Database Reference
In-Depth Information
CHAPTER 2
■ ■ ■
Hidden Initialization
Parameters
T
he view
V$PARAMETER
provides access to documented initialization parameters. This view is
built on top of the X$ fixed tables
X$KSPPI
and
X$KSPPCV
(more on X$ fixed tables in Part IV). All
hidden parameters start with one or two underscores.
V$PARAMETER
is built in such a way that
it lists documented parameters only. There are 540 undocumented parameters in Oracle9
i
Release 2, 1,124 in Oracle10
g
Release 2, and 1,627 in Oracle11
g
. Their names and a short
description may be retrieved with the following query (file
hidden_parameters.sql
):
SQL> SELECT ksppinm name,
ksppstvl value,
ksppdesc description
FROM x$ksppi x, x$ksppcv y
WHERE (x.indx = y.indx)
AND x.inst_id=userenv('instance')
AND x.inst_id=y.inst_id
AND ksppinm LIKE '\_%' ESCAPE '\'
ORDER BY name;
The source code depot contains a complete list of undocumented parameters for the releases
Oracle9
i
, Oracle10
g
, and Oracle11
g
. Due to the large amount of undocumented parameters, it
is impossible to investigate what they all do and to document the results. I have chosen
_TRACE_
FILES_PUBLIC
and
_ASM_ALLOW_ONLY_RAW_DISKS
as examples of two useful hidden parameters
and discuss both in detail in this chapter. Of course there are many more, which may be useful
under certain circumstances. Some may be set to work around bugs (e.g.,
_DISABLE_RECOVERABLE_
RECOVERY
), while others may have an effect on performance (e.g.,
_ROW_CACHE_CURSORS
,
_ASM_
AUSIZE
). Still others may be used to salvage data from a database when the documented
recovery procedures fail due to invalid backup procedures or the loss of the current online
redo log (
_ALLOW_RESETLOGS_CORRUPTION
,
_OFFLINE_ROLLBACK_SEGMENTS
). Normally hidden
parameters should only be set under the supervision of Oracle Support Services.
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