When an update request is successfully posted to the server, Firebird creates and writes to disk a reference linking the original row image as seen by the transaction —sometimes called a delta—and a new version of the row, incorporating the requested changes. The original and new row images are referred to as record versions. When a new record version is created by a newer transaction than the one that created the “live” version, other transactions will not be able to update or delete the original unless the owner transaction of the new version rolls back. The versioning process is described in detail in the previous chapter.
Until the transaction is eventually committed, it does not touch the “live” version again until the commit occurs. Within its own context, it treats the posted version as if it were the latest committed version. Meanwhile, other transactions continue to “see” the latest committed version. In the case of “snapshot” transactions that started before our transaction, the latest committed version that they see may be older than the one seen by our transaction and by other transactions that either started later or are in READ COMMITTED isolation.
If the table that has an update posted to it has foreign keys linked to it, the server creates deltas of the rows from those tables that “belong to” the updated row. Those dependent rows, and any that are dependent on them through foreign keys, are thus made inaccessible for update by other transactions too, for the duration of the transaction.
Firebird Related Interview Questions
|RDBMS Interview Questions||MySQL Interview Questions|
|Linux Interview Questions||Mac OS X Deployment Interview Questions|
|Windows Administration Interview Questions||Windows Server 2003 Interview Questions|
|SQL Interview Questions||NoSQL Interview Questions|
|Advanced C++ Interview Questions|
Introduction To Client/server Architecture
About Firebird Data Types
Date And Time Types
Blobs And Arrays
From Drawing Board To Database
Creating And Maintaining A Database
Firebird’s Sql Language
Expressions And Predicates
Querying Multiple Tables
Ordered And Aggregated Sets
Overview Of Firebird Transactions In
Programming With Transactions
Introduction To Firebird Programming
Developing Psql Modules
Error Handling And Events
Security In The Operating Environment
Configuration And Special Features
Interactive Sql Utility (isql)
Database Backup And Restore (gbak)
Housekeeping Tool (gfix)
Understanding The Lock Manager
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