How to delete unused tables and columns

Introduction
Starcounter only deletes data when explicitly told to. In code, data is deleted with the Delete method or with DELETE SQL queries, read more in Data manipulation - delete. 
For example, if a database class is deleted from the code, the underlying table and the data in that table will not be deleted even if it's not referenced by any database class. The reason for this is that there's no way for Starcounter to determine if the data is abandoned and will never be used again. 
To delete tables and columns that will not be used, you can mark that they can be dropped with SQL and then delete them by executing the command staradmin start cleaner.
staradmin start cleaner launches the cleaning process by looking up all DROP TABLE or ALTER TABLE commands that were executed since the last time it ran. It will run until all those commands have been processed and then terminate. This will permanently delete the tables and columns.
Data that is deleted with start cleaner can't be recovered. Backup your data before deleting anything to have a safe point to revert to. 
How to drop a table
Tables should be dropped if it's not referenced by a database class and the data in the table is not useful anymore. The reason to do this is to free up memory. For this example, we have a table UselessTable that is not needed anymore:
[Database] 
public class UselessTable
{
    public string Nothing { get; set; }
}

1. Delete the database class from the code
In order to delete a table, the database class that references it has to be deleted. Thus, we delete the class UselessTable from the code and all the references to the class.
2. Restart the application
To delete the reference between the database class and the table, rebuild and restart the application. If you don't do this, the Administrator will throw ScErrColumnIsMapped (SCERR15014).
3. Run SQL command DROP TABLE
To mark that the underlying table should be deleted with all its data, run this command in the Administrator: 
DROP TABLE MyApp.UselessTable
If you try to run DROP TABLE without deleting all the references to the table in all the apps that are running, Starcounter will throw  ScErrTableIsReferenced (SCERR15012). 
4. Run the cleaner
To permanently delete UselessTable, execute staradmin start cleaner, when it's finished, it will print:  Done (nothing more to clean).
If you add UselessTable back to the code, executing SELECT * FROM MyApp.UselessTable will yield no results. The data has successfully been deleted.
Running staradmin start cleaner might produce transaction conflicts with running applications, thus, run the cleaning when the load is low to avoid performance drops.
How to drop a column
Deleting unused columns works similarly to how unused tables are deleted: a SQL command is used to mark that it should be dropped, and staradmin start cleaner deletes the data and column that has been marked.
For this example, IrrelevantFact should be deleted from the Person table:
[Database] 
public class Person 
{
  public string Name { get; set; }
  public string IrrelevantFact { get; set; }
}

1. Delete the property from the database class
To drop a column, the corresponding property has to be deleted. In this case, the class will then look like this:
[Database] 
public class Person 
{
  public string Name { get; set; }
}

2. Restart the application
To delete the reference between the property and the column, rebuild and restart the application. If you don't do this, the Administrator will throw ScErrColumnIsMapped (SCERR15014).
3. Stop the app
Since the ALTER TABLE command can't be executed while the table it alters is referenced in any running app - you have to stop the app before running ALTER TABLE, you can do this by running staradmin stop app MyApp.
4. Run SQL command DROP COLUMN
In the Administrator, run this query to mark that the column should be dropped:
ALTER TABLE MyApp.Person DROP COLUMN IrrelevantFact
5. Run the cleaner
To delete the data and the column, run this command:
staradmin start cleaner
After this, the column is deleted and all the data that was stored in it is permanently deleted.
Summary
Data definition language queries (DDL) are executed in the Administrator to delete tables or columns. Running the queries by themselves does not delete the data - you also have to run the cleaner to delete the data. To run DDL queries with either DROP COLUMN or DROP TABLE, you can't have the affected tables loaded. This means that the apps with references to those tables can't run unless the references to the tables are removed in the app.
To help you with find unused tables and columns you can use the tool WhatsNotReferenced.  It checks, when you start the database, for all tables and columns that are not in use by any currently running application
Again, be careful when permanently deleting data. If the data is of any value, you should make a backup before cleaning it.
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Outline

Introduction

How to drop a table

1. Delete the database class from the code

2. Restart the application

3. Run SQL command DROP TABLE

4. Run the cleaner

How to drop a column

1. Delete the property from the database class

2. Restart the application

3. Stop the app

4. Run SQL command DROP COLUMN

5. Run the cleaner

Summary