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Locally managed tablespaces have several advantages over the traditional dictionary-managed tablespaces. Dictionary-managed tablespaces have to constantly check the data dictionary during the course of extent management whenever an extent is allocated to an object or reclaimed from an object, Oracle will update the relevant tables in the data dictionary. If you have an OLTP system with heavy inserts and deletes, this could lead to contention for the data dictionary objects used to manage extents. In Oracle Database 10g, local tablespaces are the default. Since locally managed tablespaces offer superior performance, I use them exclusively in this book and ignore the management of dictionary-managed tablespaces.

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elif [ $days_since_change -ge $notify -a $days_since_change -le $max ] then exp=$(($max-$days_since_change))

Any time an Oracle object needs to grow in size, space is added to the object in terms of extents When you create locally managed tablespaces, you have two options for managing the extent sizes: you can let the database automatically choose the extent size (by selecting the AUTOALLOCATE option) or you can specify that the tablespace be managed with uniform-sized extents (the UNIFORM option) If you choose the UNIFORM option, you specify the actual size of the extents by using the SIZE clause If you omit the SIZE clause, Oracle will create all extents with a uniform size of 1MB, but you can choose a much larger uniform extent size if you wish You can t change the extent size once you create the tablespace.

If you think that all the segments in a tablespace are approximately of the same size, and that they ll grow in a similar fashion, you can choose the UNIFORM extent size option If you do this, you can select a few extent sizes, create all your tablespaces with one of these uniform extent sizes, and allocate objects to the tablespaces based on their size Traditionally, Oracle DBAs worried about the number of extents in a segment You should be more concerned about the size of the extents, though, since extent size has a bearing on the read and write performance of a segment For example, if you choose a very small UNIFORM extent size, the database can t prefetch data or do multiblock reads, thus adversely impacting performance.

Oracle suggests the following extent size guidelines, if you wish to set the extent sizes yourself: 64KB for small segments 1MB or medium segments 64MB for large segments Under the AUTOALLOCATE option, Oracle will manage the extent size automatically The extent size starts at 64KB and is progressively increased to 64MB by the database The database automatically decides what size the new extent for an object should be, based on the segment s growth pattern Interestingly, Oracle will increase the extent size for an object automatically as the object grows! Autoallocate is especially useful if you aren t sure about the growth rate of an object and you would like Oracle to deal with it..

If you know the exact space requirements for your objects, you can choose the UNIFORM extents option, which leads to efficient use of all available space. For example, say you know that your largest tables will consume a lot of space and will therefore need a very high extent size. Create a tablespace with a very large uniform size for such tables. If you aren t sure what extent size will be best, AUTOALLOCATE will let the database determine the extent size but it may waste some space due to the varying size of extents.

We then report warning notifications to the output file and to the debugging e-mail address, provided it has been configured.

Oracle recommends that unless all the objects in a tablespace are of the same size, you should use the

AUTOALLOCATE feature. In addition to the simplicity of management, the AUTOALLOCATE option for extent sizing can potentially save you a significant amount of disk space, compared to the UNIFORM extent size option.

Automatic Choice of Compilation Model: Avoid Warning 4793!