Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 14: Transactions and Locks
The syntax is simple enough:
SAVE TRAN[SACTION] [| ]
The thing to remember about save points is that they are cleared on ROLLBACK — that is, even if you
save five save points, once you perform one ROLLBACK they are all gone. You can start setting new save
points again and rolling back to those, but whatever save points you had when the ROLLBACK was
issued are gone.
SAVE TRAN can get extremely confusing and I can’t recommend it for the beginning user, but keep it in
mind as being there.
How the SQL Ser ver Log Works
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You definitely must have the concept of transactions down before you try to figure out how SQL Server
tracks what’s what in your database. You see, what you think of as your database is only rarely a complete
version of all the data. Except for rare moments when it happens that everything has been written to disk,
the data in your database is made up of not only the data in the physical database file(s), but also any
transactions that have been committed to the log since the last checkpoint.
In the normal operation of your database, most activities that you perform are logged to the transaction
log, rather than written directly to the database. A checkpoint is a periodic operation that forces all dirty
pages for the database currently in use to be written to disk. Dirty pages are log or data pages that have
been modified after they were read into the cache, but the modifications have not yet been written to
disk. Without a checkpoint the log would fill up and/or use all the available disk space. The process works
something like the diagram in Figure 14-1.
Don’t mistake all this as meaning that you have to do something special to get your data out of the
cache. SQL Server handles all of this for you. This information is only provided here to facilitate your
understanding of how the log works and, from there, the steps required to handle a transaction. Whether
something is in cache or not can make a big difference to performance, so understanding when things
are logged and when things go in and out of the cache can be a big deal when you are seeking maximum
performance.
Note that the need to read data into a cache that is already full is not the only reason that a checkpoint
would be issued. Checkpoints can be issued under the following circumstances:
❑ By a manual statement (using the CHECKPOINT command)
❑ At normal shutdown of the server (unless the WITH NOWAIT option is used)
❑ When you change any database option (for example, single user only, dbo only, and so on)
❑ When the Simple Recovery option is used and the log becomes 70 percent full
❑ When the amount of data in the log since the last checkpoint (often called the active portion of
the log) exceeds the size that the server could recover in the amount of time specified in the
recovery interval option