Or does a transaction log contain only differences?
Are there differences in the log file between these two scripts:
DECLARE @f1 INT ,
@f2 INT
CREATE TABLE #t ( int F1, int f2 )
UPDATE #t
SET f1 = ISNULL(@f1, f1) ,
f2 = ISNULL(@f2, f2)
--or
IF @f1 IS NOT NULL
UPDATE #t
SET f1 = ISNULL(@f1, f1)
IF @f2 IS NOT NULL
UPDATE #t
SET f2 = ISNULL(@f2, f2)
Only if the update is part of the clustered key.
Have a read of this article titled "The impact of non-updating updates" which discusses this issue.
Summary
SQL Server contains a number of optimisations to avoid unnecessary logging or page flushing when processing an UPDATE operation that will not result in any change to the persistent database.
- Non-updating updates to a clustered table generally avoid extra logging and page flushing, unless a column that forms (part of) the cluster key is affected by the update operation.
- If any part of the cluster key is ‘updated’ to the same value, the operation is logged as if data had changed, and the affected pages are marked as dirty in the buffer pool. This is a consequence of the conversion of the UPDATE to a delete-then-insert operation.
- Heap tables behave the same as clustered tables, except they do not have a cluster key to cause any extra logging or page flushing. This remains the case even where a non-clustered primary key exists on the heap. Non-updating updates to a heap therefore generally avoid the extra logging and flushing (but see below).
- Both heaps and clustered tables will suffer the extra logging and flushing for any row where a LOB column containing more than 8000 bytes of data is updated to the same value using any syntax other than ‘SET column_name = column_name’.
- Simply enabling either type of row versioning isolation level on a database always causes the extra logging and flushing. This occurs regardless of the isolation level in effect for the update transaction.
