Isolation Levels

An important aspect of transaction processing is the database server's ability to isolate an operation. ANSI standards define four levels of isolation. Each higher level provides transactions a greater degree of isolation from other transactions, and thus a greater assurance that the database remains internally consistent.

The isolation level controls the degree to which operations and data in one transaction are visible to operations in other, concurrent transactions. IQ snapshot versioning supports the highest level of isolation. At this level, all schedules may be serialized.

Snapshot versioning maintains this high level of isolation between concurrent transactions by following these rules:
The level of isolation that Sybase IQ provides prevents several types of inconsistencies. The ones most commonly encountered are listed here:

Sybase IQ protects you from all of these inconsistencies by ensuring that only one user can modify a table at any given time, by keeping the changes invisible to other users until the changes are complete, and by maintaining time-stamped snapshots of data objects in use at any time.

While IQ allows you to set the isolation level to 0, 1, 2, or 3 (comparable to ANSI levels 1, 2, 3, or 4) using SET OPTION ISOLATION_LEVEL, there is no reason to do so. All users execute at isolation level 4, even if you set a different level. There is no performance advantage to setting a lower isolation level.

For more information on preventing concurrent transactions from accessing or modifying tables, see the LOCK TABLE statement in Reference: Statements and Options.