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Prefixed

From time to time the question about whether local indexes on partitioned tables should be prefixed or non-prefixed appears on the Oracle forums and mailing lists.

It’s possible that I’m wrong – although no-one has come up with a counter-example to the statement I keep repeating – but the whole prefixed/non-prefixed thing for local indexes dates back to a limitation in the optimizer somewhere in the 8.0 time line where Oracle couldn’t do partition elimination on indexes properly but the overhead of the error it made could be dramatically reduced (in most cases) by sticking the partition key at the start of the index.

The guideline for local indexes are the same as they would be for a non-partitioned index on a non-partitioned heap table – the partitioning column(s) are just columns in the table that might be worth including in the index, and the order of the index columns is dictated by the important queries that have to access the table.

For further comments, there’s a note I wrote (which I’ve just been reminded of) on the OTN database forum that adds a little detail to this argument.