When a query tries to modify database, original data entries are kept in separate location and then changes are applied to datafiles. The data which is kept in separate location is called Undo data. Undo data is used to maintain data integrity and data consistency. Oracle uses Undo data in following situations (more…)
When query is submitted to oracle server, various processing steps are performed to execute query. Following main steps are involved in this process.
1- Query Parsing
2- Optimizer determines most efficient way to execute query and generates an execution plan.
3- Finally SQL engine executes query.
In 1st step, SQL server checks syntax of query and outputs error if found.
After parsing of query, optimizer generates execution plan. Execution plan produced by optimizer depends on two main possible goals (more…)