We have a SQL Server 2012 Enterprise live transactional database that is now growing over 1G per month and is becoming a size problem for us. It is currently at 23G. Character type fields are all Unicode and I have calculated a savings of 5G in space converting only 2 such fields averaging 206 characters each to non-Unicode, and almost 10G in space if we convert a few more of them from nchar and nvarchar to char and varchar types. These fields will never have a requirement to hold Unicode characters that cannot be in the SQL_Latin1_General_CP1_CI_AS collation as they come in as plain ASCII originally and will always do so per the protocol standard.
I’m the software architect and chief C# developer though only a DBA hack or I would not have designed our database to have Unicode fields for high volume tables that did not need Unicode for those fields when the database was created 3 years ago. I want to correct this mistake now before we finalize converting to an AlwaysOn environment to help with various performance and backup issues.
After downsizing these two or more fields, we would like to shrink the database one time to take advantage of the space savings for full backups, and for seeding an AlwaysOn environment.
Questions are –
What is the safest and most efficient conversion technique for downsizing columns from nchar/nvarchar to char/varchar types? Esp. when there are multiple fields in the same table to be converted. I tested doing an “add new column, set new=old, drop old, rename old to new” for both of the main two fields I want to convert from nvarchar(max) to varchar(max), and it took 81 minutes on our test server (4 virtual core, 8G memory) before running out of disk space even though there was 8G left on the disk, and the db has unlimited size set (Could not allocate space for object 'dbo.abc'.'PK_xyz' in database 'xxx' because the 'PRIMARY' filegroup is full). I did delete an old database before it finished after getting a disk warning so maybe it did not count that new space. Regardless it was too slow. And this was on just the two largest of these columns (12.6M rows) and only ran 2 to 3% CPU busy so seemed not very efficient, and indicated unacceptable downtime if we were to convert even these two fields much less any additional fields. Average field size for these two fields was only 206 characters or 412 bytes each. Another technique I plan to try is to create the new table def in a new schema, select into it from the old table, then move tables amongst schema and delete the old table. I have a FK and indexes to contend with on the table.
If I figure out how to do #1 efficiently within an acceptable maint window, what is the safest practice for doing a one-time shrink and end up with organized/rebuilt indexes and updated Statistics? I understand the logic of not doing regular shrinks and that sometime it can actually increase the size.
Is there any third party tool that could take a backup and restore it into a new database with the modified field definitions or otherwise convert certain field types?
Any suggestions and best practices most welcome.