SQL Anywhere allows you to control the degree to which the operations in one transaction are visible to the operations in other concurrent transactions. You do so by setting a database option called the isolation level.
SQL Anywhere also allows you to control the isolation levels of individual tables in a query with corresponding table hints.
SQL Anywhere provides the following isolation levels:
|This isolation level...||Has these characteristics...|
1 Snapshot isolation must be enabled for the database by setting the allow_snapshot_isolation option to On for the database.
The default isolation level is 0, except for Open Client, jConnect, and TDS connections, which have a default isolation level of 1.
Lock-based isolation levels prevent some or all interference. Level 3 provides the highest level of isolation. Lower levels allow more inconsistencies, but typically have better performance. Level 0 (read uncommitted) is the default setting.
The snapshot isolation levels prevent all interference between reads and writes. However, writes can still interfere with each other. Few inconsistencies are possible and contention performance is the same as isolation level 0. Performance not related to contention is worse because of the need to save and use row versions.
In general, each isolation level is characterized by the types of locks needed and by how locks held by other transactions are treated. At isolation level 0, the database server needs only write locks. It makes use of these locks to ensure that no two transactions make modifications that conflict. For example, a level 0 transaction acquires a write lock on a row before it updates or deletes it, and inserts any new rows with a write lock already in place.
Level 0 transactions perform no checks on the rows they are reading. For example, when a level 0 transaction reads a row, it does not check what locks may or may not have been acquired on that row by other transactions. Since no checks are needed, level 0 transactions are fast. This speed comes at the expense of consistency. Whenever transactions read a row that is write locked by another transaction, they risk returning dirty data. At level 1, transactions check for write locks before they read a row. Although one more operation is required, these transactions are assured that all the data they read is committed.
All isolation levels guarantee that each transaction executes completely or not at all, and no updates are lost.
The isolation is between transactions only: multiple cursors within the same transaction cannot interfere with each other.
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