Sql query when fired for first time

Question

Here is my query. I installed database and software using DBCA. Now i got connected to sql. Now my doubt is when I fired SELECT * FROM EMP from scott user, for the first time, what are the different steps followed to fectch the data. I mean how the user gets connected to the database and which background process works at what stages. Please give detailed explanation. Thanks.

Answer 1

When connecting to the database the client makes a request to the listener, and the listener checks the availability of the requested database service and responds to the client with an ACCEPT message, which indicates connection dispatching.
After that, the listener sends a RESEND message back to the client and uses this time to check the database service. The client recieves the RESEND message and sends the same request again to the Listener.
After recieving the ACCEPT message the connection is established, and this connection is different depending if you are using a Dedicated Server or Shared Server configuration.
When you issue the query:
SELECT * FROM emp;Oracle checks in the Shared Pool, in the Data Dictionary cache if the object exists and if you have sufficient privileges to access it (user Scott in this case). It then checks the Library cache if this statement has been executed before. If it did, another parse will not be necessary. Then the SQL statement is parsed (if necessary), interpreted and executed.
The DBRW (Database Writer) background process then checks in the Buffer Cache if the blocks which contain the information from the EMP table are already there. If they are, a physical read is not necessary and it fetches the rows from the Buffer Cache.
If they are not, DBWR reads the data files, retrieves the data from them, store the related blocks in the Buffer Cache (so in the next executions of this query a physical read will not be needed) and then the data is shown to the user.
There are other details envolved, but in a general view that is what happens.