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Consider a traditional, row-oriented database. Indexes are known to improve query performance. They can greatly reduce I/O costs by avoiding the need to perform table scans since they directly contain the data you need to answer a query or contain pointers to such data. If you have a query that accesses only two out of thirty columns from a large table, and you have an index on these two columns, then you can use the indexes to avoid scanning all of the data in the table.