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《Oracle Concepts》是介绍oracle数据库的基础概念的入门书籍，学习这些概念可以为更深层次的学习打好基础。
| metric | 指标（变化率） | The rate of change in a cumulative statistic |
| mounted database | 已装载数据库 | An database instance that is started and has the database control file open. |
| multitenant architecture | 多租户架构 | The architecture that enables an Oracle database to function as a multitenant container database (CDB), which means that it can contain multiple PDBs. A PDB is a portable collection of schemas, schema objects, and nonschema objects that appears to an Oracle Net client as a traditional Oracle database non-CDB). |
| multithreaded Oracle Database model | 多线程Oracle数据库模型 | A model that enables Oracle processes to execute as operating system threads in separate address spaces. In threaded mode, some background processes on UNIX and Linux run as processes containing one thread, whereas the remaining Oracle processes run as threads within processes. |
| multitier architecture | 多层架构 | An architecture in which one or more application servers provide data for clients and serves as an interface between clients and database servers. |
| multiversion consistency model | 多版本一致性模型 | A model that enables the database to present a view of data to multiple concurrent users, with each view consistent to a point in time. |
| multiversioning | 多版本功能 | The ability of the database to simultaneously materialize multiple versions of data. |
| mutex (mutual exclusion object) | 互斥对象 | A low-level mechanism that prevents an object in memory from aging out or from being corrupted when accessed by concurrent processes. |
| natural key | 自然键 | A meaningful identifier made of existing attributes in a table. For example, a natural key could be a postal code in a lookup table. |
| network database | 网络数据库 | A type of database, similar to a hierarchical database, in which records have a many-to-many rather than a one-to-many relationship. |
| network encryption | 网络加密 | Encrypting data as it travels across the network between a client and server. |
| non-CDB | 非CDB | An Oracle database that is not a multitenant container database (CDB). Before Oracle Database 12c, all databases were non-CDBs. Starting in Oracle Database 12c, every database must be either a CDB or a non-CDB. |
| noncircular reuse record | 非循环重用记录 | A control file record that contains critical information that does not change often and cannot be overwritten. Examples of information include tablespaces, data files, online redo log files, and redo threads. Oracle Database never reuses these records unless the corresponding object is dropped from the tablespace. |
| nondeferrable constraint | 非可延迟约束 | A constraint whose validity check is never deferred to the end of the transaction. Instead, the database checks the constraint at the end of each statement. If the constraint is violated, then the statement rolls back. |
| null | 空 | Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data. |
| object link | 对象链接 | In a PDB, an internal mechanism that points to data (not metadata) in the root. For example, AWR data resides in the root. Each PDB uses an object link to point to the AWR data in the root, thereby making views such as DBA_HIST_ACTIVE_SESS_HISTORY and DBA_HIST_BASELINE accessible in each separate container. |
| ORDBMS (object-relational database management system) | 对象关系型数据库管理系统 | An RDBMS that implements object-oriented features such as user-defined types, inheritance, and polymorphism. |
| object table | 对象表 | An special kind of table in which each row represents an object. |
| object type | 对象类型 | A schema object that abstracts a real-world entity such as a purchase order. Attributes model the structure of the entity, whereas methods implement operations an application can perform on the entity. |
| object view | 对象视图 | A virtual object table. Each row in the view is an object, which is an instance of a user-defined data type. |
| OLAP (Online Analytical Processing) | 在线分析处理 | OLAP is characterized by dynamic, dimensional analysis of historical data. |
| OLAP page pool | OLAP页池 | The pool in the UGA that manages OLAP data pages, which are equivalent to data blocks. The page pool is allocated at the start of an OLAP session and released at the end of the session. |
| OLTP (Online Transaction Processing) | 在线事务处理 | OLTP systems are optimized for fast and reliable transaction handling. Compared to data warehouse systems, most OLTP interactions involve a relatively small number of rows, but a larger group of tables. |
| online redo log | 在线重做日志 | The set of two or more online redo log files that record all changes made to Oracle Database data files and control file. When a change is made to the database, Oracle Database generates a redo record in the redo buffer. The log writer process (LGWR) process writes the contents of the redo log buffer to the online redo log. |
| online redo log group | 在线重做日志组 | An online redo log file and its redundant copies. |
| operating system block | 操作系统数据块（最小读写单元） | The minimum unit of data that the operating system can read or write. |
| operator | 操作程序/运算符 | 1. In memory management, operators control the flow of data. Examples include sort, hash join, and bitmap merge operators. 2. In SQL, an operator manipulates data items called operands or arguments and returns a result. Keywords or special characters represent the operators. For example, an asterisk (*) represents the multiplication operator. |
| optimizer | 优化程序 | Built-in database software that determines the most efficient way to execute a SQL statement by considering factors related to the objects referenced and the conditions specified in the statement. |
| optimizer statistics | 优化程序统计数据 | Details about the database its object used by the optimizer to select the best execution plan for each SQL statement. Categories include table statistics such as numbers of rows, index statistics such as B-tree levels, system statistics such as CPU and I/O performance, and column statistics such as number of nulls. |
| Oracle architecture | Oracle架构 | Memory and process structures used by Oracle Database to manage a database. |
| Oracle Application Express | Oracle Application Express | A Web application development tool for Oracle Database. Oracle Application Express uses built-in features such as user interface themes, navigational controls, form handlers, and flexible reports to accelerate application development. |
| Oracle ASM (Oracle Automatic Storage Management) | Oracle自动存储管理 | A volume manager and a file system for database files. Oracle ASM is Oracle's recommended storage management solution, providing an alternative to conventional volume managers and file systems. |
| Oracle ASM allocation unit | Oracle ASM分配单元 | The fundamental unit of allocation within an ASM disk group. An allocation unit is the smallest contiguous disk space that Oracle ASM allocates. One or more allocation units form an Oracle ASM extent. |
| Oracle ASM disk | Oracle ASM磁盘 | A storage device that is provisioned to an Oracle ASM disk group. An Oracle ASM disk can be a physical disk or partition, a Logical Unit Number (LUN) from a storage array, a logical volume, or a network-attached file. |
| Oracle ASM disk group | Oracle ASM磁盘组 | One or more Oracle ASM disks managed as a logical unit. I/O to a disk group is automatically spread across all the disks in the group. |
| Oracle ASM extent | Oracle ASM扩展 | A section of an Oracle ASM file. An Oracle ASM file consists of one or more file extents. Each Oracle ASM extent consists of one or more allocation units on a specific disk. |
| Oracle ASM file | Oracle ASM文件 | A file stored in an Oracle ASM disk group. The database can store data files, control files, online redo log files, and other types of files as Oracle ASM files. |
| Oracle ASM instance | Oracle ASM实例 | A special Oracle instance that manages Oracle ASM disks. Both the Oracle ASM instance and the database instances require shared access to the disks in an Oracle ASM disk group. Oracle ASM instances manage the metadata of the disk group and provide file layout information to the database instances. |