In database management systems, particularly SQL databases, cursors are used to retrieve and manipulate data in a set row-by-row. Cursor types can affect how the data is accessed and modified. Here are the three primary cursor types: STATIC, KEYSET, and DYNAMIC, along with their usage and differences:
Static Cursors:
1. Definition: A static cursor creates a complete copy of the data set when the cursor is opened. This means the result set is fixed and does not change as the underlying data changes.
2. Use Case: Ideal when you need a consistent snapshot of data that won't change while you're processing it, such as generating reports.
3. Behavior:
- You can scroll through the result set.
- Changes made to the database after opening the cursor will not be reflected in the static cursor.
- Any updates made via the static cursor are typically reflected in the database only when you explicitly call an update function.
Keyset Cursors:
1. Definition: A keyset cursor creates a result set based on the unique identifiers (keys) of the rows returned by the query. The rows represented by the keys are settled when the cursor is opened, but the values of those rows can change.
2. Use Case: Useful when you need to see changes made by other transactions while querying data for updates, but you don't want the set of rows to change.
3. Behavior:
- When you open the cursor, it fetches the keys of the result set, and as you navigate through it, you can see changes made to the data.
- You can see updates made to the rows by other transactions, but you cannot see newly inserted or deleted rows — only the rows that match the keys initially fetched.
Dynamic Cursors:
1. Definition: A dynamic cursor allows full access to the result set, adapting to changes in the underlying data. This means you can see new rows, updates to existing rows, and deletions all in real-time.
2. Use Case: Ideal for applications where real-time data views are necessary, such as user interfaces that require live updates of data.
3. Behavior:
- The result set can change as you scroll through it, showing new rows or reflecting changes made by other users.
- It allows operations like insertions, deletions, and updates, and these changes can be seen immediately.
Summary of Differences:
- Data Snapshot:
- Static: Fixed data snapshot.
- Keyset: Fixed set of rows (by keys), but values can change.
- Dynamic: Real-time view, reflecting all changes.
- Scrollability:
- All three cursor types allow scrolling through rows, but the visible rows differ based on the type as explained above.
- Updates from Others:
- Static: No updates reflected from external changes.
- Keyset: Can see updates to data but not structural changes (insertions/deletions).
- Dynamic: Full reflection of all changes in real-time.
In practice, the choice of cursor type depends on the specific requirements of the application in terms of data consistency, real-time needs, and performance considerations.