oracle x$ tables

前言

如果对动态性能视图（v$_)感兴趣的话，会发现这些视图都是访问的内部结构表（x$开头），也就是常说的fixed tables。要进一步分析这类表，你首先得了解几个重要的x$表。

• x$kqfta - 关于x$结构的标题信息，名称、大小、字段数量等
• x$kqfvi - 动态性能视图的名称列表
• x$kqfvt - 定义x$kqfvi中各个视图的select语句
• x$kqfdt - 衍生表：在x$kqfta结构中的一些衍生出来的同义词和视图
• x$kqfco - 每个x$kqfta中的表的字段列表

v$fixed_view_definition

这些x$表名称晦涩难懂，不过可以看看哪些动态性能视图用到了这些fixed table，通过v$fixed_view_definition或者v$fixed_table就能查到定义。

SQL> @v V$FIXED_VIEW_DEFINITION

VIEW_NAME                      TEXT
------------------------------ ----------------------------------------------------------------------------------------------------
GV$FIXED_VIEW_DEFINITION       select i.inst_id,kqfvinam,kqftpsel, i.con_id   from x$kqfvi i, x$kqfvt t where i.indx = t.indx
V$FIXED_VIEW_DEFINITION        select  VIEW_NAME , VIEW_DEFINITION, CON_ID from GV$FIXED_VIEW_DEFINITION where inst_id =
                               USERENV('Instance')

通过查看v$fixed_view_definition视图的定义发现，V$FIXED_VIEW_DEFINITION最终是查询的GV$FIXED_VIEW_DEFINITION视图，而带G开头的视图才是查询的x$表，GV$视图代表的查询RAC环境中的GLOBAL，而V$视图则表示查询本节点当中的信息。

同理看看v$fixed_table的构成

SQL> @v v$fixed_table

VIEW_NAME                      TEXT
------------------------------ ----------------------------------------------------------------------------------------------------
GV$FIXED_TABLE                 select inst_id,kqftanam, kqftaobj, 'TABLE', indx, con_id from x$kqfta union all select
                               inst_id,kqfvinam, kqfviobj, 'VIEW', 65537, con_id from x$kqfvi union all select inst_id,kqfdtnam,
                               kqfdtobj, 'TABLE', 65537, con_id from x$kqfdt

V$FIXED_TABLE                  select  NAME , OBJECT_ID , TYPE , TABLE_NUM, CON_ID from GV$FIXED_TABLE where inst_id =
                               USERENV('Instance')

通过上面的视图构成SQL可以知道，fixed table 其实本质上就是由基础内存结构（x$kqfta），基于这些内存结构构造的视图（x$kqfvi）以及衍生出来的表（x$kqfdt）union all组成。

那么什么是衍生表呢，有两种方式来回答这个问题。第一直接通过查询x$kqfdt，另一种就是找到哪里用到它了，这里是完整的x$kqfdt清单（19.16.0.0）

col KQFDTNAM for a40
col KQFDTEQU for a40
select * from x$kqfdt;

ADDR                   INDX    INST_ID     CON_ID   KQFDTOBJ KQFDTNAM                                 KQFDTEQU
---------------- ---------- ---------- ---------- ---------- ---------------------------------------- ----------------------------------------
00000000146F9940          0          1          0 4294952735 X$KSLLTR_CHILDREN                        X$KSLLTR
00000000146F9968          1          1          0 4294952736 X$KSLLTR_PARENT                          X$KSLLTR
00000000146F9990          2          1          0 4294956013 X$KSLLTR_OSP                             X$KSLLTR
00000000146F99B8          3          1          0 4294956014 X$KSLWSC_OSP                             X$KSLWSC
00000000146F99E0          4          1          0 4294951073 X$KCVFHONL                               X$KCVFH
00000000146F9A08          5          1          0 4294951074 X$KCVFHMRR                               X$KCVFH
00000000146F9A30          6          1          0 4294951440 X$KCVFHALL                               X$KCVFH
00000000146F9A58          7          1          0 4294951056 X$KGLTABLE                               X$KGLOB
00000000146F9A80          8          1          0 4294951057 X$KGLBODY                                X$KGLOB
00000000146F9AA8          9          1          0 4294951058 X$KGLTRIGGER                             X$KGLOB
00000000146F9AD0         10          1          0 4294951059 X$KGLINDEX                               X$KGLOB
00000000146F9AF8         11          1          0 4294951060 X$KGLCLUSTER                             X$KGLOB
00000000146F9B20         12          1          0 4294951061 X$KGLCURSOR                              X$KGLOB
00000000146F9B48         13          1          0 4294952684 X$KGLCURSOR_CHILD_SQLID                  X$KGLOB
00000000146F9B70         14          1          0 4294952680 X$KGLCURSOR_CHILD_SQLIDPH                X$KGLOB
00000000146F9B98         15          1          0 4294952683 X$KGLCURSOR_CHILD                        X$KGLOB
00000000146F9BC0         16          1          0 4294953372 X$KGLCURSOR_PARENT                       X$KGLOB
00000000146F9BE8         17          1          0 4294953759 X$KGLSQLTXL                              X$KGLOB
00000000146F9C10         18          1          0 4294956135 X$ALL_KQLFXPL                            X$KQLFXPL
00000000146F9C38         19          1          0 4294953124 X$KKSSQLSTAT_PLAN_HASH                   X$KKSSQLSTAT
00000000146F9C60         20          1          0 4294953231 X$ZASAXTD1                               X$ZASAXTAB
00000000146F9C88         21          1          0 4294953232 X$ZASAXTD2                               X$ZASAXTAB
00000000146F9CB0         22          1          0 4294953233 X$ZASAXTD3                               X$ZASAXTAB
00000000146F9CD8         23          1          0 4294951597 X$JOXFS                                  X$JOXFT
00000000146F9D00         24          1          0 4294951598 X$JOXFC                                  X$JOXFT
00000000146F9D28         25          1          0 4294951599 X$JOXFR                                  X$JOXFT
00000000146F9D50         26          1          0 4294951621 X$JOXFD                                  X$JOXFT
00000000146F9D78         27          1          0 4294952364 X$JOXOBJ                                 X$JOXFT
00000000146F9DA0         28          1          0 4294952365 X$JOXSCD                                 X$JOXFT
00000000146F9DC8         29          1          0 4294952366 X$JOXRSV                                 X$JOXFT
00000000146F9DF0         30          1          0 4294952367 X$JOXREF                                 X$JOXFT
00000000146F9E18         31          1          0 4294952368 X$JOXDRC                                 X$JOXFT
00000000146F9E40         32          1          0 4294952369 X$JOXDRR                                 X$JOXFT
00000000146F9E68         33          1          0 4294952370 X$JOXMOB                                 X$JOXFM
00000000146F9E90         34          1          0 4294952371 X$JOXMIF                                 X$JOXFM
00000000146F9EB8         35          1          0 4294952372 X$JOXMIC                                 X$JOXFM
00000000146F9EE0         36          1          0 4294952373 X$JOXMFD                                 X$JOXFM
00000000146F9F08         37          1          0 4294952374 X$JOXMMD                                 X$JOXFM
00000000146F9F30         38          1          0 4294952375 X$JOXMAG                                 X$JOXFM
00000000146F9F58         39          1          0 4294952376 X$JOXMEX                                 X$JOXFM
00000000146F9F80         40          1          0 4294956138 X$ALL_ASH                                X$ASH
00000000146F9FA8         41          1          0 4294956123 X$ALL_KESWXMON                           X$KESWXMON
00000000146F9FD0         42          1          0 4294956126 X$ALL_KESWXMON_PLAN                      X$KESWXMON_PLAN

43 rows selected.

同一个fixed表衍生出多个不同的fixed表，这里以X$KGLOB举例，它衍生出11个不同的fixed表，这样做的目的可能在于oracle想表示出：不同的衍生表以相互不兼容的方式代表同一源fixed表的不同组成部分，举一个例子：

SQL_ID  cgs3s9kvnt9hz, child number 0
-------------------------------------
select  child_number, plan_hash_value, sql_text from V$sql where sql_id
= '123456789'

Plan hash value: 1346707802

-------------------------------------------------------------------------------------------
| Id  | Operation               | Name                      | E-Rows |E-Bytes| Cost (%CPU)|
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |                           |        |       |     1 (100)|
|*  1 |  FIXED TABLE FIXED INDEX| X$KGLCURSOR_CHILD (ind:2) |      1 |   549 |     0   (0)|
-------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(("KGLOBT03"='123456789' AND "INST_ID"=USERENV('INSTANCE')))

查询v$sql视图，执行计划里显示的是查询X$KGLCURSOR_CHILD表。有趣的是X$KGLCURSOR_CHILD在x$kqfta表中查不到，只能在x$kqfdt中查到

SQL> select * from x$kqfta where KQFTANAM='X$KGLCURSOR_CHILD';

no rows selected

X$表索引

除此之外，这个执行计划还有个特别的地方，就是在访问X$KGLCURSOR_CHILD表时，后面还接了一个(ind:2)，走了FIXED TABLE FIXED INDEX的访问路径。要知道x$kglob表是一个表示Library Cache的内存结构，所以这个索引并不是我们理解的传统意义上的索引，仅仅表示Oracle根据谓词判断出有一个特别有效的访问路径，也就是通过filter里的"KGLOBT03"=‘123456789’。既然有ind:2了，那么是否还有其他访问路径呢。可以通过视图v$index_fixed_columns来查看

SQL> select * from v$indexed_fixed_column where table_name = 'X$KGLOB';

TABLE_NAME                     INDEX_NUMBER COLUMN_NAME                    COLUMN_POSITION     CON_ID
------------------------------ ------------ ------------------------------ --------------- ----------
X$KGLOB                                   1 KGLNAHSH                                     0          0
X$KGLOB                                   2 KGLOBT03                                     0          0

注意INDEX_NUMBER列表示使用了KGLOBT03字段作为第2索引来访问表，也与执行计划中的ind:2保持一致。也可以通过其他方式来验证KGLNAHSH字段对应v$sql.hash_value，KGLOBT03字段对应v$sql.sql_id。

select  child_number, plan_hash_value, sql_text from V$sql where
hash_value = '123456789'

Plan hash value: 2133195615

-------------------------------------------------------------------------------------------
| Id  | Operation               | Name                      | E-Rows |E-Bytes| Cost (%CPU)|
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |                           |        |       |     1 (100)|
|*  1 |  FIXED TABLE FIXED INDEX| X$KGLCURSOR_CHILD (ind:1) |      1 |   554 |     0   (0)|
-------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(("KGLNAHSH"=123456789 AND "INST_ID"=USERENV('INSTANCE')))

X$表数据从哪来

X$表通常展示内存里的数据，所以无论你什么时候来查询X$表，都是通过访问内存结构然后解析成可读的信息展示出来。

比如要查询V$SESSION视图用到的X$KSUSE表

SQL> SELECT addr, indx, ksuudnam FROM x$ksuse WHERE rownum <= 5;

ADDR                   INDX KSUUDNAM
---------------- ---------- --------------------
00000000A8B13588          1 SYS
00000000A8B15D70          2 SYS
00000000A8B18558          3 SYS
00000000A8B1AD40          4 SYS
00000000A8B1D528          5 SYS

查询上面的内存地址是存放在哪个内存区域

SQL> @fcha 00000000A8B13588

LOC KSMCHPTR           KSMCHIDX   KSMCHDUR KSMCHCOM           KSMCHSIZ KSMCHCLS   KSMCHTYP KSMCHPAR
--- ---------------- ---------- ---------- ---------------- ---------- -------- ---------- ----------------
SGA 00000000A8B13578          1          1 session             1951272 perm              0 00000000A80CD000

这个表示X$KSUSE读取的持久化分配的内存空间SGA里的一部分，那试下其他的看看

SQL> select addr,indx from x$kcccp where rownum<=5;

ADDR                   INDX
---------------- ----------
00007F5886AA9C00          0
00007F5886AA9C00          1
00007F5886AA9C00          2
00007F5886AA9C00          3
00007F5886AA9C00          4

SQL> @fcha 00007F5886AA9C00

LOC KSMCHPTR           KSMCHIDX   KSMCHDUR KSMCHCOM           KSMCHSIZ KSMCHCLS   KSMCHTYP KSMCHPAR
--- ---------------- ---------- ---------- ---------------- ---------- -------- ---------- ----------------
UGA 00007F5886AA9378                       kxsFrame4kPage         4152 freeabl           0 00

注意x$kcccp表读取的是UGA内存，这里就会引出另外一个问题了。有些X$表并不是直接读取内存当中存放的信息，而是通过一些辅助函数。所以当你去查询这些X$表时，会首先来调用一个辅助函数，它会去获取所需的源数据，然后将其拷贝至你的UGA内存区域当中，按照X$表的格式解析和展示出来。如果对访问x$kcccp表的过程做trace的话，会发现主要是在等待control file sequential read事件（为了获取检查点信息）。

SQL> @snapper stats,gather=w 2 1 964
Sampling SID 964 with interval 2 seconds, taking 1 snapshots...

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    SID, USERNAME  , TYPE, STATISTIC                                                 ,         DELTA, HDELTA/SEC,    %TIME, GRAPH       , NUM_WAITS,  WAITS/SEC,   AVERAGES
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    964, SYS       , WAIT, control file sequential read                              ,            38,    18.54us,      .0%, [          ],         5,       2.44,      7.6us average wait
    964, SYS       , WAIT, SQL*Net message to client                                 ,             3,     1.46us,      .0%, [          ],         2,        .98,      1.5us average wait
    964, SYS       , WAIT, SQL*Net message from client                               ,       2600456,      1.27s,   126.9%, [WWWWWWWWWW],         2,        .98,       1.3s average wait

--  End of Stats snap 1, end=2023-03-30 16:04:54, seconds=2

所以访问这个X$表不仅仅只是被动的读取内存结构，其辅助函数首先会为会话分配一些运行时的内存（UGA中的kxsFrame4kPage块），然后拷贝结果到这块内存区域，所以X$数组和偏移量解析代码能够读取并展示给查询引擎。

这里X$表中的ADDR地址信息不一定代表源数据的最终结果，而是在解析过程中被解析的最终数组是什么样子。有时候解析的数据结构就是最终的源数据，有时候辅助函数需要做一些实实在在的工作（比如获取latchs然后遍历完X$KSMSP的链表，或者直接做一些物理读比如上面的读取控制文件信息）。

现在重复查询x$kcccp表

SQL> select addr,indx from x$kcccp where rownum<=5;

ADDR                   INDX
---------------- ----------
00007F5886AB8770          0
00007F5886AB8770          1
00007F5886AB8770          2
00007F5886AB8770          3
00007F5886AB8770          4

SQL> select addr,indx from x$kcccp where rownum<=5;

ADDR                   INDX
---------------- ----------
00007F5886AB97A8          0
00007F5886AB97A8          1
00007F5886AB97A8          2
00007F5886AB97A8          3
00007F5886AB97A8          4

可以看到即使我们查询的是同样的数据，但是每次返回的ADDR都不一样。这并不代表源端的controlfile内存信息重新加载了，这个addr只是表示在执行SQL过程中所使用到的临时SQL游标的内存存放地址，因为这个就是辅助函数最后解析到的内存结构，所以就导致每次执行时得到的地址都不一样。

只要内存地址是在SGA当中，我们就可以说这个ADDR反映的是源数据实际存放的位置。但是当其在UGA或者PGA当中时，这也许只是代表了临时游标的内存地址。

附录

表名	描述
x$activeckpt	active checkpoint
x$bh	buffer header
x$ckptbuf	checkpoint buffer (queue)
x$dbgalertext	debug alert extented
x$dbglogext	debug log extended
x$dbgricx, x$dbgrifx, x$dbgrikx, x$dbgripx	debug tables
x$dbkece	debug kernel error, critical error
x$dbkefefc	debug kernel error, fatal error flood control
x$dglparam	data guard logical parameters
x$diag_alert_ext	diagnostics alert extended
x$diag_hm_run, x$diag_vhm_run	diagnostics health monitor runs
x$diag_ips_configuration	diagnostics incident packaging service configuration
x$dnfs_meta	dNFS metadata
x$dra_failure	data recovery advisor failures
x$drm_history, x$drm_history_stats	dynamic remastering history, stats
x$jskjobq	job scheduling ?, job queue
x$k2gte, x$k2gte2	kernel 2-phase commit, global transaction entry
x$kbrpstat	kernel backup recovery process(?) statistics
x$kcbbes	kernel cache, buffer ?
x$kcbbf	kernel cache, buffer buffer_handles
x$kcbfwait	kernel cache, buffer file wait
x$kcbkpfs	kernel cache, buffer ckpt prefetch statistics
x$kcbkwrl	kernel cache, buffer write list
x$kcbldrhist	kernel cache, buffer load direct read history
x$kcbobh	kernel cache, buffer, objectqueue buffer header
x$kcboqh	kernel cache, buffer, object queue header
x$kcbsw	kernel cache, buffer statistics why
x$kcbuwhy	kernel cache, buffer why
x$kcbwbpd	kernel cache, buffer workingset buffer pool descriptor
x$kcbwds	kernel cache, buffer workingset descriptors
x$kcbwh	kernel cache, buffer where/why
x$kcccf	kernel cache, controlfilemanagement control file
x$kcccp	kernel cache, controlfile checkpoint progress
x$kccdi	kernel cache, controlfilemanagement database information
x$kccle	kernel cache, controlfile logfile entry
x$kccnrs, x$kccrsp	kernel cache, controlfile non-guaranteed restorepoint; kernel cache, controlfile restore point
x$kcfis*	kernel cache, file intelligent scan
x$kclcrst	kernel cache, (RAC) lock, consistent read statistics
x$kclfh	kernel cache, (RAC) lock file hashtable
x$kclfi	kernel cache, (RAC) lock file index
x$kclfx	kernel cache, (RAC) lock (element) freelist statistics
x$kcluh	kernel cache, (RAC) lock undo header
x$kclui	kernel cache, (RAC) lock undo index
x$kcmscn	kernel cache, maximum SCN
x$kcrfstrand	kernel cache, redo file strand
x$kcrfx	kernel cache, redo file context
x$kcrrlns	kernel cache, recovery process LNS
x$kdxst	kernel data, index status
x$kdxhs	kernel data, index histogram
x$kewrtb	kernel server (manageability), workload repository tables
x$kfdat	kernel file, disk allocation table?
x$kffxp	kernel file, file extent map
x$kfklib	kernel file, - library
x$kghlu	kernel generic, heap LRUs
x$kglcursor	kernel generic, librarycache cursor
x$kgllk	kernel generic, librarycache lock
x$kglob	kernel generic, librarycache object
x$kglpn	kernel generic, librarycache pin
x$kglrd	kernel generic, librarycache readonly dependency
x$kglst	kernel generic, librarycache statistics
x$kgltr	kernel generic, librarycache translation
x$kgskvft	kernel generic, service, ?? fixed table
x$kjxm	kernel RAC cross-instance (?) messaging
x$kjznhangs, x$kjznhangses	kernel RAC diag node hang session
x$kmgsct	kernel memory, granule scoreboard ?
x$kmgstfr	kernel memory, granule - transfer
x$knstmvr	kernel replication, statistics materialized view refresh
x$kqdpg	kernel query, dictionary PGA
x$kqfco	kernel query, fixed table columns
x$kqfdt	kernel query, fixed derived table
x$kqfp	kernel query, fixed package
x$kqfsz	kernel query, fixed size (size of fixed objects in current version of Oracle)
x$kqfta	kernel query, fixed table
x$kqfvi	kernel query, fixed view
x$kqlfsqce	kernel query, librarycache fixedtable sql cursor environment
x$kqrpd	kernel query, rowcache parent definition
x$kqrsd	kernel query, rowcache subordinate definition
x$krbmsft	kernel recovery, - search file ?
x$krcfh, x$krcfde, x$krcfbh, x$krcbit	kernel recovery, changetracking file, header, descriptor, bitmap header, bitmap block
x$ksbdd	kernel service, background detached (process) definition
x$ksbsrvdt	kernel service, background server detached (process)
x$ksbtabact	kernel service, background - action
x$ksimsi	kernel service, instance management serial (and) instance (numbers)
x$ksipc_info and x$ksipc_proc_stats	kernel service IPC info and process stats
x$ksi_reuse_stats	kernel service, instance, reuse stats
x$ksled, x$kslei, x$ksles	kernel service, lock, event descriptors, events for instance, events for session
x$kslemap	kernel service, lock, event map
x$kslhot	kernel service, lock, hot (blocks)
x$ksllclass	kernel service, lock, latch class
x$ksllw	kernel service, lock, latch where
x$kslpo	kernel service, latch posting
x$ksmdd	kernel service, memory segmented (array) definition
x$ksmfs	kernel service, memory fixed SGA
x$ksmfsv	kernel service, memory fixed SGA variables
x$ksmhp	kernel service, memory heap
x$ksmjch, x$ksmjs	kernel service, memory, java chunks, java (pool) statistics
x$ksmlru	kernel service, memory LRU
x$ksmls	kernel service, memory large (pool) statistics
x$ksmmem	kernel service, memory
x$ksmns	kernel service, memory numa (pool) statistics
x$ksmpgdst	kernel service, memory PGA detailed statistics
x$ksmpp	kernel service, memory pga heap
x$ksmsp	kernel service, memory sga heap
x$ksmspr	kernel service, memory shared pool reserved
x$ksmsp_dsnew	kernel service, memory shared pool, - statistics new
x$ksmsp_nwex	kernel service, memory shared pool ?
x$ksmss	kernel service, memory sga statistics
x$ksmsst, x$ksmstrs	kernel service, memory, sga streams (pool), streams (pool) statistics
x$ksmssinfo	kernel service, memory sga OS (level) info
x$ksmup	kernel service, memory uga heap
x$ksolsfts	kernel service, object level statistics, fts?
x$ksppcv	kernel service, parameter, current (session) value
x$ksppi	kernel service, parameter, parameter info
x$ksppsv	kernel service, parameter, system value
x$kspspfh	kernel service, parameter spfile header
x$ksrcctx, x$ksrcdes	kernel service, (intra-instance) broadcast, channel context, channel description
x$ksrchdl	kernel service, (intra-instance) broadcast, channel ?
x$kstex	kernel service, trace execution
x$ksulop	kernel service, user long operation
x$ksulv	kernel service, user locale value
x$ksupgp, x$ksupgs	kernel service, user, process group, process group sniped
x$ksupl, x$ksuru	kernel service, user, process (resource) limit, resource usage
x$ksuprlat	kernel service, user process latch
x$ksuse	kernel service, user session
x$ksusecon	kernel service, user session connection
x$ksusm	kernel service, user session migration
x$ksuvmstat	kernel service, user virtual memory statistics
x$kswsastab	kernel service, workgroup services, service table
x$ksxafa	kernel service, execution, - file affinity
x$ksxm_dft	kernel service, execution, modification dml frequency tracking
x$ksxpclient	kernel service, ipc, client
x$ksxpif	kernel service, ipc, interface
x$ksxpping	kernel service, ipc, ping
x$ksxp_stats	kernel service, ipc, stats
x$ktcn*	kernel transaction, change notification *
x$ktcxb	kernel transaction, control, transaction object
x$ktfbfe	kernel tablespace, file bitmap free extent
x$ktfbhc	kernel tablespace, file bitmap header control
x$ktfbnstat	kernel tablespace, file bigfile - stat
x$ktfbue	kernel tablespace, file bitmap used extent
x$ktifb, x$ktiff, x$ktifp, x$ktifv	kernel transaction, in-memory flush, ?
x$ktprhist	kernel transaction, parallel (transaction) recovery history
x$ktsimapool	kernel transaction, - in-memory pool
x$ktsj*	kernel transaction, space job(?)
x$ktskstat	kernel transaction/tablespace, segment shrink statistics
x$ktslchunk	kernel transaction/tablespace, space LOB chunk
x$ktspstat	kernel tablespace, space statistics
x$ktsso	kernel transaction, sort segment
x$ktugd	kernel transaction, undo global data
x$ktuqqry	kernel transaction, undo - query
x$kturhist	kernel transaction, undo recovery history
x$ktusmst	kernel transaction, undo system managed, statistics
x$ktuxe	kernel transaction, undo transaction entry
x$kvii, x$kvit	kernel (performance) view, instance, initialization, transitory (parameters)
x$kwqbpmt	kernel OLTP queue ?
x$kxdbio_stats, x$kxdcm*, x$kxdrs	kernel Exadata, block (level) intelligent operations stats, callback for metrics, resilvering
x$kxfbbox	kernel execution, fast (parallel process) black box
x$kxfpbs	kernel execution, fast (parallel) process batch size
x$kxfpcds, x$kxfpcms, x$kxfpcst	kernel execution, fast (parallel) process, coordinator, dequeue stats, message stats, (query) stats
x$kxfpinstload	kernel execution, fast (parallel) process instance load
x$kxfpsds, x$kxfpsms, x$kxfpsst	kernel execution, fast (parallel) process, slave, dequeue stats, message stats, (query) stats
x$kxsbd	kernel execution, SQL bind data
x$kxttstecs, x$kxttstehs, x$kxttsteis, x$kxttstets	kernel execution, temporary table stats, column stats, histograms, index stats, table stats
x$kywm*	kernel - workload management
x$kzspr, x$kzsro	kernel security, session, privilege, role
x$le	lock element
x$lobsegstat, x$lobstat, x$logstathist	LOB (segment) stats, history
x$logbuf_readhist	Log buffer read histogram
x$messages	(background process) messages
x$modact_length	(sql) module action length (limit)
x$qesmmiwt	query execution, sql memory management ?
x$qesmmsga	query execution, sql memory management ?
x$qksbgses, x$qksbgsys	query compilation service, bug session or system
x$qksceses, x$qkscesys	query compilation service, compilation environment, session or system
x$skgxp_connection, x$skgxp_port	OS kernel generic interface IPC, connections, ports
x$targetrba	target RBA
x$trace	trace
x$uganco	user global area, network connection
x$xplton, x$xpltoo	explain plan sql trace(?) operation name, option
x$xs_sessions	? sessions
x$zasa*	?