Specifies the algorithm used during a delete in a WD index.
0 – 3
0
DBA permissions are not required to set this option. Can be set temporary, for an individual connection, or for the PUBLIC group. Takes effect immediately.
This option chooses the algorithm used during a delete operation in a WD index. When this option is not set or is set to 0, the delete method is selected by the cost model. The cost model considers the CPU related costs as well as I/O related costs in selecting the appropriate delete algorithm. The cost model takes into account:
Rows deleted
Index size
Width of index data type
Cardinality of index data
Available temporary cache
Machine related I/O and CPU characteristics
Available CPUs and threads
Allowed values for WD_DELETE_METHOD:
0: The delete method is selected by the cost model. Cost model only selects either mid or large method for deletion.
1: Forces small method for deletion. Small method is useful when the number of rows being deleted is a very small percentage of the total number of rows in the table. Small delete can randomly access the index, causing cache thrashing with large datasets.
2: Forces large method for deletion. This algorithm scans the entire index searching for rows to delete. Large method is useful when the number of rows being deleted is a high percentage of the total number of rows in the table.
3: Forces mid method for deletion. Mid method is a variation of the small method that accesses the index in order and is generally faster than the small method.
This statement forces the large method for deletion from a WD index:
SET TEMPORARY OPTION WD_DELETE_METHOD = 2
For more details about these delete methods, see “Optimizing delete operations” in Chapter 3, “Optimizing Queries and Deletions” in the Performance and Tuning Guide.
