Note the error and time, and contact your system administrator. Valid expressions are constants, constant expressions, and in some contexts variables.
The transaction log is a critical component of the database. If there is a system failure, you will need that log to bring your database back to a consistent state.
Warning Never delete or move this log unless you fully understand the ramifications of doing so. Tip Known good points from which to begin applying transaction logs during database recovery are created by checkpoints.
Operations supported by the transaction log The transaction log supports the following operations: Recovery of all incomplete transactions when SQL Server is started.
Rolling a restored database, file, filegroup, or page forward to the point of failure. Supporting high availability and disaster recovery solutions: Always On availability groups, database mirroring, and log shipping. Individual transaction recovery If an application issues a ROLLBACK statement, or if the Database Engine detects an error such as the loss of communication with a client, the log records are used to roll back the modifications made by an incomplete transaction.
Recovery of all incomplete transactions when SQL Server is started If a server fails, the databases may be left in a state where some modifications were never written from the buffer cache to the data files, and there may be some modifications from incomplete transactions in the data files.
When an instance of SQL Server is started, it runs a recovery of each database. Every modification recorded in the log which may not have been written to the data files is rolled forward. Every incomplete transaction found in the transaction log is then rolled back to make sure the integrity of the database is preserved.
Rolling a restored database, file, filegroup, or page forward to the point of failure After a hardware loss or disk failure affecting the database files, you can restore the database to the point of failure.
You first restore the last full database backup and the last differential database backup, and then restore the subsequent sequence of the transaction log backups to the point of failure. As you restore each log backup, the Database Engine reapplies all the modifications recorded in the log to roll forward all the transactions.
When the last log backup is restored, the Database Engine then uses the log information to roll back all transactions that were not complete at that point. Supporting transactional replication The Log Reader Agent monitors the transaction log of each database configured for transactional replication and copies the transactions marked for replication from the transaction log into the distribution database.
For more information, see How Transactional Replication Works.
Supporting high availability and disaster recovery solutions The standby-server solutions, Always On availability groups, database mirroring, and log shipping, rely heavily on the transaction log.
In an Always On availability groups scenario, every update to a database, the primary replica, is immediately reproduced in separate, full copies of the database, the secondary replicas.
The primary replica sends each log record immediately to the secondary replicas which applies the incoming log records to availability group databases, continually rolling it forward.
For more information, see Always On Failover Cluster Instances In a log shipping scenario, the primary server sends the active transaction log of the primary database to one or more destinations. Each secondary server restores the log to its local secondary database.
For more information, see About Log Shipping.
In a database mirroring scenario, every update to a database, the principal database, is immediately reproduced in a separate, full copy of the database, the mirror database.
The principal server instance sends each log record immediately to the mirror server instance which applies the incoming log records to the mirror database, continually rolling it forward. For more information, see Database Mirroring. The transaction log is implemented as a separate file or set of files in the database.
The log cache is managed separately from the buffer cache for data pages, which results in simple, fast, and robust code within the SQL Server Database Engine. For more information, see Transaction Log Physical Architecture.
The format of log records and pages is not constrained to follow the format of data pages. The transaction log can be implemented in several files.• • • Passing variable to Child package from Parent package in SSIS SSIS Interview Questions Copy/Rename a file using File System Task in SSIS Column .
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An index in a database is a list of values in a table with the storage locations of rows in the table that contain each value. One day our unit tests started failing on Linux.
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