Performing index scans in a database

摘要

Methods and apparatus, including computer program products, implementing and using techniques for performing an index scan in a database system. A sequence of pages visited during an index scan are dynamically tracked. The tracking includes tracking the number of page accesses requiring a wait for input or output. It is determined whether prefetching of an object is desirable. The determination being made at least in part based on the number of page accesses requiring a wait for input or output. If prefetching is desirable, a prefetching method is selected among a set of available prefetching methods. An effectiveness of the selected prefetching method is continuously monitored by tracking the sequence of pages accessed and available access methods. A different prefetching method is selected from the set of available prefetching methods if the different prefetching method has a better effectiveness for a portion of the index that is being scanned.

主权项

1. A computer-implemented method for performing an index scan in a database system, the method comprising:
dynamically tracking a sequence of pages visited during an index scan, wherein the tracking includes:
tracking a total number of pages visited during an index scan,tracking a number of page accesses requiring a wait for input or output, anddetermining a percentage of page accesses requiring a wait for input or output; comparing the percentage of page accesses requiring a wait for input or output to a threshold value; in response to determining that the percentage of pages requiring a wait for input or output exceeds the threshold value, selecting a prefetching method among a set of available prefetching methods; continuously tracking a sequence of pages accessed and available access methods; and dynamically selecting a different prefetching method from the set of available prefetching methods in response to detecting that the different prefetching method provides reduced scanning time compared to the currently selected prefetching method for a portion of the index that is being scanned.