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OraLearner asked

What are the advantages & disadvantages of clusters ?

What are the main advantages and disadvantages of clusters ? Thank you.

oracleclusters
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Andrew Mobbs answered

Hash clusters are a physical storage option that groups multiple tables together based on a hash value of the cluster key (one or more fields shared by all tables in the cluster). That means you can have incredibly efficient joins on the cluster key, and very fast lookups. Normal (index) clusters achieve the same thing with a B-Tree index rather than a hash. Hash clusters give an O(1) lookup, but are best used for fixed size data, index clusters give O(log N) lookup, but are more flexible.

When I last looked though, Clusters had quite a lot of restrictions on what operations can be performed on them. Essentially the code for them doesn't really appear to have been kept up-to-date since about Oracle 8. I haven't checked this against 11g though.

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Thank you very much. This is interesting. In concept, somewhat like SQL Server's indexed views and maybe more useful, although completely different in implementation. I can see the relevance to the TRUNCATE limitation. +1! - KDBA :-)
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