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Indexes in Oracle

As always, I apologize if this is a stupid question (actually two questions). I am not a database administrator, so I know little about indexes. My questions:

  • Is there any clipping point (by the number of rows) at which the index would be pointless? For example, is there any use for an index in a lookup table with 10-20 rows?

  • I read something about index coverage in Oracle, and the concept makes sense that data can be obtained directly from the index, and you don't need a trip to the table. How to determine if an index is a coverage index? Is this the value set when creating the index, or the default based on the rows that the index is in?

Hope this makes sense.

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2 answers

Richard Foote has a series of blog posts on indexes for small tables . The short answer is probably not (but the long answer is much more interesting).

A coverage index is a generic term for an index that contains all columns that are part of a SELECT or WHERE list for a table. It is not a property of an index — any index can be a coverage index for some query. This is what is characteristic of the query and the indexes available to the optimizer.

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