5.7.3 Characteristics of each type of grouping
The following table describes the characteristics of each type of grouping.
|
No. |
Grouping type |
Benefits |
Disadvantages |
|
|---|---|---|---|---|
|
1 |
Hash grouping |
Local hash grouping |
Grouping is performed at a higher speed if the data required for grouping can fit in the hash grouping area for each SQL processing real thread, such as when there are only a few groups. |
Processing performance decreases when there are many groups. |
|
2 |
Global hash grouping |
Grouping is performed at a higher speed if the data required for grouping can fit in the hash table area. |
Processing performance decreases when grouping requires a large amount of data, and a shortage of space occurs in the hash table area, because the data is first stored in the work table. |
|
|
3 |
Sort grouping |
Grouping is possible even if no hash grouping area or hash table area is allocated. |
If there are many retrieval results, a large amount of data is stored in the work table. Processing performance decreases because the data stored in the work table is sorted, and then is grouped. |
|