An index probe access method will take advantage of the keys of the index to directly position to the index key entries that match any selection within the query. This allows the query to quickly subset the number of rows that need to be considered for the table. By minimizing the number of rows that need to be considered from the table, the paging associated with the table can be reduced in order to return the selected rows in the query.
This access method is generally associated with a separate table probe operation in order to retrieve the table data needed for the selected rows. The random paging for the table in this access method is performed synchronously without any pre-fetch scheduling logic.
The advantage of the being able to minimize the number of rows that need to be considered through selection against the index is offset by the added cost of synchronously paging against the table for the selected rows. This is generally why this is only an efficient implementation when processing a smaller percentage of the total rows in the table.
For more information, see Database Performance and Query Optimization in the
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