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SQL Anywhere 11.0.0 » SQL Anywhere Server - SQL Usage » Designing and Creating Databases » Using transactions and isolation levels » Isolation levels and consistency » Typical types of inconsistency

 

Cursor instability

Another significant inconsistency is cursor instability. When this inconsistency is present, a transaction can modify a row that is being referenced by another transaction's cursor. Cursor stability ensures that applications using cursors do not introduce inconsistencies into the data in the database.

Example

Transaction A reads a row using a cursor. Transaction B modifies that row and commits. Not realizing that the row has been modified, Transaction A modifies it.

Eliminating cursor instability

SQL Anywhere provides cursor stability at isolation levels 1, 2, and 3. Cursor stability ensures that no other transactions can modify information that is contained in the present row of your cursor. The information in a row of a cursor may be the copy of information contained in a particular table or may be a combination of data from different rows of multiple tables. More than one table will likely be involved whenever you use a join or sub-selection within a SELECT statement.

For information about programming SQL procedures and cursors, see Using procedures, triggers, and batches.

Cursors are used only when you are using SQL Anywhere through another application. For more information, see Using SQL in applications.

A related but distinct concern for applications using cursors is whether changes to underlying data are visible to the application. You can control the changes that are visible to applications by specifying the sensitivity of the cursor.

For more information about cursor sensitivity, see SQL Anywhere cursors.