DBA-LES01 ------------------------------------------------- ---------------------------------------- Oracle Database and Oracle Instance: General For, Oracle Database corresponds to Oracle Instance, an Oracle Database corresponds to an Oracle Instance. However, in Oracle's Parallel Environment (OPS, RAC), an Oracle Database can correspond to multiple Oracle Instance. One of us usually said is actually three parts: 1.oracle software: Total local hard disk. Composed of Oracle's software programs. 2. Oracle Instance: After the database is started, the database disappears after turning off the database. It is a collection of memory structures used by Oracle and the background process. 3. Oracle Database: There is a local hard disk, or on the storage device. Composed of Oracle data files.
PGA.Sort_area_size When a session is established with Oracle, a private sorting area is assigned to the session in memory. If the connection is a dedicated connection, a Program Global Area (PGA) is allocated in memory according to the size of the sort_AREA_SIZE parameter in INIT.ORA. If the connection is established by multi-threaded servers, the sorted space is assigned in Large_Pool. Unfortunately, for all sessions, it must be the same, we can't assign an additional sorting area for more sorted operations. Therefore, the designer must make a balance, while disk sorts are disk sorts, while disk sorts are disk sorting, there will be some waste on those who do not need a large sorting task. . Of course, when the sorted space needs exceeds the size of sort_area_size, the disk sort will be made in the TEMP tablespace. Disk sorting is about 14,000 times more slower than memory. Sort is completed, before the row returns, Oracle releases memory by the value given by the parameter sort_AREA_RETAINED_SIZE; when all the rows returned, Oracle released all the memory
Rollback segment.scn instance analysis: a query returns the following 5 lines ID Name ------------------------ 1 Shanghai2 beijing3 gugangzhou4 shenzhen5 hanzhou User A From 12: 00 start running, at 12:05 End 12:01 User B execute an UPDATE statement, updated the ID is 2 records beijing should be tianjing. And submit. At this time, the query of users A will not The SCN of Tianjing. 12: 00 query time SCN is N and the user B update makes the system's SCN turns N 1 When the user A queries the record of ID = 2, it is found that his SCN has been larger than the start of the query. SCN, he will find the record of SCN = N in the rollback segment data buffer. Return it.
SMON: System Monitrarthomatic or Recovers The Instance - Rolls Forward Changes in The Redo Logs - Opens The Database for User Access - Rolls Back Uncommitted Transactions
