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I work on Oracle systems where I two type of indices exists in any table:

  1. Unique index (Based on any search key which is unique key).
  2. Normal index (Index made on any search key which is not candidate or primary key).

But now I came to know about two new type of indices from a university documentation:

  1. Primary index
  2. Secondary index

How are primary and secondary indices different from each other? Are they the same as unique and normal index respectively in Oracle?

I also want to know about below two indices:

  1. Dense index
  2. Sparse index

Can we consider dense and sparse indices as type of primary and secondary indices?

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Unique index and index are keywords for type of index. But primary, secondary , dense and sparse are conceptual types. –  Mani Sep 20 '13 at 9:13

2 Answers 2

** Primary index** 
A primary index is an index on a set of fields that includes
the unique primary key for the field and is guaranteed not to contain duplicates.
Also Called a **Clustered index**. 
eg. Employee ID can be Example of it. 

Secondary index
A Secondary index is an index that is not a primary index and may have duplicates.
eg. Employee name can be example of it. Because Employee name can have similar values.

**Dense Index**
Dense index
Index record appears for every search­ key value in the file.
Dense indexes point directly to individual records.
**Sparse index**
contains index records for only some search ­key values.
Applicable when records are sequentially ordered on search ­key.
Just as with book indexes,
sparse database indexes don’t point to individual records, but to ‘pages'

Thank you.

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A secondary index is just any index that is not the primary index (of which there can be only one). It can still be unique.

A dense index is an index where every record is contained in the index, even if it does not have a relevant value, whereas a sparse index contains only record with relevant values. I am not sure if this is a meaningful distinction when talking about Oracle database indexes, except that you could say that a normal index is "sparse" in the sense that it does not contain entries where all columns are null. That would make a bitmap index "dense", I suppose.

A primary key index always has to be dense.

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