly written statement , e.g converting literal once and not the whole column

Bad query as it will convert literal for all row : Select * from employee where to_ch(salary) =sal  

Good query as it convert only 1 row : select * from employee where salary = to_number(sal)

• Cursor sharing 

• Hardware resources high utilization 
• CPU, Memory, IO, Network 
• Data fragmentation 
• Logical contention 
• Row lock contention 
• Block update contention 
• Index contention