Order-by and group-by columns

You can place an index on the ordering column or columns of a table. The database server then uses the index that to sort the query results in the most efficient manner.

When a large quantity of rows must be ordered or grouped, the database server must put the rows in order. One way that the database server performs this task is to select all the rows into a temporary table and sort the table. But, as explained in Queries and the query optimizer, if the ordering columns are indexed, the optimizer sometimes reads the rows in sorted order through the index, thus avoiding a final sort.

Because the keys in an index are in sorted sequence, the index really represents the result of sorting the table. By placing an index on the ordering column or columns, you can replace many sorts during queries with a single sort when the index is created.

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