Basic concepts of Oracle Database

1. Oracle Server is a data management system (RDBMS) that provides open, comprehensive, and almost perfect information management. Oracle is composed of an Oracle instance and an Oracle database.

The Oracle server consists of two parts: the Oracle database and the Oracle instance.

(1) the Oracle database is the file that actually stores data on the hard disk. These files are organized together into a logical whole, that is, the Oracle database. Therefore, in Oracle's view, the database refers to the logical set of files on the hard disk. It is necessary to cooperate with instances in the memory to provide data management services.

(2) An Oracle instance is a data structure in the physical memory class. It consists of a shared memory pool and multiple background processes. The shared memory pool can be accessed by all processes, if you want to access the data in the database (that is, the files on the hard disk), you must use the instance to achieve this. You cannot directly read the files on the hard disk. An instance can operate databases. At any time, an instance can only be associated with one database, but one database may be accessed by multiple instances.

When a database file is read to the memory, it is an Instance. An Instance divides the occupied memory into multiple blocks. The database we operate on is the Instance in the memory, the database cannot be operated directly. When we obtain a Connection through JDBC to connect to this Instance, the Instance will open up a memory (PGA) for this Connection, and there will be a PGA when a Connection comes in. One Instance can have multiple PGA segments, but only one global SGA segment. The relationship between instances and databases can be many-to-one. That is, a database can correspond to multiple instances. This is the cluster (RAC). If a machine crashes, an Instance is paralyzed, we can use the Instance in another host to continue providing database connections.

We only add, delete, modify, and query the database in PGA. When we perform the commit operation, the operation is submitted to SGA, And the SGA is still in the memory, when the data in SGA is submitted to the database is determined by the server. This method is called two-phase commit. As long as the data is still in SGA, we still have the opportunity to modify the data. This is the principle of flashback.

2. tablespace. tablespace is a logical concept, and data file is a physical concept. A tablespace consists of multiple data files. A data file can belong to only one tablespace. The tablespace corresponds to the PGA In the instance. The common tablespace is users.

3. Oracle-related services: OracleDBConsoleorcl: Manages Oracle Web interface servers. OracleJobSchedulerORCL: scheduled service for Oracle. OracleOraDb10g_home1liSQL * Plus: A Web server for interface painting. OracleOraDb10g_home1TNSListener: Oracle client listening service. If the client connects to Oracle, it must start the service. For example, JDBC (jdbc: oralce: this: @ localhost: 1521: orcl) has a low performance, and Oracle (jdbc: Oracle: oci: @ loaclhost: 1521: orcl) performance is much higher.