Tablespace and Extents : A Simple qn

Question

Hi,
I have a simple qn. I need to understand in detail about extents in tablespace creation script.
I want to create a new tablespace XXXYYYZZZ in my QA database, I can just get the syntax in google or oracle guide and execute.
1. CREATE TABLESPACE "XXXYYYZZZ" DATAFILE '+DATAQA' SIZE 5120M
2. REUSE AUTOEXTEND ON NEXT 8K MAXSIZE 8192M
3. LOGGING
4. EXTENT MANAGEMENT LOCAL UNIFORM SIZE 524288K
5. SEGMENT SPACE MANAGEMENT MANUAL;
I need to understand the following:
(a). ON NEXT 8K on line 2 and UNIFORM SIZE 524288K on line 4
(b). What are the factors should we consider before tablespace creation - the NEXT extent of 8K and/or UNIFORM SIZE of 524288K
(c) If I created the above tablespace, Is there a way to change the storage parameters with respect to extents, if so how? How to do the change in minimum time.
Thanks
~ Its never late to clear the doubts ~

Answer 1

1. It's not 'on next'. It's actually "autoextend on... next Xk'
In other words, you've switched on the data file autoextension feature. Then you specify what the datafile will grow by each time it decides it needs to autoextend. 8KB is a maniacally daft number to put in at this point: your datafile will be forever inching larger, causing waits as it does so. Instead, you want to bite the bullet: if you have to autoextend, make sure you do in decent-sized chunks. Something more like 'autoextend on next 100M', for example.
The size a datafile grows by has nothing at all, automatically, to do with how the space inside the file is allocated to segments, which is what the uniform size clause controls. Your example syntax states that segments acquire space in chunks ("extents") of 512MB (if my maths is as good as I'd like to think it was!!) Which is certainly another reason why increasing the size of a datafile by a mere 8K a go when a segment is demanding a new 512MB chunk would be a very poor way to configure things.
If the datafile is allowed to grow in 100M lots as I suggested, you'd still end up requesting 5 or 6 datafile extensions to meet the demands of one segment needing one extra 512MB extent that can't be accomodated in the existing space. So if you're going to have 512MB extents, you'd be better off with a NEXT 1024M sort of autoextension clause.
There's a commonsensical relationship between the two clauses, in other words, but it's one you have to work out for yourself. If you demand 512MB extents and datafile growth in 8K increments, nothing in Oracle will stop you doing it, but it will result in poor performance later on.
Once you've created a locally managed tablespace with a daft UNIFORM SIZE clause, there's nothing you can do to change that, except drop it and start again. Which is why I'd recommend not bothering with it at all. Just say on line 4, 'extent management local' and you'll get the default 'autoallocate' extent allocation policy, which works just fine in 9.2 and anything thereafter.