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To quote wikipedia, "An index is a data structure that improves the
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speed of data retrieval operations on a database table at the cost of slower
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writes and increased storage space. Indexes can be created using one or more
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columns of a database table, providing the basis for both rapid random
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lookups and efficient access of ordered records. The disk space required to
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store the index is typically less than that required by the table (since
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indexes usually contain only the key-fields according to which the table is
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to be arranged, and exclude all the other details in the table), yielding
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the possibility to store indexes in memory for a table whose data is too
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large to store in memory."