Sybase 12.4.2 manual Isolation levels, Primary keys and locking, 302

Isolation levels

GRANT, REVOKE, and SET OPTION are not restricted

While the commands GRANT, REVOKE, and SET OPTION are also considered DDL operations, they cause no concurrency conflicts, and so are not restricted. GRANT and REVOKE always cause an automatic commit; SET OPTION causes an automatic commit except when it is specified as TEMPORARY. GRANT and REVOKE are not allowed for any user currently connected to the database. SET OPTION affects all subsequent SQL statements sent to the database server, except for certain options that do not take effect until after you restart the database server. See the Adaptive Server IQ Reference Manual for details of setting options.

Primary keys and locking

Because only one user can update a table, primary key generation does not cause concurrency conflicts.

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:

•Transaction management maintains a snapshot of committed data at the time each transaction begins.

•A transaction can always read, as long as the snapshot version it uses is maintained.

•A transaction's writes are reflected in the snapshot it sees.

•Once a transaction begins, updates made by other transactions are invisible to it.

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