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Category: Transactions

Log Chains

Dave Mason discusses broken log chains:

There are a handful of activities that have broken my SQL Server backup routines. Sometimes a DIFFERENTIAL (or LOG) backup would fail because there was no FULL backup. Other times, a LOG backup would fail because the log chain was broken. Some of those activities include:

  1. Creating a new database.

  2. Changing the recovery model to SIMPLE.

  3. Restoring/reverting a database from a snapshot.

  4. Any other activity that breaks the transaction log chain.

Dave discusses an easy way of figuring out if you’ve just performed an activity which breaks the log chain.

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Identifying Object Names Using fn_dblog()

Frank Gill digs into the transaction log to find object names used in CREATE and ALTER statements:

This is the object id of the view that was created.  So, Jes’s question was answered.  But this led me to one of my other favorite SQL Server topics: string manipulation.  The following script will identify all transactions for a particular Transaction Name and return the object name affected.  The comments provide additional information about the functionality.

Click through to check out Frank’s script.

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Analyze Transaction Log Hierarchy

Paul Randal has written code to determine how much log an operation has generated:

Over the weekend there was a discussion on the MVP distribution list about the sys.dm_tran_database_transactions DMV and how one cannot use it to accurately determine how much log an operation has generated because it doesn’t provide a roll-up of the sub-transaction metrics to the outer transaction. This makes the output somewhat non-intuitive.

The discussion prompted me to write some code I’ve been meaning to do since 2012, when SQL Server 2012 introduced a field in LOP_BEGIN_XACT log records that tracks the transaction ID of the parent transaction, allowing the hierarchy of transactions to be investigated.

The actual code is at the bottom of the article, and is available in a zip file here.

It provides two stored procs, sp_SQLskillsAnalyzeLog and sp_SQLskillsAnalyzeLogInner, with the former making use of the latter, and the latter calling itself recursively.

I’d consider this squarely in the advanced troubleshooting realm.  Definitely read the whole thing and Paul’s code.

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Transaction Log Is Full

Andy Galbraith diagnoses an “ACTIVE_BACKUP_OR_RESTORE” issue:

The way FULL backups work in SQL Server, the transaction log is not released for re-use during a FULL backup, even if regular LOG backups are occurring or the database is in SIMPLE recovery.  This is due to the fact that the portion of the LOG that is used during the FULL has to be persisted during the FULL in order to be backed up at the end of the FULL – that is, the FULL backup includes the data at the start of the FULL (23:30) *plus* the LOG used until the end of the FULL (in the case of the 12/04-12/05 backup, the LOG used from 23:30 to 07:11).  This is the meaning of the ACTIVE_BACKUP_OR_RESTORE message – the LOG is waiting for the end of the active FULL backup before it can be released for re-use, which in this case was causing the LOG/LDF file to grow to fill its mount point.

This is interesting analysis and a reminder that even though you’re in Simple recovery mode, SQL Server still uses a transaction log and it’s just as important.

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Watch Named, Nested Transactions

Gail Shaw finishes her outstanding series on transactions:

The error was thrown by the ROLLBACK statement. As such, the transaction is still open, the locks are held and the transaction log space can’t be reused. Unless the application that called this was checking for open transactions, that transaction could potentially be left open for quite some amount of time, causing blocking and/or the transaction log to grow.

It’s not just that someone in the future might call the code from another stored proc within a transaction, it’s also that it might be that the code is called from an application which started a transaction. Or called from SSIS which started a transaction. It’s very hard to ensure that code is never called from within an existing transaction

Read the whole thing.

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Naming Transactions

Gail Shaw asks, why name transactions?

So what conclusion can we come to here? Pretty much that naming of transactions has one real use, as a form of documentation. Instead of putting a comment above an BEGIN TRANSACTION we can give the transaction a name that indicates what the transaction does, That’s about the only real use.

With two exceptions.

The one reason I have to name transactions:  name and shame.

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Shaw On Transactions, Part 2

Gail Shaw has part 2 of her transactions series up:

Again, exactly the desired behaviour. The changes made in the outer procedure were committed, the changes in the inner procedure, the procedure where the error was thrown, were rolled back.

Used correctly, savepoints can be a powerful mechanism for managing transactions in SQL Server. Unfortunately they’re not well known and as such their use can also make code much harder for later developers to debug.

I’ve used conditional transactions fairly regularly (procedures can have calling parent procedures, or sometimes can be called on their own), but never savepoints.

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Nested Transactions Aren’t

Friends don’t let friends nest transactions:

Before getting into the details, I need to make one thing clear. Nested transactions are a lie. They do not exist in SQL Server.

This is part 1 of a three-part series by Gail Shaw.  Read the whole thing.  Also read Paul Randal:

Nested transactions do not actually behave the way the syntax would have you believe. I have no idea why they were coded this way in SQL Server – all I can think of is someone from the dim and distant past is continually thumbing their nose at the SQL Server community and going “ha – fooled you!!”.

Nested transactions are somebody’s attempt at trolling.  They succeeded.

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