Re: Oracle internals.

In article <9v0nr00279n_at_drn.newsguy.com>, Thomas says...
>
>In article <_OPQ7.20640$wL4.36406_at_rwcrnsc51>, "Jim says...
>>
>>Howard is correct. You are asking more of a meta question with regards to
>>the System tablespace. The system tablespace is a little special. One
>>might ask how on Earth does Oracle know what files comprise the database?
>>This information starts in the init.ora file.(version 8i and prior) In that
>>file is an entry that says control_files=(...)
>>This points to the files that hold the information about what files Oracle
>>knows about and their state (e.g. SCN numbers ect..). Lose the control file
>>and you have a bear of a time getting the whole thing running which is why
>>one should have multiple copies of the control file on multiple disk devices
>>and keep a backup. Anyway, I think the system tablespace is special and if
>>you go hunting through the sql.bsq file you can see how it is set up and a
>>features that are not documented and things you should not muck with. The
>>sql.bsq file is executed when you create the database. It knows how to
>>initially build the system catalogs so the whole thing can work. Which is
>>probably why you can't have a locally managed tablespace for the system
>>tablespace (at least not in 8i). Oracle puts some things in the system
>>tablespace in some specific areas so it can bootstrap itself. I consider it
>>an academic curiosity as to exactly how it works; interesting but I wouldn't
>>muck around with it (e.g. try to read or change it directly).
>>
>>Think of a header of the file in the system tablespace as being static
>>pointers large enough to initialize the system. e.g. starting at block 5 to
>>block 45 is the first block for the UET$ table etc. The system would just
>>need a little bit of static information in known areas (probably the first x
>>blocks in the system tablespace) to set the whole thing up for a more
>>dynamic table driven system.
>>
>>Again Howard is correct. Specifically, a database block belongs to one
>>database object and it does not have multiple database objects.
>>Jim
>
>not so -- CLUSTERS give you the ability to store data from many segments
>(tables) on the same database block. The oracle data dictionary is littered
>with them (you use them all of the time).
>

as HJR pointed out -- technically its not many segments (there is the index cluster segment) but...

>a database block may be used by MANY objects simultaneously. happens everyday
>with the data dictionary. It is a way to store "pre-joined" data.....
>

is still true -- two tables (or more) can occupy the same block, but they aren't technically segments anymore.

sorry for the confusion

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>>
>>"Andrej" <andrej611_at_wp.pl> wrote in message
>>news:9v0g9g$ji8$1_at_korweta.task.gda.pl...
>>> > > I am looking for the following info concering Oracle internals:
>>> > > - how does Oracle know what blocks are assigned to what tables ? does
>>is
>>> > > blindly scan throug all the blocks ?
>>> >
>>> > No, it holds that information in data dictionary tables. Specifically,
>>> UET$
>>> > records what extents exist, and who owns them. Tables such as OBJ$ and
>>> TAB$
>>> > also come into play. In locally managed tablespace, a bitmap at the
>>> header
>>> > of the tablespace indicates whether a block is reserved for use or not.
>>> > Data dictionary entries still say which object owns which block.
>>>
>>> But data dictionary is also a bunch of TABLE(s). So your asnwer produces
>>an
>>> infinite loop ... it knows where the table blocks are by querying a table.
>>>
>>> I am trying to find out how Oracle recognizes what tables ARE in a SYSTEM
>>> tablespace datafiles and how are blocks internaly linked with them - for
>>> example what blocks make UET$.
>>>
>>> > >So where is a
>>> > > glue that holds a blocks/extents of a segment toegether ?
>>> >
>>> > There is no such substance.
>>>
>>> There must be. Otherwise Oracle wouldn't work at all. A single block holds
>>a
>>> data about rowids in the header. But how is data distionary build so that
>>> Oracle knows where to look for rows making a single schema object ?
>>>
>>>
>>>
>>>
>>>
>>
>>
>
>--
>Thomas Kyte (tkyte@us.oracle.com) http://asktom.oracle.com/
>Expert one on one Oracle, programming techniques and solutions for Oracle.
>http://www.amazon.com/exec/obidos/ASIN/1861004826/
>Opinions are mine and do not necessarily reflect those of Oracle Corp
>

--
Thomas Kyte (tkyte@us.oracle.com)             http://asktom.oracle.com/ 
Expert one on one Oracle, programming techniques and solutions for Oracle.
http://www.amazon.com/exec/obidos/ASIN/1861004826/  
Opinions are mine and do not necessarily reflect those of Oracle Corp