Oracle X$ Tables

Table Name Acronym Expanded Comments
x$bh buffer header The most common use of this table is to find the object and the file# and block# of its header when there's high cache buffers chains latch contention: select obj, dbarfil, dbablk from x$bh a, v$latch_children b where a.hladdr = b.addr for the said latch (whose sleeps you think are too high). You can also use this table to see if a specific buffer has too many clones: select dbarfil, dbablk, count(*) from x$bh group by dbarfil, dbablk having count(*) > 2. Note that obj column matches dba_objects.data_object_id, not object_id. For performance reason, don't merge dba_extents with the query of x$bh that has a group by, unless you use in-line view and no_merge hint (see J. Lewis Practical Oracle8i, p.215) The tch column, touch count, records how many times a particular buffer has been accessed. Its flag column is explained by J. Lewis; explanation of state, mode and indx can be found in Anjo Kolk's paper. Tim is time the buffer touch happened (276392.996). Lru_flag is about the buffer's hot/cold feature (Ref and 221860.999); 8 is often used to find hot blocks.
x$dbgalertext debug alert extented One use is to find old alert.log text long after you recycled the physical file: select originating_timestamp, message_text from x$dbgalertext. The message_id and message_group columns are also interesting and are not available in alert.log.
x$dbglogext debug log extended In, related to In-Memory DB only?
x$dbkece debug ?? critical error Base table of v$diag_critical_error but includes facility dbge (Diagnostic Data Extractor or dde)
x$dglparam data guard logical parameters contains invisible parameters, shows whether they're dynamic
x$diag_alert_ext diagnostics alert extended same as x$dbgalertext but has more lines, slower to query
kernel 2-phase commit, global transaction entry See Note:104420.1. Find sessions coming from or going to a remote database; in short, x$k2gte.k2gtdses matches v$session.saddr, .k2gtdxcb matches v$transaction.addr. select /*+ ordered */ substr(s.ksusemnm,1,10)||'-'|| substr(s.ksusepid,1,10) origin, substr(g.k2gtitid_ora,1,35) gtxid, substr(s.indx,1,4)||'.'|| substr(s.ksuseser,1,5) lsession, s.ksuudlna username, substr(decode(bitand(ksuseidl,11), 1,'ACTIVE', 0, decode( bitand(ksuseflg,4096) , 0,'INACTIVE','CACHED'), 2,'SNIPED', 3,'SNIPED', 'KILLED'),1,1) status, e.kslednam waiting from x$k2gte g, x$ktcxb t, x$ksuse s, x$ksled e where g.k2gtdxcb=t.ktcxbxba and g.k2gtdses=t.ktcxbses and s.addr=g.k2gtdses and e.indx=s.ksuseopc; It's better than checking for DX locks for outgoing sessions (since a DX lock only shows up in v$lock for the current distributed transaction session). X$k2gte2 is the same as x$k2gte except on k2gtetyp which may show 2 for 'TIGHTLY COUPLED' instead of 0 for 'FREE'. One use of x$k2gte[2] is the clearly translated global transaction ID in k2gtitid_ora as opposed to the hex numbers in v$global_transaction.globalid.
x$kbrpstat kernel backup recovery? stat RMAN related
x$kcbbes kernel cache, buffer ? Check incremental checkpoints (259586.1)
x$kcbbf kernel cache, buffer buffer_handles Jonathan Lewis ("_db_handles")
x$kcbfwait kernel cache, buffer file wait A commonly used query breaks down the contents of v$waitstat into per-datafile statistics: select name, count, time from v$datafile df, x$kcbfwait fw where fw.indx+1 = df.file#
x$kcbkpfs kernel cache, buffer ckpt prefetch statistics Tanel Poder
x$kcbkwrl kernel cache, buffer write list each row for the write list of one DBWR
x$kcbldrhist kernel cache, buffer load direct read history  
x$kcbobh kernel cache, buffer, objectqueue buffer header 10g and up. Tanel Poder
x$kcboqh kernel cache, buffer, object queue header See above
x$kcbsw kernel cache, buffer statistics why Note:34405.1: select kcbwhdes, why0+why1+why2 "Gets", "OTHER_WAIT" from x$kcbsw s, x$kcbwh w where s.indx=w.indx and s."OTHER_WAIT">0 order by 3 (That works for 10g only; for newer versions, see x$kcbuwhy); Ref1 ("statistics about the way these [x$kcbwh] functions have been used")
x$kcbuwhy kernel cache, buffer why For 11g and up, select kcbwhdes, why0+why1+why2 "Gets", "OTHER_WAIT" from x$kcbsw s, x$kcbwh w, x$kcbuwhy w2 where s.indx=w.indx and w.indx=w2.indx and s."OTHER_WAIT">0
x$kcbwbpd kernel cache, buffer workingset buffer pool descriptor See 183770.999 for relationship to x$bh and x$kcbwds. Some people use this query to find how many blocks of a segment are in each buffer pool: select decode(pd.bp_id,1,'KEEP',2,'RECYCLE',3,'DEFAULT',4,'2K SUBCACHE',5,'4K SUBCACHE',6,'8K SUBCACHE',7,'16K SUBCACHE',8,'32K SUBCACHE','UNKNOWN') subcache, bh.object_name,bh.blocks from x$kcbwds ds, x$kcbwbpd pd, (select /*+ use_hash(x) */ set_ds, object_name, count(*) BLOCKS from obj$ o, x$bh x where o.dataobj#=x.obj and x.state!=0 and o.owner#!=0 and'&mytable' group by set_ds, bh where ds.set_id>=pd.bp_lo_sid and ds.set_id<=pd.bp_hi_sid and pd.bp_size!=0 and ds.addr=bh.set_ds
x$kcbwds kernel cache, buffer workingset descriptors See above. Also see Ref1, Ref2, Ref3. Total row count in this table is _db_block_lru_latches, although only db_writer_processes rows have real numbers.
x$kcbwh kernel cache, buffer where/why See x$kcbsw for SQL. Ref1 ("different functions that may be used to perform different types of logical I/O"), Ref2
x$kcccf kernel cache, controlfilemanagement control file In 10gR1, to find controlfile size as viewed at OS level but from inside Oracle, select cfnam, (cffsz+1)*cfbsz from x$kcccf. cfbsz is the controlfile log block size; should report the same as the command dbfsize controlfile ($ORACLE_HOME/bin/dbfsize is available on UNIX, regardless Oracle version.) In 10gR2, block size and file size are both in v$controlfile although Reference manual misses them.
x$kcccp kernel cache, controlfile checkpoint progress S. Adams and K Gopalakrishnan use this view to find how much the current redo log is filled. Eygle studied instance heartbeat, column cphbt.
x$kccdi kernel cache, controlfilemanagement database information  
x$kccle kernel cache, controlfile logfile entry lebsz may be used to show redo logfile block size, usually 512; should report the same as the command dbfsize redologfile ($ORACLE_HOME/bin/dbfsize is available on UNIX only)
x$kccnrs, x$kccrsp kernel cache, controlfile non-guaranteed restorepoint; kernel cache, controlfile restore point Base tables of v$restore_point, for non-guaranteed and guaranteed restore points. Retain records of them after they were dropped
x$kcfis* kernel cache, file intelligent scan Exadata smart scan related. Note that views for cell servers per se are x$kxdcm_* (kernel Exadata cell module) plus x$cell_name
x$kclcrst kernel cache, (RAC) lock, consistent read statistics base table of v$cr_block_server or v$bsp, used to troubleshoot global cache cr requests
x$kclfh kernel cache, (RAC) lock file hashtable  
x$kclfi kernel cache, (RAC) lock file index  
x$kclfx kernel cache, (RAC) lock (element) freelist statistics See Ref1, Ref2, 1492990.1. If lwm is too low, you may see 'gc freelist' wait.
x$kcluh kernel cache, (RAC) lock undo header  
x$kclui kernel cache, (RAC) lock undo index  
x$kcmscn kernel cache, maximum SCN Cur_scn is the same as v$database.current_scn. Cur_max_scn should be 16384*seconds since 1988 (1376995.1). This view may be related to the SCN headroom problem.
x$kcrfstrand kernel cache, redo file strand 10g and up. Info about redo strands. Non-zero pvt_strand_state_kcrfa_cln (and strand_num_ordinal_kcrfa_cln=3735928559 or DEADBEEF in hex) means a transaction is using this private strand. (Private strands may be disabled in RAC or if supplemental logging is on, but multistrand redo is still used.) Strand_size_kcrfa is the strand size (meaningful only if last_buf_kcrfa<>'00'; Ref). Also see 18164614, Ref.
x$kcrfx kernel cache, redo file context "columns bfs (buffer size) and bsz (block size). Dividing bfs by bsz gives mxr (the maximum number of blocks to read size)" (from Anjo Kolk's paper)
x$kcrrlns kernel cache, recovery process LNS Related to LNS (redo transport) processes, NSSn (sync) and NSAn (async).
x$kdxst kernel data, index status used in catalog.sql to create index_stats
x$kdxhs kernel data, index histogram Used in catalog.sql to create index_histogram
x$kewrtb kernel event?, workload repository tables See Note:555124.1
x$kfdat kernel file, disk allocation table? Only populated in ASM instance. See Note:351117.1 and Steve Shaw and Julian Dyke Pro Oracle Database 10g RAC on Linux, pp.232-3. Column v_kfdat is 'V' for allocated and 'F' for free. For most ASM-related x$ tables, read Luca Canali.
x$kffxp kernel file, file extent map Only populated in ASM instance. You can check how many extents are allocated for each datafile on which disk, e.g. select, d.path, d.group_number, d.disk_number, count(*) from v$asm_alias a, v$asm_disk d, v$asm_file f, x$kffxp x where a.group_number = x.group_kffxp and a.file_number = x.number_kffxp and d.group_number = x.group_kffxp and d.disk_number = x.disk_kffxp and f.group_number = a.group_number and f.file_number = a.file_number and f.type = 'DATAFILE' group by, d.path, d.group_number, d.disk_number, f.bytes order by 1;
x$kfklib kernel file, ? library You can tell from inside ASM instance whether you're using ASMLib and its version.
x$kghlu kernel generic, heap LRUs  
x$kglcursor kernel generic, librarycache cursor Base table for v$sql, v$sqlarea. Fixed view based on x$kglob according to x$kqfdt. See Note 1 or x$kglob for more details. One use of this table is for finding partially parsed SQLs because they cause parse failures (viewable in v$sysstat or v$sesstat). Their kglobt02 (command type) is 0, kglobt09 (child number) is 65535 for the child, SQL text length is cut to 20 chars, kglobt17 and kglobt18 (parsing and user schema) are 0 or 2147483644 (for 32-bit Oracle) depending on if it's parent or child, and obviously miss heap 6 (cursor body). Find them by select kglnaobj, kglnatim, kglobts0, kglnahsh from x$kglcursor where kglobt02 = 0 (kglobts0 is module; you can further restrict by kglnatim i.e. first_load_time).
x$kgllk kernel generic, librarycache lock Used in catblock.sql to build dba_kgllock. kgllkuse or kgllkses maps to v$session.saddr, kgllkpnc call pin, kgllkpns session pin, kgllkmod lock held (0: no lock; 1: null; 2: shared; 3: exclusive), kgllkflg (allegedly 8 for pre-10g or 2048 for 10g meaning SQL being run, Ref; 256 for broken kgl lock in 10g or 1 in 9i, Ref), kgllkspn savepoint. If you get library cache lock or pin wait, kgllkhdl matches v$session_wait.p1raw (handle address), and kglnaobj is the first 80 characters of the object name. Note:122793.1 has this SQL for our convenience: select * from x$kgllk lock_a where kgllkreq = 0 and exists (select lock_b. kgllkhdl from x$kgllk lock_b where kgllkses = '&saddr_from_v$session' /* blocked session */ and lock_a.kgllkhdl = lock_b.kgllkhdl and kgllkreq > 0). Kgllkadr column is shown in event 10270 trace files to find SQLs in session cursor cache (Ref). X$kgllk.kglhdpar matches x$kglob.kglhdpar if there's a KGL lock on the object.
x$kglob kernel generic, librarycache object To find library cache object for wait events library cache pin or lock and pipe get or put, match kglhdadr with v$session_wait.p1raw. kglhdflg is partially explained in Note:311689.1 (for permanent keeping). kglhddmk may be data object load mask; can be used to identify the number of the loaded heap, counted from 0 (see comment of 06/12/01 in Bug:1164709). Steve Adams' objects_on_hot_latches.sql finds the way Oracle links a library cache object (based on kglnahsh) to a specific library cache child latch. x$kglob, and in case of cursors x$kglcursor too, can be used to find library cache objects that are partially built therefore not visible in v$sql(XXX), v$open_cursor, v$object_dependency. (Try typing select *; and enter, then check these views!) Kglobhd[0-7] is heap descriptor address and kglobhs[0-7] is its size; can join to x$ksmhp.ksmchds to see heap components.
x$kglpn kernel generic, librarycache pin used in catblock.sql to build dba_kgllock. Some columns are simiarly explained for x$kgllk.
x$kglrd kernel generic, librarycache readonly dependency kglnacnm (container name?) is PL/SQL program unit or anonymous block while kglnadnm (dependent name?) is the individual SQLs inside the PL/SQL unit. Ref; this may be the way to differentiate between user recursive SQLs (code in PL/SQL, trigger, etc.) from system-generated recursive SQLs (data dictionary check etc.). (See also v$object_dependency, but that doesn't show relation between PL/SQL block and its contents.) In 11g, v$sql.program_id may be used to tie the constituent SQL to its containing PL/SQL stored object (not anonymous block).
x$kgltr kernel generic, librarycache translation Maps synonym translation from original (kgltrorg) to final (kgltrfnl) address, All 3 address columns map to x$kglob.kglhdadr.
x$kgskvft kernel generic, service, ?? fixed table Base table of v$blocking_quiesce. If the blocking session is not in SYS_GROUP consumer group according to v$rsrc_session_info, v$blocking_quiesce ignores it. Workaround is to directly query x$kgskvft. (Ref; Bug 7832504)
x$kjxm kernel RAC cross-instance (?) messaging Stats about messages sent and received on RAC instances (not about data buffers which would be in x$kclcrst i.e. v$cr_block_server a.k.a v$bsp). The kjxmname column is interesting as well as the stats.
x$kjznhangs, x$kjznhangses kernel RAC diag node hang session Base tables of v$hang_info and v$hang_session_info so column names can be deciphered. Retain info after the hang.
x$kmgsct kernel memory, granule scoreboard ? Base table of v$sga_dynamic_components, v$sga_current_resize_ops etc., probably used to be named x$ksmgst and x$ksmgsc in 9i.
x$kmgstfr kernel memory, granule ? transfer Maybe another way of representing SGA and memory components resizing operations. Ts: time; startaddr and end: addresses before and after resizing; donor and receiver: x$kmgsct.grantype
x$knstmvr kernel replication, statistics materialized view refresh Base table of v$mvrefresh. Stores MV refresh history info, such as session SID and serial#. Un-exposed columns reftype_knstmvr, groupstate_knstmvr and total_* are useful; see the query in Note:258021.1.
x$kqdpg kernel query, dictionary PGA Row cache cursor statistics, columns explained in "How can you tune it?" section of Tuning the _row_cache_cursors Parameter. Note this is PGA. Need to dump another process's PGA to view it.
x$kqfco kernel query, fixed table columns One use is to find all fixed tables given a column name, e.g. select kqftanam, kqfconam, kqfcoidx from x$kqfco c, x$kqfta t where t.indx=c.kqfcotab and kqfconam='KGLHDADR', or like part of the column name. If kqfcoidx is 0, the column is not indexed.
x$kqfdt kernel query, fixed derived table Contains x$kglcursor, x$kgltable etc. which are based on x$kglob; effectively these are views of other x$ tables, but Oracle couldn't call them views because they already had x$kqfvi.
x$kqfp kernel query, fixed package Used in catprc.sql to build disk_and_fixed_objects view. Each object has two rows, one package and one package body.
x$kqfsz kernel query, fixed size (size of fixed objects in current version of Oracle)  
x$kqfta kernel query, fixed table Base table of v$fixed_table, whose object_id (indx of x$kqfta) matches obj# of tab_stats$, the table dbms_stats.gather_fixed_objects_stats inserts stats into.
x$kqfvi kernel query, fixed view  
x$kqlfsqce kernel query, librarycache fixedtable sql cursor environment Base table of v$sql_optimizer_env. One use is to find all parameters including underscore ones in the environment of a SQL cursor by not restricting on column kqlfsqce_flags as v$sql_optimizer_env does.
x$kqrpd kernel query, rowcache parent definition Column kqrpdosz is size of this parent rowcache object, not exposed in v$rowcache_parent although shown in rowcache dump.
x$kqrsd kernel query, rowcache subordinate definition Column kqrsdosz is size of this subordinate rowcache object, not exposed in v$rowcache_subordinate although shown in rowcache dump.
x$krcfh, x$krcfde, x$krcfbh, x$krcbit kernel recovery, changetracking file, header, descriptor, bitmap header, bitmap block Alex Gorbachev
x$ksbdd kernel service, background detached (process) definition Base table of v$bgprocess. Column ksbddfile in 12c associates the process with a header file. But you may wish to see the actual internal names for the processes, in and, which are more detailed and available in earlier versions as well.
x$ksbsrvdt kernel service, background server detached (process) Probably background process slaves.
x$ksbtabact kernel service, background ? action Actions performed by certain background processes and their timeout values
x$ksimsi kernel service, instance management serial (and) instance (numbers) Base table of v$active_instances. The un-exposed ksimisum column is instance incarnation number, matching "Reconfiguration started ... new inc ..." in alert.log.
x$ksipc_info and x$ksipc_proc_stats kernel service IPC info and process stats These two 12c tables do not exist as I checked although v$fixed_table has their names. They probably would contain stats for the new IPC0 background process. Also related to the new _ksipc* parameters.
x$ksi_reuse_stats kernel service, instance, reuse stats 12c only. Not sure why it's called this name. Apparently it's about enqueue resources. The name column is a wondeful alternative brief description for each enqueue or lock, compared with v$lock_type.description: select a.type, a.description, from v$lock_type a, x$ksi_reuse_stats b where a.type = b.resname order by 1
x$ksled, x$kslei, x$ksles kernel service, lock, event descriptors, events for instance, events for session Base tables for v$event_name, v$system_event, and v$session_event, respectively. Benefit of querying x$ksles: (1) When ksleswts (wait count) is 0, v$session_event won't have the row but x$ksles still has them with non-zero kslestim (time waited micro) or kslesmxt (max wait time in micro); (2) Since kslesmxt is in microsec, it could be non-zero even if v$session_event.max_wait is 0. x$kslei has benefit (2) over v$system_event. In 12c, the new column ksleddsp of x$ksled provides a better event name, such as "db single block read" for the perpetually confusing "db file sequential read": select kslednam, ksleddsp from x$ksled where kslednam != ksleddsp
x$kslemap kernel service, lock, event map "Indx = event number...Basically map events to a small number of useful classes like I/O waits" (Ref)
x$kslhot kernel service, lock, hot (blocks) Set _db_hot_block_tracking to true and track hot blocks in buffer cache. It's an alternative and probably better way than checking touch count. (Ref)
x$ksllclass kernel service, lock,, latch class "describes the 8 classes", "Specify which latch belongs to which class" with _latch_class_ (Ref)
x$ksllw kernel service, lock, latch where Base table of v$latch_misses. But column ksllwlbl is not exposed in any view. It's said to record "the 'Why' meaning for some 'Where'" (Ref) or "Unit to guard" (Ref).
x$kslpo kernel service, latch posting Bug:653299 says it "tracks which function is posting smon". Ksllwnam column (the part before semicolon if it exists) can match v$latch_misses.location to identify the latch that uses this function. Column ksllwlbl is explained in the entry for x$ksllw.
x$ksmdd kernel service, memory segmented (array) definition Ref
x$ksmfs kernel service, memory fixed SGA One of the base tables of v$sgastat. Shows sizes of fixed SGA, buffer cache, log buffer, shared I/O pool (for SecureFile LOBs), and in 12c, data transfer cache. Even though some of these can be dynamically resized in modern versions of Oracle, any component in shared memory not in some kind of pool (v$sgastat where pool is null) is left in this "fixed" SGA table.
x$ksmfsv kernel service, memory fixed SGA variables detailing fixed SGA: select a.ksmfsnam, a.ksmfstyp, a.ksmfssiz, b.ksmmmval from x$ksmfsv a, x$ksmmem b where a.ksmfsadr = b.addr and a.ksmfsnam like... (Ref. p.82, Oracle Internal Services). For a latch, get ksmfsnam by matching x$ksmfsv.ksmfadr with x$kslld.kslldadr. You can see SGA parameters in ksmfsnam column and get their values with oradebug dumpvar varname or all values with oradebug dumpsga
x$ksmhp kernel service, memory heap S. Adams, "What it returns depends on which heap descriptor you join to it. It is effectively a function returning the contents of an arbitrary heap that takes the heap descriptor as its argument." You need to join this table to another one on heap descriptor ksmchds, such as in v$sql_shared_memory (joining to x$kglcursor), or to x$ksmsp (on column ksmchpar), or kglobhd[0-6] of x$kglob or x$kglcursor_child, and possibly need to use use_nl hint. Example, example.
x$ksmjch, x$ksmjs kernel service, memory, java chunks, java (pool) statistics X$ksmjch shows each chunk of java pool except for free area. (Nor interesting to me though)
x$ksmlru kernel service, memory LRU Refer to Metalink Notes 61623.1 and 43600.1 for details. Note that query on this table can only be done once; subsequent query returns no rows unless large chunk shared pool allocations happened in the interim.
x$ksmls kernel service, memory large (pool) statistics  
x$ksmmem kernel service, memory Entire SGA memory map. Each row shows memory content for 8 bytes (on 64-bit Oracle). Due to memory guard pages, you can only select from x$ksmmem specifying a specific indx or addr (addr=hextoraw('...')), or by joining to another table on addr column; otherwise the session may hang or throws ORA-3113 (Windows doesn't seem to have this problem). One usage is to find the value for an SGA variable, e.g. select ksmmmval from x$ksmfsv a, x$ksmmem b where a.ksmfsadr=b.addr and ksmfsnam='kzaflg_' to see if audit is enabled (what kzaflg_ means), which is equivalent to oradebug peek command, or for this particular purpose, even more simply, oradebug dumpvar sga kzaflg_.
Indx is SGA index, i.e. the difference of SGA address and sgabeg (which is x$ksmmem.addr where indx = 0) divided by architecture word size (4 for 32-bit, 8 for 64-bit machines). E.g., the value stored at address 0000000060001F40 on a 64-bit machine whose sgabeg is 0x60000000 can be calculated as:
select (to_number('0000000060001F40','xxxxxxxx') - to_number('60000000','xxxxxxxx')) /8 from dual;
select ksmmmval from x$ksmmem where indx = 1000;
x$ksmns kernel service, memory numa (pool) statistics  
x$ksmpp kernel service, memory pga heap PGA heap (variable area). PGA subheaps: select /*+use_nl(h,p)*/ h.ksmchds,p.ksmchcom, h.ksmchcom ksmchnam,h.ksmchsiz, h.ksmchcls,h.ksmchpar from x$ksmhp h,x$ksmpp p where h.ksmchds = p.ksmchpar and p.ksmchcls like '%recr' and p.ksmchpar != hextoraw('00');
x$ksmsp kernel service, memory sga heap The 3rd argument of ORA-4031 tells you which section of shared (or java or large) pool is short of memory. It matches x$ksmsp.ksmchcom (or v$ SGA heaps: select /*+use_nl(h,s)*/ sess.sid, sess.username, h.ksmchds, h.ksmchcom ksmchnam, h.ksmchsiz, h.ksmchcls,h.ksmchpar from x$ksmhp h,x$ksmsp s,v$session sess where h.ksmchds = s.ksmchpar and s.ksmchcls like '%recr' and s.ksmchpar != hextoraw('00') and h.ksmchown = sess.saddr; SGA subheaps: select /*+use_nl(h,s)*/ h.ksmchds,s.ksmchcom,h.ksmchcom ksmchnam, h.ksmchsiz,h.ksmchcls,h.ksmchpar from x$ksmhp h,x$ksmsp s where h.ksmchds = s.ksmchpar and s.ksmchcls like '%recr'and s.ksmchpar != hextoraw('00'); You can sort on ksmchptr to get a map of memory pieces. In ksmchcom, the hex number after SQLA^ is the SQL hash value.
x$ksmspr kernel service, memory shared pool reserved  
x$ksmsp_dsnew kernel service, memory shared pool, ? statistics new One row summarizes subpools and durations. Dscnt_kghdsnew is subpool count (distinct dsidx_ksmnwex in x$ksmsp_nwex). Cursiz_kghdsnew is total duration count (row count of x$ksmsp_nwex).
x$ksmsp_nwex kernel service, memory shared pool ? A new efficient fixed table shows subpools and durations. See 396940.1.
x$ksmss kernel service, memory sga statistics The 3rd argument of ORA-4031 tells you which section of shared (or java or large) pool is short of memory. It matches x$ksmss.ksmssnam (or v$
x$ksmsst, x$ksmstrs kernel service, memory, sga streams (pool), streams (pool) statistics  
x$ksmssinfo kernel service, memory sga OS (level) info This 12c table shows how OS level shared memory segments are used (which segment is used by what component of SGA), in effect matching the rows of `ipcs -m' with those of v$sga. It also tells you whether and which segments are using HugePages, so you don't have to check /proc/pid/smaps to see that and is of course more detailed than just seeing the brief message in alert.log.
x$ksmup kernel service, memory uga heap UGA heap (variable area). UGA subheaps: select /*+use_nl(h,s)*/ h.ksmchds,u.ksmchcom,h.ksmchcom ksmchnam,h.ksmchsiz,h.ksmchcls,h.ksmchpar from x$ksmhp h,x$ksmup u where h.ksmchds = u.ksmchpar and u.ksmchcls like '%recr' and u.ksmchpar != hextoraw('00');
x$ksolsfts kernel service, object level statistics, fts? Base table of v$segstat and v$segment_statistics. Fts_stmp records the last time fts_staval was updated, fts_preval the previously recorded value. Fts_inte greater than 0 reveals some less known types of statistics. Note that value in v$segstat or v$segment_statistics is cumulative; e.g., if "row lock waits" is non-zero, the waits may not be happening right now.
x$ksppcv kernel service, parameter, current (session) value Base table of v$parameter and v$parameter2. See comments on x$ksppi.
x$ksppi kernel service, parameter, parameter info Base table of v$parameter, v$system_parameter and v$system_parameter2. Often used to see undocumented parameters: select a.ksppinm Parameter, a.ksppdesc Description, b.ksppstvl "Session Value", c.ksppstvl "Instance Value" from x$ksppi a, x$ksppcv b, x$ksppsv c where a.indx = b.indx and a.indx = c.indx and a.ksppinm like '\_%' escape '\' order by 1. You can also see if the parameter is dynamically changeable with the query in v$fixed_view_definition (nicely formatted here) by manually removing the part that excludes underscore parameters.
x$ksppsv kernel service, parameter, system value Base table of v$system_parameter and v$system_parameter2. See comments on x$ksppi.
x$kspspfh kernel service, parameter spfile header You can see when you made the last change to the spfile: select kspspfhmodtime/86400+to_date('19870630','yyyymmdd') from x$kspspfh. I don't know what's special about that time.
x$ksrcctx, x$ksrcdes kernel service, (intra-instance) broadcast, channel context, channel description Base tables of the undocumented v$channel_waits, which is used to find the big contributors to "reliable message" wait event. You can find the last message publishing time by select a.name_ksrcdes, b.totpub_ksrcctx, b.waitcount_ksrcctx, b.waittime_ksrcctx, lastpub_ksrcctx/86400+to_date('19700101','yyyymmdd') from x$ksrcdes a, x$ksrcctx b where b.name_ksrcctx=a.indx and b.waitcount_ksrcctx>0. Column id_ksrcdes of x$ksrcdes provides alternative keywords you can use to search for.
x$ksrchdl kernel service, (intra-instance) broadcast, channel ? Column ctxp_ksrchdl matches p1 of 'reliable message' (context) and owner_ksrchdl matches ksspaown of x$ksuse (base table of v$session). Last message time is lastmsg_ksrchdl/86400+to_date('19700101','yyyymmdd').
x$kstex kernel service, trace execution Base table of v$execution, a table documented poorly and probably wrong for a long time. The definition in v$fixed_view_definition probably should restrict on id instead of op (where id=10), official documentation should call FUNCTION function, PID pid (as v$, and the view should expose sid as session ID. This table together with x$trace provides info about KST trace. Unfortunately it seems to have stopped working in 11g and up.
x$ksulop kernel service, user long operation Base table of v$session_longops. Column ksulotgt, probably for total gets?, is not exposed.
x$ksulv kernel service, user locale value Base table of v$nls_valid_values.
x$ksupgp, x$ksupgs kernel service, user, process group, process group sniped X$ksupgp.ksupgpnm!='DEFAULT' may suggest session leaking (Ref). X$ksupgs is the base table of undocumented v$detached_session showing sessions killed (without immediate option) but not cleaned.
x$ksupr kernel service, user process  
x$ksuse kernel service, user session T. Poder finds hidden recursive session based on ksuseflg.
x$ksusecon kernel service, user session connection In 11g, check client version with
with x as (select distinct to_char(ksuseclvsn,'xxxxxxx') v
 from x$ksusecon where ksusenum = &sid)
select decode(v, '       0', 'no version provided: 10g or lower, or background process?',
 to_number(substr(v,1,2),'xx') || '.' || --maj_rel
 to_number(substr(v,3,1),'x') || '.' || --mnt_rel
 to_number(substr(v,4,2),'xx') || '.' || --ias_rel
 to_number(substr(v,6,1),'x') || '.' || --pat_set
 to_number(substr(v,7,2),'xx')) client_version -- port_mnt
from x;
[Update 2013-07] Not needed in 12c because v$session_connect_info.client_version works fine.
x$ksuvmstat kernel service, user virtual memory statistics In 10g and up, base table providing physical_memory_bytes to v$osstat (and VM paging stats on Windows). But on Linux up to Oracle, this number is system free memory in kilobytes (grep MemFree /proc/meminfo); on other OSes or or up on Linux, it is "Total number of bytes of physical memory".
x$kswsastab kernel service, workgroup services, Base table of v$services and a few other service-related views. v$services may need x$kswsastab.kswsastabpflg=0 restriction; otherwise stopped services linger in the view till instance bounce.
x$ktcxb kernel transaction, control object Base table of v$transaction. Four bits of ktcxbflg column, exposed as v$transaction.flag, are explained in v$fixed_view_definition. Metalink Mark Bobak and Melissa Holman explain the bit for isolation level. Since v$transaction is empty without a transaction, you can directly query x$ktcxb to find sessions with serializable isolation level: select * from v$session where taddr in (select ktcxbxba from x$ktcxb where bitand(ktcxbflg,268435456) <> 0). Other bits of ktcxbflg not shown in v$fixed_view_definition are: bit 1 read write and read committed, 4(?) read only, 13 using private strand (Ref).
x$ktfbfe kernel transaction, file bitmap free extent Free extent bitmap in file header for LMT (equivalent to fet$ in DMT); check dba_free_space view definition
x$ktfbhc kernel transaction, file bitmap header control Summarizes free space with one row per datafile (Ref); check dba_data_files or dba_temp_files view definition
x$ktfbue kernel transaction, file bitmap used extent Used extent bitmap in file header for LMT (equivalent to uet$ in DMT)
x$ktifb, x$ktiff, x$ktifp, x$ktifv kernel transaction, ? flush ? Probably related to in-memory undo. See 7th bullet of IMU.txt.
x$ktprxrt kernel transaction, parallel transaction rollback Fairlie Rego
x$ktsso kernel transaction, sort segment Base table of v$sort_usage (or $tempseg_usage). From, ktssosqlid provides SQL ID for the SQL associated with this temp segment usage, not exposed in the v$ views. See Bug 17834663 and description.
x$ktuxe kernel transaction, undo transaction entry "get the SCN of the most recently committed (local) transaction" with select max(ktuxescnw * power(2, 32) + ktuxescnb) from x$ktuxe (Ref); select * from x$ktuxe where ktuxecfl = 'DEAD' and ktuxesta = 'ACTIVE' "shows transaction dead waiting for cleanup" (Ref)
x$kxfpsds kernel execution, fast process slave dequeue statistics Current list of reasons for parallel execution dequeuing, as explained for wait event "parallel query dequeue wait" in Anjo Kolk's paper.
x$kxsbd kernel execution, SQL bind data Base table of v$sql_bind_data. Column kxsbdof2 (or shared_flag2 of v$sql_bind_data) is oacfl2 (not oacflg2 as in Note:39817.1) in SQL trace. "System-generated binds have a value of 256 in the SHARED_FLAG2 column". According to Bug 4359367, when it's 0x300, the bind variable is marked as unsafe (affecting cursor_sharing=similar). Note:296377.1 has more on its value.
x$kzsprv kernel security, session privilege Session-specific. Base table for v$enabledprivilege, which is base table of session_privs
x$kzsro kernel security, session role used in many SQL scripts in ?/rdbms/admin
x$le lock element See the definition of GV$BH for its relationship with x$bh. Note that v$gc_element differs slightly from v$lock_element and documentation for v$gc_element is really for v$lock_element.
x$le_stat lock element status  
x$logbuf_readhist Log buffer read histogram 951152.1
x$messages (background process) messages May be the place where processes store and fetch messages. Related to _messages parameter, messages latch, and "rdbms ipc (message|reply)" wait events.
x$qksceses, x$qkscesys query compilation service, compilation environment, session or system Base tables for v$ses_optimizer_env and v$sys_optimizer_env, respectively. There're so many optimizer parameters the two documented views are missing that sometimes you need to query these base tables directly. Select pname_qksceserow from x$qksceses minus select name from v$ses_optimizer_env to have a feel of the missing ones. Or subtract v$ from x$qkscesys.pname_qkscesyrow.
x$targetrba target RBA Ref
x$trace   Beginning with 9i, x$trace records event tracing info. 10g RAC bdump/cdmp_time directory has trw files that contain the same info (the trace file seems to have columns TimeInMicroSec:?, OraclePid, SID, event, OpCode, TraceData). select event, count(*) from x$trace group by event shows what events are enabled internally (not shown in v$parameter). oerr ora eventID on UNIX shows the event name. RAC databases should have GES and GCS related events set. select pid, count(*) from x$trace group by pid shows how many events have been trapped by each oracle process (including those that exited). select sid, pid, count(*) from x$trace where (sid, pid) in (select sid, pid from v$session s, v$process p where s.paddr = p.addr) group by sid, pid order by 1, 2 shows the numbers for each currently existing session (I think without the where clause, exited sessions would be included). select event, op, time, seq#, data from x$trace where sid = &sid and pid = &pid order by time shows traced events for a session in question. Below 11g, op column indicates various operations, such as 7 for wait, 11 for latch post (896098 ).
x$uganco user global area, network connection Base table of v$dblink. Since it's about UGA, each session has different content. After you end your distributed transactions (which includes distributed queries) and close database links, v$dblink no longer shows the entries. But x$uganco still has them, with ncoflg set to 8320 and hstflg set to 0. Unfortunately this is not very useful because you can't see the UGA content from a different session.

Note 1 x$kglcursor columns

These columns are not exposed in v$sql based on comparison in v$fixed_view_definition in (for column match between x$kglcursor and v$sql, view browser source):
kglhdamk: always 0;
kglhddmk: 0,1,65,253,...; some kind of masks (kglhdkmk, keep mask, is exposed as v$sql.kept_versions);
kglhdexc: executions but not used any more; see documentation for v$db_object_cache.executions which is x$kglob.kglhdexc; 9i uses kglobt05 and 10g uses kglobt48; but kglobt05 and kglobt48 may be 0 perhaps on heap 6 flush(?) while kglhdexc keeps the old value?
kglhdflg: (Ref) in my 9i DB:

SQL> select to_char(kglhdflg,'xxxxxxxx'), count(*) from x$kglcursor group by to_char(kglhdflg,'xxxxxxxx');

--------- ----------
 10010000        193
 10010001         39
 12010000         49
 50010000         95

kglhdnsp, kglhdobj, kglhdpmd: parent handle namespace, object, pin mode;
kglnadlk: (DB link, Ref);
kglnaptm: previous time; probably deprecated, all null;
kglobflg: Ref;
kglobhd[0-7]: heap descriptors 0 to 7, can be passed to x$ksmhp.ksmchds, can be used to perform the heapdump_addr dumps (Bug:2247763)
kglobhs7: heap 7 size
kglobt22 kglobt23 kglobt24 kglobt25 kglobt26 kglobt27
kglobtl0 kglobtl1
kglobtn1 kglobtn2 kglobtn3 kglobtn4 kglobtn5

Note 2 Links

Rama Velpuri Original source of possibly all x$ table web pages
Julian Dyke X$ tables of different versions
Frank Naude Covers some tables not covered by me
Egor Starostin Definitions in v$fixed_view_definition nicely formatted for 10.2 and 11.2

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