Sql-server – Which command Deletes and Frees Space

The impact will depend on a number of things including your dependencies and indexes on the table. If there are few dependencies (i.e. foreign keys) and few indexes then the delete will go faster. In any case it is best to do this type of operation "off peak" hours when there are the fewest number of users on the system.

If you're in the pinch right now then you'll have to perform the delete now. The system will be a bit slower than usual while the delete is taking place.

You should also understand that due to the way database and log files grow in SQL Server you will not suddenly have a bunch of disk space to work with. To free up more disk space you need to look into other things (in the short term) such as log truncation and db shrinking. Then, figure out a better long-term disk solution.

Space is available inside the database because data has been moved around. Perhaps you have very high levels of page splits, or have recently deleted a large portion of data that had previously caused the data file to grow.

SQL Server does not shrink database files automatically when you've freed up space within them, because the logical assumption is that if you've used that space once, you'll use it again. Autogrow can be an expensive event and unnecessary if you've only freed up space temporarily (what were you able to do with all that free space in the meantime?). For the same reasons, you shouldn't try to temporarily reclaim space, either. Just let SQL Server use the 18 GB of available space as you add more data. If you think you will need more than 18 GB of additional space going forward (in which case, you will need to add file(s) on other disks, or move to a bigger disk).

sp_spaceused (and in turn the UI dialog you're looking at) may return more space than is possible because of synchronization issues in the metadata about your tables/indexes/files. In order to make sure it reflects accurate space, run this:

DBCC UPDATEUSAGE(0);

I have also seen scenarios where rebuilding indexes was required in order to rectify the counts/space, but I haven't seen that specific case since SQL Server 2000.

(I suspect this isn't a simple case where your database already spans multiple disks, or you surely would have mentioned that in the question.)

That all said, the fact that when you shrink the data file expands almost immediately leads me to believe that you are actually using the space, but must also be performing big deletes or updates that are freeing it up (which is when you see 18 GB free). Unfortunately it's impossible for us to see exactly why the data file is expanding and then clearing itself out - perhaps you have transactions where you are truncating / re-populating large tables, performing massive archive operations, etc.