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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Index Tablespace Issues

When a large data warehouse has been created, it is not uncommon to see yearly aggregate values purged and re-computed on a scheduled basis. When this happens, we know that the table and index extending will not be a problem since the tablespace will coalesce all free extents within the tablespace.

But what about the indexes? After a large purge, the indexes can become out of balance, and additional I/O may be required to access particular records. The logical remedy to out-of-balance indexes would be to drop and rebuild all of the Oracle indexes. However, this poses another problem. To understand this process, let’s examine a snapshot of an index tablespace prior to dropping all indexes in the tablespace (Figure 8.8)

Figure 8.8 An Oracle Index Tablespace as seen by Oracle Tablespace Manager

Now, lets drop all of the indexes, and look at the tablespace after the drop (Figure 8.9).

Figure 8.9 An Oracle Index Tablespace after dropping an index

Here we see that there are lots of empty extents in the tablespace, since the empty storage has not been coalesced. Now, if we are on Oracle 7.2 or above we can issue the ALTER TABLESPECE XXX COALESCE command (Figure 8.10). Now, the tablespace is clear, and the index can easily be re-created to reside in a single extent.

Figure 8.10  An Oracle Index Tablespace after coalescing the tablespace

This is an excerpt from "High Performance Data Warehousing", copyright 1997. To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:



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