X$
X$表是Oracle数据库的核心部分，用于跟踪数据库内部信息，维护数据库的正常运行。
CREATE OR REPLACE VIEW BH AS SELECT * FROM SYS.X$BH; CREATE OR REPLACE PUBLIC SYNONYM X$BH FOR BH;
数据字典表
数据字典表用以存储表、索引、约束以及其他数据库结构的信息。这些对象通常是以“$”结尾（例如：TAB$、OBJ$、TS$等）。Bsq是非常重要的一个文件，其中包含了数据字典的定义以及注释说明，深入学习Oracle数据库的用户都应该仔细阅读该文件，该文件位于$ORACLE_HOME/rdbms/admin目录下（在Oracle 11g中bsq文件被分别归类到不通的.bsq文件）。
静态数据字典视图
静态数据字典视图是由catalog.sql脚本创建（在$ORACLE_HOME/rdbms/admin下）。静态数据字典中的视图可以分为三类，它们分别由三个前缀构成：USER_*、ALL_*和DBA_*。
动态性能视图
动态性能视图基于从数据库内存结构构建的虚拟表。因此，它们不是存储在数据库中的常规表。由于数据动态更新，所以，动态性能视图的读一致性不能保证。
SYS是这些动态性能表的所有者，这些表的名字都以V_$开头，基于这些表的视图被创建后，Oracle还会为视图创建公共同义词。同义词名称以V$开头，例如，视图V$DATAFILE包含数据库数据文件的信息，而V$FIXED_TABLE包含数据库中所有动态性能表和视图的信息。NOMOUNT状态不能查询所有的V$视图。因为动态性能视图不是真实表，所以数据依赖于数据库和实例的状态。例如，当数据库处于NOMOUNT时，可以查询V$INSTANCE和V$BGPROCEP。但是，在MOUNT数据库之前，不能查询V$DATAFILE。
只有赋予了SELECT ANY DICTIONARY权限的用户才可以查询所有的V$视图。
v$动态视图主要是针对SGA和数据库实例进程的显示。在这个过程中，Oracle使用了DMA（Direct Memory Access）技术，可以高效的获取这些分析数据。
数据库启动时，Oracle动态创建X$表，在此基础之上，Oracle创建了GV$和V$视图。
系统数据字典视图定义有关的几个视图
DICTIONARY视图记录了全部数据字典表的名称和解释，它有一个同义词DICT。
DICT_COLUMNS视图记录了全部数据字典表里字段名称和解释。
如果想知道数据字典中的USER_INDEXES视图中各字段的详细含义，那么可以用如下SQL语句：
SQL> SELECT COLUMN_NAME, COMMENTS FROM DICT_COLUMNS WHERE TABLE_NAME = 'USER_INDEXES'; COLUMN_NAME COMMENTS ------------------------------ ---------------------------------------------------------------------------------------------------- INDEX_NAME Name of the index INDEX_TYPE TABLE_OWNER Owner of the indexed object TABLE_NAME Name of the indexed object TABLE_TYPE Type of the indexed object UNIQUENESS Uniqueness status of the index: "UNIQUE", "NONUNIQUE", or "BITMAP" COMPRESSION Compression property of the index: "ENABLED", "DISABLED", or NULL PREFIX_LENGTH Number of key columns in the prefix used for compression TABLESPACE_NAME Name of the tablespace containing the index INI_TRANS Initial number of transactions MAX_TRANS Maximum number of transactions INITIAL_EXTENT Size of the initial extent in bytes NEXT_EXTENT Size of secondary extents in bytes MIN_EXTENTS Minimum number of extents allowed in the segment MAX_EXTENTS Maximum number of extents allowed in the segment PCT_INCREASE Percentage increase in extent size PCT_THRESHOLD Threshold percentage of block space allowed per index entry INCLUDE_COLUMN User column-id for last column to be included in index-only table top index FREELISTS Number of process freelists allocated in this segment FREELIST_GROUPS Number of freelist groups allocated to this segment PCT_FREE Minimum percentage of free space in a block LOGGING Logging attribute BLEVEL B-Tree level LEAF_BLOCKS The number of leaf blocks in the index DISTINCT_KEYS The number of distinct keys in the index AVG_LEAF_BLOCKS_PER_KEY The average number of leaf blocks per key AVG_DATA_BLOCKS_PER_KEY The average number of data blocks per key CLUSTERING_FACTOR A measurement of the amount of (dis)order of the table this index is for STATUS Whether the non-partitioned index is in USABLE or not NUM_ROWS Number of rows in the index SAMPLE_SIZE The sample size used in analyzing this index LAST_ANALYZED The date of the most recent time this index was analyzed DEGREE The number of threads per instance for scanning the partitioned index INSTANCES The number of instances across which the partitioned index is to be scanned PARTITIONED Is this index partitioned? YES or NO TEMPORARY Can the current session only see data that it place in this object itself? GENERATED Was the name of this index system generated? SECONDARY Is the index object created as part of icreate for domain indexes? BUFFER_POOL The default buffer pool to be used for index blocks FLASH_CACHE The default flash cache hint to be used for index blocks CELL_FLASH_CACHE The default cell flash cache hint to be used for index blocks USER_STATS Were the statistics entered directly by the user? DURATION If index on temporary table, then duration is sys$session or sys$transaction else NULL PCT_DIRECT_ACCESS If index on IOT, then this is percentage of rows with Valid guess ITYP_OWNER If domain index, then this is the indextype owner ITYP_NAME If domain index, then this is the name of the associated indextype PARAMETERS If domain index, then this is the parameter string GLOBAL_STATS Are the statistics calculated without merging underlying partitions? DOMIDX_STATUS Is the indextype of the domain index valid DOMIDX_OPSTATUS Status of the operation on the domain index FUNCIDX_STATUS Is the Function-based Index DISABLED or ENABLED? JOIN_INDEX Is this index a join index? IOT_REDUNDANT_PKEY_ELIM Were redundant primary key columns eliminated from iot secondary index? DROPPED Whether index is dropped and is in Recycle Bin VISIBILITY Whether the index is VISIBLE or INVISIBLE to the optimizer DOMIDX_MANAGEMENT If this a domain index, then whether it is system managed or user managed SEGMENT_CREATED Whether the index segment has been created ORPHANED_ENTRIES INDEXING AUTO Whether the index is automatically created CONSTRAINT_INDEX 61 rows selected.
V$FIXED_TABLE可以查询数据库中所有底层的表。V$FIXED_TABLE视图显示数据库中所有动态性能表、视图和导出表。由于某些V$表（例如V$ROLLNAME）涉及底层的表，因此，没有列出.
V$FIXED_VIEW_DEFINITION这个视图包含所有固定视图（以V$起头的视图）的定义
