Fast select for fetch first n rows with order by

Abstract

Fetching an ordered first N rows of a table, includes: reading a row in the table; determining that the read row qualifies as the first N rows of the table for rows read so far, and storing data of the read row; and determining an order of data of qualifying rows and storing the order. Only data in rows that qualify to be among the first N rows are ordered and stored. This provides a significantly more efficient processing. It eliminates tournament tree sorts, corresponding work file read and write I/O's, and associated CPU time. This reduces the time for the running of a query and improves the performance of other applications sharing the use of work files. Further, the improved performance is particular significant when the buffer sizes are within a page of records or rows, although multiple pages may be used.

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