For several years I have happily relied on @@IDENTITY to return the latest identity of a row when inserting data. However, I have started using the OUTPUT clause and have discovered the entertainment to be had in getting the latest identity when writing to two or more tables with what can appear to be one …
Background Untested backups. An awful lot is said about taking regular backups, although an inordinate number of people out there appear to ignore it. What is less talked about is actually checking that the backups are usable – it’s no good having a series of backups for that fateful day to discover that when they’re …
Recently I have had to extract user’s details from Active Directory (AD) for certain security groups. Having looked through a slew of internet resources it is obvious that the work required to do this has changed little over many years – and it looks like is was deliberately designed to make it difficult. I’d like …
Partitioned views have been around since before SQL Server 2005 (whereupon partitioned tables were introduced) but this does not mean that they aren’t without their uses now.
A heap is a table that has no clustered index – it may have non-clustered indexes but without a clustered index it is a heap. When a row is updated within the heap, if the new data size means a row cannot be stored in the original location, SQL Server moves the row to another …
Optimistic locking requires the use of row versioning, where a copy of the data about to be modified is stored in TempDB. The advantage of this approach is that the number of locks are reduced and the opportunities for blocking are also reduced. The downside is that the resources required for data modifications are increased, …
In a previous article (here) I described how the isolation level read committed works, with some examples. There is also an optimistic version of this – read committed snapshot, which uses row versions. The behaviour of read committed and read committed snapshot is similar and repeating the examples used previously will show the similarities and …
The previous articles on isolation levels were using pessimistic locking. Two further isolation levels exist that use optimistic locking and are row-versioning based. Snapshot Isolation With snapshot isolation level no locks are placed upon the data when it is read and transactions that write data don’t block snapshot isolation transactions from reading data. Instead, when …
In the previous article it was shown that phantom reads and skipped rows were possible with the repeatable read isolation level. Serializable is the highest isolation level and will not allow phantom reads or skipped rows.
In my previous article I showed how read committed would allow rows to be updated that it had already processed in a query, because it releases locks as soon as it has processed that row or page (depending upon the type of lock taken). A second query using the same criteria would then show different …
Notes on SQL 