The Sorting tab on the Database Manager Summary drilldown uses graphs to show sort activity within the selected database and to contrast this activity to overall sorting activity in the instance.
An application query often requires that retrieved rows be put in a specific order—that is, be sorted—to satisfy a SQL sort request (as with the ORDER BY clause), to construct an index, or to perform a hash join. Sort heap is a block of memory that the database manager allocates to an application as workspace whenever the application requires a sort. An important factor in the performance of any sort is the size of its sort heap, which is determined by one of these parameters:
SORTHEAP—Database configuration parameter
SHEAPTHRES—Database manager configuration parameter.
A sort consists of these steps:
The actual sort process—If the information being sorted cannot fit into the sort heap, some information is written to temporary database tables. This incident is called a sort overflow. Sorts that do not overflow perform better than those that do.
Return of the sorted rows—If sorted information can be returned directly without requiring a temporary table to store the final, sorted list of data, the sort is referred to as a piped sort. If the sorted information requires a table to temporarily store the data before it is returned, the sort is referred to as a non-piped sort. A piped sort always performs better than a non-piped sort.
Sorting problems, such as overflows, non-piped sorts, less than optimal sort heap allocation size, might indicate a need to adjust the SORTHEAP and SHEAPTHRES parameters.
Use the following graph to determine sort activity and problems over consecutive monitoring intervals:
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