RE: what is the data dictionary?

From: Goulet, Richard <>
Date: Tue, 22 Sep 2009 08:24:03 -0400
Message-ID: <>


    No creating an object as sys does not make it part of the data dictionary and besides it is very bad practice to do so. Any object that is owned by sys is not exported from the database by the exp utility thereby making it non transportable. If your really interested in what makes up the data dictionary look in sql.bsq as it has the basic data dictionary. Warning: Venturing down into this layer is NOT for the faint of heart as things get really cryptic fast. Even data within one of these tables is not obvious as to it's meanings. Many of the dba_ views use decodes, bitand, and other similar functions to turn them into meaningful data, not information. Also be careful of stuff that you pick up on the internet. Some people like to try and demonstrate a great understanding of what Oracle is doing, sometimes to disastrous results. A certain person who owns a consulting firm with his last name as part & parcel of the company name is rather infamous for his erroneous publishing. Basically if it's not published in the Oracle Reference manual by Oracle then your venturing into their proprietary area and what happens is totally unknown. Many years back the aforementioned person published a way to rename objects in the database by twiddling with the x$ tables. More than one database was ruined beyond help because of that. Many of us who have been around this product for many a year (since 1985 for me) have come to the conclusion that piddling around in that area is a case of no reward for much effort.  

Dick Goulet
Senior Oracle DBA/NA Team Lead
PAREXEL International  

From: Martin Berger [] Sent: Tuesday, September 22, 2009 4:44 AM To: Goulet, Richard
Subject: Re: what is the data dictionary?


thank you for the answer.
Until now I assume the Data Dictionary is a set of tables (e.g. obj$ tab$, ...) and views representing them (DBA_OBJECTS, DBA_TABLES, ...). They all belong to schema SYS (ok, some synonyms may belong to PUBLIC?) But if I create a table sys.myobj$ in tablespace SYSTEM and a vie DBA_MYOBJ on this table, does they belong to the data dictionary also? Or is there any other quality how the Oracle kernel decides which objects to treat as DATA DICTIONARY and which not?

I disagree a little about the v$ (or x$) views: From my point of view, V$bh is (also) there to tell me WHAT is in the cache (beside some other infos like the corresponding latches, buckets etc.)

There is a lot material available about buffer cache, but I didn't find anything similar about row cache. Is this because it's not available or noone is interrested in this data structure and its affects?



            First off the data dictionary is listed under those views that start with DBA or USER. They point you at what your or others tables/objects are and their properties. As far as any view that starts with V$ those are dynamic performance views of internal data structures. They are not there to tell you what is in the caches, but how they are behaving.          

	Dick Goulet 
	Senior Oracle DBA/NA Team Lead 
	PAREXEL International 



[] On Behalf Of Martin Berger
	Sent: Monday, September 21, 2009 4:16 PM
	Subject: what is the data dictionary?
	Hi List, 

	once again I stumble about two realy dumb (and related)
	1) what is the data dictionary (or how can I identify objects
which belongs to it)?
	2) is there any spyhole to view into the row cache (like v$bh
for the buffer cache)?

        According to the documentation, the data dictionary is htm#CNCPT2033

        "A read-only collection of database tables and views containing reference information about the database, its structures, and its users."

        The row cache is defined with htm#CNCPT44459

        "A memory area in the shared pool
.htm#CBAIACIC> that holds data dictionary
.htm#CHDJCHJA> information. The data dictionary cache is also known as the row cache because it holds data as rows instead of buffers, which hold entire data blocks."

        Even v$rowcache seems not as featured as I'd like it to be.           

        This question is not based on any need, just pure curiosity.           

	 thank you for any hint,


Received on Tue Sep 22 2009 - 07:24:03 CDT

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