Beginning Oracle Database 11g Administration From Novice to Professional

60
CHAPTER 3 ORACLE ARCHITECTURE
4. The user’s program sends an SQL statement to the dedicated server for processing.
5. The dedicated server prepares the SQL statement for execution. First it verifies
that the SQL statement is syntactically correct. Then it verifies that the tables
mentioned in the SQL statement actually exist and that the user has permission to
access those tables. Then it creates a query execution plan for the SQL statement.
The query execution plan is saved in the plan cache in the shared SQL area so that
it can be reused later by the same user or another user.
6. The dedicated server checks whether the required blocks of data are in the buffer
cache. If the needed blocks are not in the buffer cache, the dedicated server retrieves
them from the data files.
7. If the SQL statement does not involve any modifications to the data, the required
data rows are transmitted to the user. If the SQL statement involves modification
to the data, the dedicated server first copies the relevant blocks of data to a rollback
segment. It also creates undo and redo entries in the log buffer. The log writer
wakes up periodically and copies any accumulated undo and redo entries to the
redo log files.
8. If the SQL statement involved modification to the data, the user’s program sends
a command to the dedicated server process. The dedicated server process
puts the instruction in the log buffer and waits for confirmation from the
log writer that it has recorded the instruction in the redo log files—this
guarantees the durability of the modifications.
9. The database writer wakes up periodically and copies any modified data blocks
that it finds in the data cache to the data files. The checkpoint process wakes up
periodically and initiates a systematic synchronization procedure on all data files
that includes updating the system change number in the heading of each data file.
10. If the undo and redo entries fill up the current redo log file, the log writer closes
the file and opens the next one. The archiver subsequently makes a copy of the
closed file in case a data file ever gets damaged and needs to be repaired.
11. The user’s program disconnects from the database. This terminates the dedicated
server process.
Summary
The information in this chapter is a summary of the contents of Oracle 11g Concepts. Here
is a short summary of the concepts we touched on: