Transactions and Versioning

The Sybase IQ approach to transaction processing, called snapshot versioning, has implications for performance and other aspects of database administration.

• Transactions and Versioning Overview
  Transaction processing ensures that logically related commands execute as a unit.
• Versioning Prevents Inconsistencies
  Without versioning, concurrent read and write operations could cause inconsistencies in the database.
• How Locking Works
  All Sybase IQ locks occur automatically, based on the type of operation a user requests.
• Tools for Managing Locks
  While locking and unlocking occurs automatically, Sybase IQ helps you manage locks by means of stored procedures, the IQ monitor, and database and server options.
• 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.
• Checkpoints, Savepoints, and Transaction Rollback
  Besides permitting concurrency, transaction processing plays an important role in data recovery. Database recovery always recovers every committed transaction. Transactions that have not committed at the time of a database failure are not recovered.
• Performance Implications of Snapshot Versioning
  Snapshot versioning should have a minimal impact on performance. The flexibility you gain by being able to update the database while other users read from it far outweigh any negative effects. There are certain resource issues you should be aware of, however:
• Cursors in Transactions
  A cursor allows you to return the results of a SELECT in the form of a data type called a cursor.