HP 3000 Manuals HP 3000 Manuals
Dropping and Recreating Hash Structures You may wish to modify a hash structure for any of the reasons you need to modify an ordinary table. In addition, you may need to recreate the hash structure when it is no longer large enough for the data in the table. The SQLMON Static Hash screen contains information about each hash structure you have defined. By examining the OVERFLOW CHAIN LNGTH, AVGOVERFLOW, and MAXOVERFLOW fields on the Static Hash screen, you can determine the efficiency of hash structures. The AVGOVERFLOW value is the average number of page accesses required to retrieve a particular row. MAXOVERFLOW is the maximum number of accesses; therefore, it is a worst case value. For most hash structures, the normal value of AVGOVERFLOW is between 1.05 and 1.10. This means that approximately 5 to 10% of the primary pages have one overflow page. A value of 1.5 means that half the primary pages have one overflow page. Use this information to decide whether unloading and reloading the hash structure might improve performance. To alter the hash structure, simply unload the data, then drop the table and recreate it with the desired modifications, including, if necessary, a new number of primary pages. If you add to the number of primary pages, make sure there are enough empty TABLE or MIXED DBEFiles in the appropriate DBEFileSet with the required pages available.