Oracle hidden utilities undocumented parameters

Each new release of Oracle brings new hidden utilities. They are sometimes used by internal Oracle development staff and left in Oracle for emergency support. Many of these undocumented utilities are very powerful but can be complex.

A hidden utility is not a utility that is physically hidden in the Oracle software. Rather, a hidden utility is an executable or PL/SQL package that is either undocumented or where the documentation is difficult to find. For example, some PL/SQL packages are never loaded by the Oracle installer, yet their definitions remain in the operating system files.

The main directories of interest regarding Oracle utilities are the following:

• $ORACLE_HOME/bin -- This contains the binary executables used by the Oracle server. Most of the tools discussed in this book reside here.
• $ORACLE_HOME/plsql/demo -- This contains a useful collection of SQL scripts related to many utilities, including dbms_profiler.
• $ORACLE_HOME/rdbms/admin -- This contains many SQL scripts used for creating PL/SQL packages and their required environments.
• $ORACLE_HOME/sqlplus/admin -- This contains scripts used with autotrace and other utilities.
• $ORACLE_HOME/otrace/admin -- This is the administration directory for the Oracle Trace diagnostic tool.
• $ORACLE_HOME/otrace/sysman -- This is used by utilities such as oemctl and the Oracle Management Server (OMS).
• $ORACLE_HOME/otrace/lib -- This contains facility files used with the oerr utility.

Many of these undocumented utilities such as TKPROF have surfaced from the obscure and entered mainstream Oracle toolkits while others remain hidden inside the O/S.

Finding hidden utilities

The easiest way to find hidden utilities is to look for new packages within Oracle. The following query compares the packages in Oracle8i with new packages in Oracle9i, using a database link:

select 
   object_name 
from 
   dba_objects@oracle9i 
where 
   owner      = 'SYS' 
and 
   object_type = 'PACKAGE BODY' 
minus
select 
   object_name 
from  
   dba_objects@oracle8i
where 
   owner     = 'SYS'
and 
   object_type = 'PACKAGE BODY';

This query will quickly display all package bodies owned by SYS that exist in Oracle9i but not in Oracle8i.

Finding hidden executables

Discovering new binaries entails comparing two directories in the operating system and ignoring the duplicates entries. The dircmp UNIX command can be used to find only new entries within two directories:

dircmp -- s 
/u01/aoracle/product/8.1.7/bin 
/u01/oracle/product/9.0.3/bin

Here we compare the bin directory of 8.1.7 with Oracle 9.0.2. Note that the -- s option of the dircmp command tells UNIX to eliminate the matches from the output.

Hidden initialization parameters

Hidden initialization parameters are very dangerous because their use is undocumented, but they can be very valuable if you are careful. We can use a SQL*Plus query to quickly find any new initialization parameters in a new version of Oracle:

select 
   name
from 
    v$parameter@oracle9i 
minus 
select 
   name
from 
   v$parameter@oracle8i
; 

Every version of Oracle has special undocumented initialization parameters. These undocumented initialization parameters are usually only used in emergencies and only under the direction of a senior DBA or Oracle support. Because the undocumented parameters begin with an underscore "_", we can write a query against the X$ foxed tables to easily extract them:

COLUMN parameter           FORMAT a37
COLUMN description         FORMAT a30 WORD_WRAPPED
COLUMN "Session Value"     FORMAT a10
COLUMN "Instance Value"    FORMAT a10
SET LINES 100
SET PAGES 0
SPOOL undoc.lis

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 '/'
/

Remember, not everyone knows about the undocumented parameters, and few know how or when to use them. Oracle does not allow DBAs to use many of these parameters unless specifically directed by Oracle support. DBAs should be aware that use of certain undocumented parameters will result in an unsupported system.

In many cases, the undocumented parameters were either documented in previous releases or will be in future releases. Of course, it is difficult to use the undocumented parameters that have never been documented, and never will be, safely. When in doubt, get guidance from Oracle support. We will take a closer look at some of these parameters in our next installment.

Hidden v$ views

New V$ views indicate a new functionality within the database, and not all of the views make it into the Oracle documentation. Here is a quick query to locate new views:

select 
   view_name
from 
    v$fixed_view_description@oracle9i 
minus 
select 
   view_name
from 
   v$fixed_view_description@oracle8i
; 

Hidden system events

With each new release of Oracle, system events and system statistics are changed. For example, here is a query to show changed system statistics for the database writer process between Oracle8i and Oracle9i:

sql> select distinct name from v$sysstat where name like 'DBWR%'

NAME
----------------------------------------------------------------
DBWR Flush object call found no dirty buffers
DBWR Flush object cross instance calls
DBWR buffers scanned
DBWR checkpoint buffers written
DBWR checkpoint write requests
DBWR checkpoints
DBWR cross instance writes
DBWR free buffers found
DBWR incr. ckpt. write requests
DBWR lru scans
DBWR make free requests
DBWR revisited being-written buffer
DBWR skip hot writes
DBWR summed scan depth
DBWR timeouts
DBWR transaction table writes
DBWR undo block writes

Here is the listing from Oracle9i.  Note the changed statistics.

SQL> select distinct name from v$sysstat where name like 'DBWR%';

NAME
----------------------------------------------------------------
DBWR buffers scanned
DBWR checkpoint buffers written
DBWR checkpoints
DBWR cross instance writes
DBWR free buffers found
DBWR fusion writes
DBWR lru scans
DBWR make free requests
DBWR revisited being-written buffer
DBWR summed scan depth
DBWR transaction table writes
DBWR undo block writes

By the same token, we van use the v $event_name view to quickly locate all new system events in Oracle19i.  Here is a quick query to locate new views:

select 
   name
from 
    v$event_name@oracle9i 
minus 
select 
   name
from 
   v$event_name@oracle8i
; 

Undocumented SQL hints

Getting a list of Oracle hints for SQL tuning is often difficult. The Oracle hint list is inside the Oracle executable and you can extract the Oracle hint list easily with UNIX commands. In UNIX, you can use grep and strings to get them directly from the Oracle executable:

   strings $ORACLE_HOME/bin/oracle > hints.lst

Here are all of the 124 Oracle9i hints on release 9.0.2:

ALL_ROWS	AND_EQUAL	ANTIJOIN
APPEND	BITMAP	BUFFER
BYPASS_RECURSIVE_CHECK	BYPASS_UJVC	CACHE
CACHE_CB	CACHE_TEMP_TABLE	CARDINALITY
CHOOSE	CIV_GB	COLLECTIONS_GET_REFS
CPU_COSTING	CUBE_GB	CURSOR_SHARING_EXACT
DEREF_NO_REWRITE	DML_UPDATE	DOMAIN_INDEX_NO_SORT
DOMAIN_INDEX_SORT	DRIVING_SITE	DYNAMIC_SAMPLING
DYNAMIC_SAMPLING_EST_CDN	EXPAND_GSET_TO_UNION	FACT
FIRST_ROWS	FORCE_SAMPLE_BLOCK	FULL
GBY_CONC_ROLLUP	GLOBAL_TABLE_HINTS	HASH
HASH_AJ	HASH_SJ	HWM_BROKERED
IGNORE_ON_CLAUSE	IGNORE_WHERE_CLAUSE	INDEX_ASC
INDEX_COMBINE	INDEX_DESC	INDEX_FFS
INDEX_JOIN	INDEX_RRS	INDEX_SS
INDEX_SS_ASC	INDEX_SS_DESC	INLINE
LEADING	LIKE_EXPAND	LOCAL_INDEXES
MATERIALIZE	MERGE	MERGE_AJ
MERGE_SJ	MV_MERGE	NESTED_TABLE_GET_REFS
NESTED_TABLE_SET_REFS	NESTED_TABLE_SET_SETID	NL_AJ
NL_SJ	NO_ACCESS	NO_BUFFER
NO_EXPAND	NO_EXPAND_GSET_TO_UNION	NO_FACT
NO_FILTERING	NO_INDEX	NO_MERGE
NO_MONITORING	NO_ORDER_ROLLUPS	NO_PRUNE_GSETS
NO_PUSH_PRED	NO_PUSH_SUBQ	NO_QKN_BUFF
NO_SEMIJOIN	NO_STATS_GSETS	NO_UNNEST
NOAPPEND	NOCACHE	NOCPU_COSTING
NOPARALLEL	NOPARALLEL_INDEX	NOREWRITE
OR_EXPAND	ORDERED	ORDERED_PREDICATES
OVERFLOW_NOMOVE	PARALLEL	PARALLEL_INDEX
PIV_GB	PIV_SSF	PQ_DISTRIBUTE
PQ_MAP	PQ_NOMAP	PUSH_PRED
PUSH_SUBQ	REMOTE_MAPPED	RESTORE_AS_INTERVALS
REWRITE	RULE	SAVE_AS_INTERVALS
SCN_ASCENDING	SELECTIVITY	SEMIJOIN
SEMIJOIN_DRIVER	SKIP_EXT_OPTIMIZER	SQLLDR
STAR	STAR_TRANSFORMATION	SWAP_JOIN_INPUTS
SYS_DL_CURSOR	SYS_PARALLEL_TXN	SYS_RID_ORDER
TIV_GB	TIV_SSF	UNNEST
USE_ANTI	USE_CONCAT	USE_HASH
USE_MERGE	USE_NL	USE_SEMI
USE_TTT_FOR_GSETS

In this output we see 57 undocumented SQL hints:

BYPASS_RECURSIVE_CHECK	BYPASS_UJVC	CACHE_CB
CACHE_TEMP_TABLE	CIV_GB	COLLECTIONS_GET_REFS
CUBE_GB	CURSOR_SHARING_EXACT	DEREF_NO_REWRITE
DML_UPDATE	DOMAIN_INDEX_NO_SORT	DOMAIN_INDEX_SORT
DYNAMIC_SAMPLING	DYNAMIC_SAMPLING_EST_CDN	EXPAND_GSET_TO_UNION
FORCE_SAMPLE_BLOCK	GBY_CONC_ROLLUP	GLOBAL_TABLE_HINTS
HWM_BROKERED	IGNORE_ON_CLAUSE	IGNORE_WHERE_CLAUSE
INDEX_RRS	INDEX_SS	INDEX_SS_ASC
INDEX_SS_DESC	LIKE_EXPAND	LOCAL_INDEXES
MV_MERGE	NESTED_TABLE_GET_REFS	NESTED_TABLE_SET_REFS
NESTED_TABLE_SET_SETID	NO_EXPAND_GSET_TO_UNION	NO_FACT
NO_FILTERING	NO_ORDER_ROLLUPS	NO_PRUNE_GSETS
NO_STATS_GSETS	NO_UNNEST	NOCPU_COSTING
OVERFLOW_NOMOVE	PIV_GB	PIV_SSF
PQ_MAP	PQ_NOMAP	REMOTE_MAPPED
RESTORE_AS_INTERVALS	SAVE_AS_INTERVALS	SCN_ASCENDING
SKIP_EXT_OPTIMIZER	SQLLDR	SYS_DL_CURSOR
SYS_PARALLEL_TXN	SYS_RID_ORDER	TIV_GB
TIV_SSF	UNNEST	USE_TTT_FOR_GSETS

These Oracle SQL hints can be extremely useful for solving complex SQL execution problems, and that is why Oracle created them. They generally remain undocumented because Oracle does not want someone with inadequate knowledge of optimizer internals using them because they could lead to confusing results. However, for the experienced SQL tuning professional these undocumented hints are a godsend.

Conclusion

In this first installment we have shown you how to extract powerful (but potentially dangerous) undocumented utilities, parameters, hints, packages and executables. Remember, many of these undocumented features have unknown effects and should be treated with great care.