confused terms: cover index, compound index and multi-fields index

I am confused with these 3 terms: cover index, compound index and multi-fields index.

Are they same things? or eac开发者_开发问答h of them has subtle difference?

Thanks

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• Compound index and multi-fields Index are the same. Other terms for the same are Multi-Column Index and Concatenated Index.

  That are indexes that contain more than one column.

• I suspect that Cover Index is actually Covering Index which is something entirely different, better described as Index-Only-Scan.

  That is not a property of an index, it is describing how the index is used. It means that a particular query can be satisfied with data from the index only, not needing to read the table data. (Note: An index copies data from the table).

  A single index can be a covering-index for one query, but not cover another query (that accesses columns not included (covered) in the index). Think of "The index covers the entire query."

More about database indexing: http://use-the-index-luke.com/