Check to see if the SQL already exists in the Oracle library cache. This is accomplished by parsing the SQL statement and deriving a unique hash key. The hash key is then referenced in memory. If the statement is found, then Oracle will execute the SQL.
If the statement is not found at the address, the SQL must be processed. Oracle will PARSE the SQL statement to ensure proper syntax. If there is a syntax error, the process will terminate and return the error to the user.
Oracle Tuning Reference If there is a syntax error, the SQL will be used to formulate an execution plan.
If the cost-based Optimizer is being used, Oracle statistics will be passed.
If the rule-based Optimizer is being used, Oracle dictionary entries (DBA_INDEXES) are passed.
The optimizer then completes an execution plan and then proceeds to execute the SQL statement.