Re: Oracle internals.

Andrej doodled thusly:

I know you had plenty of replies, but I've got a feeling you're still not clear on what happens.

>- how does Oracle know what blocks are assigned to what tables ? does is
>blindly scan throug all the blocks ?

Nope. A set of tables in the SYSTEM tablespace take care of this information. Typically, you get a relative start address and a length of the particular extent related to a segment in a given datafile. The relative address is a block (page) number within the datafile. These tables are scanned/looked at as needed, although most of the time they're cached.

Within a single table, the table header contains descriptions of the table itself and the address of the first data segment. Each block contains some additional header information. These mostly describe the contents of the block (used/free space,etc) and some locking info.

There is a lot more to it than this, but it should get you MOL started.

>- I know a single block can hold data for different segments. So where is a
>glue that holds a blocks/extents of a segment toegether ?

Abstracting from semantics of what is a segment: the block header says where the stuff is/starts/ends.

>- what is internal structure of a datafile ? I am not asking to know the (c)
>details but just a big picture. Once I did an append to one of a datafiles
>with tar ...

Datafiles have very much the same structure: a header section and a data area. If using the new locally managed space management, they also contain a bitmap of allocated extents. Very much like Unix file systems contain a bitmap of allocated pages/blocks.

>Please, point me to some reference on Oracle internals.

The Oracle Concepts and ORACLE DBA manuals contain a lot of this stuff. If you really want to dig into details, start there. Then go into Steve's book (already referenced) to get info on how to dump blocks so you can check the theory against the practice.

Get ready for some serious bit-digging if you wanna explore this area!

Cheers
Nuno Souto
nsouto_at_optushome.com.au.nospam Received on Mon Dec 10 2001 - 05:39:12 CST