I have just published a four-part series for SQLPerformance.com on the Halloween Problem. Some of you will never have heard of this issue, and those that have might associate it only with T-SQL UPDATE queries. In fact, the Halloween problem affects execution plans for INSERT, UPDATE, DELETE and MERGE statements. This is a topic I have been ...

In my August 2009 blog post Exploring Composable DML I introduced a new feature in SQL Server 2008 called Composable DML and also outlined one of its limitations; namely that data from the OUTPUT cannot be aggregated prior to insertion. Composable DML does have some useful scenarios however and one of those is in capturing and storing values that ...

Summary: If you use MERGE, indexed views and foreign keys, your queries can return incorrect results. Microsoft have released a fix for incorrect results returned when querying an indexed view. The problem applies to: SQL Server 2012 SQL Server 2008 R2 SQL Server 2008 The Knowledge Base article does not go into much detail, ...

A MERGE statement can fail, and incorrectly report a unique key violation when: The target table uses a unique filtered index; and No key column of the filtered index is updated; and A column from the filtering condition is updated; and Transient key violations are possible Example Tables Say we have ...

Hugo Tap asked me on Twitter earlier today whether or not there existed a SSIS Dataflow Destination component that enabled one to MERGE data into a table rather than INSERT it. Its a common request so I thought it might be useful to summarise the current state of play as regards a MERGE destination for SSIS. Firstly, there is no MERGE destination ...

This week we're going to take a look at issues with filtered indexes. This feature was a very welcome addition in SQL Server 2008 and has enjoyed widespread usage if conversations on twitter and forums are any indication. But their implementation is not perfect - bugs in SSMS and numerous restrictions on their use have certainly reduced their ...

Fellow MVP Hugo Kornelis (blog) has suggested that the proprietary UPDATE FROM and DELETE FROM syntax, which has worked for several SQL Server versions, should be deprecated in favor of MERGE.  Here is the Connect item he raised:#332437 : Deprecate UPDATE FROM and DELETE FROMAs you can see, the response is quite divided (more so than any ...

Summary: Investigating an optimiser transformation that exposes a bug in SQL Server’s MERGE implementation. I came across a Connect item today (by fellow SQL Server Central member ‘ALZDBA’) describing how using a combination of relatively new features can produce incorrect results or even an access violation inside SQL Server. Reproducing the ...

The new T-SQL MERGE statement in SQL Server 2008 seems to be finding many uses over and above its de facto UPSERT usage scenario, probably the most popular of which is its ability to update one table from another in an ANSI-compliant manner as detailed expertly by Hugo Kornelius in his blog post Let's deprecate UPDATE FROM! Today I stumbled upon ...

Just lately I’ve been using T-SQL’s MERGE statement (introduced in SQL Server 2008) and one thing that I needed to do was extract rowcounts for each DML operation (i.e. INSERT, UPDATE, DELETE) conducted by a MERGE. I was surprised to find that while @@ROWCOUNT is supported for MERGE, it only returns the total number of affected rows and there are ...