Keywords: Bulk fetch, how optimizer chooses

# About the optimizer's selection of bulk fetch

When Derby retrieves data from a conglomerate, it can fetch more than one row at a time. Fetching more than one row at a time is called bulk fetch. By default, Derby fetches 16 rows at a time.

Bulk fetch is faster than retrieving one row at a time when a large number of rows qualify for each scan of the table or index. Bulk fetch uses extra memory to hold the pre-fetched rows, so it should be avoided in situations in which memory is scarce.

Bulk fetch is automatically turned off for updatable cursors, for hash joins, for statements in which the scan returns a single row, and for subqueries. It is useful, however, for table scans or index range scans:

    SELECT *
    FROM Flights
    WHERE miles > 4

    SELECT *
    FROM Flights

The default size for bulk fetch (16 rows) typically provides good performance.

Parent topic: Derby's cost-based optimization
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