Data Warehouse Alter Tables Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Tablespace Fragmentation

Here we see that the sales tablespace has been allocated to a physical file called /Data/ORACLE/sales/sales.dbf, created at a size of 500 MB. Assuming that all tables within this tablespace use default storage, they will be initially allocated at 500 K and will extend in chunks of 50 K.

But what happens if the tablespace gets full? Processing will cease against the tablespace, and the Oracle DBA must intervene to add another data file to the tablespace with the alter tablespace command:

ALTER TABLESPACE SALES
ADD DATAFILE ‘/Data/ORACLE/sales/sales1.dbf’
SIZE 200M REUSE;

Obviously, the DBA should carefully monitor tablespace usage so that tablespaces never fill, but Oracle version 7.2 and above offer an alternative that is especially useful for data warehouses. The AUTOEXTEND command can be used to allow a data file to grow automatically on an “as needed” basis. Here are the different permutations of this command:

ALTER DATABASE DATAFILE ‘/Data/ORACLE/sales/sales.dbf’ AUTOEXTEND ON;

ALTER DATABASE DATAFILE ‘/Data/ORACLE/sales/sales.dbf’ AUTOEXTEND MAXSIZE UNLIMITED;

ALTER DATABASE DATAFILE ‘/Data/ORACLE/sales/sales.dbf’ AUTOEXTEND MAXSIZE (500M);

ALTER DATABASE DATAFILE ‘/Data/ORACLE/sales/sales.dbf’ RESIZE (600M);

When tables fragment, additional I/O will be required to access the table data, since the disk must access blocks on two noncontiguous spots on the datafile. The following script in Listing 8.14 will detect all tablespaces whose tables have taken more than 10 extents. These tablespaces will become candidates for a reorganization:

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This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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