Page History
...
Table 9-2 Operation options
| Operator | Description |
|---|---|
Operator Description
...
| Sort | The Sort operator presents the sorting of the rows returned from the previously executed operation (appears just below this operator) in a specific order, optionally eliminating duplicate entries. Sorting is required when no index that satisfies the requested ordering exists, or when an Index Scan is more expensive than sorting. It is usually used as the final step to retrieve the fetched data or a prior step for the Join or Stream Aggregate (Group By) operators. In some cases, the Sort operation requires more than the available memory, in which case a temporary table is used. This involves Disk I/O, which has an impact on performance. Sorting a large number of rows makes heavy use of the CPU, which can also affect overall server performance. The amount of memory needed for sorting can be estimated by multiplying the number of rows by the average row size. Therefore, minimizing the row size by selecting only the necessary fields will decrease the memory required for the Sort operation. |
| Nested |
...
| Joins | Nested Joins (also called Nested Loops) use the Nested algorithm to join two tables. The Nested algorithm works as follows: For each row in the outer table (the first table, appearing directly below the Nested Loop operator in the Explain tree), Sybase executes the Access Plan for the inner table (the second one). A large number of iterations may indicate a problem in choosing the Access Plan due to out-of-date statistics. |
| Merge |
...
| Joins | A Merge Join algorithm is used to join two tables by merging them. Merge Join algorithms read both tables only once; therefore it is required that the input be sorted according to the Join predicates. Merge Joins are available in Precise for Sybase from version 12.0 and above. This option is grayed-out by default. To enable it on session level, execute the set sort_merge on command. See the Sybase Help for more details on how to enable Merge Joins on server level. |
About the operators that are available in the plan tree
The table below describes the operators that are available in the Plan tree.
...