Change tracking support in SQL devices

[ashalkhakov]

We have implemented change tracking operators in MSSQL device following the MS SQL Server change tracking support:

1. CHANGETABLE_CHANGES(table,Long): table (maps to CHANGETABLE(CHANGES...) operator in SQL Server): returns some change tracking information, keyed by the clustering key of the table
2. CHANGE_TRACKING_MIN_VALID_VERSION(table): Long (maps to operator of the same name in SQL Server): returns the minimal version among all rows in the table
3. CHANGE_TRACKING_CURRENT_VERSION() (maps to operator of the same same in SQL Server); this function, when added to a table which has change tracking enabled, will yield the version of every row
   (We haven't implemented CHANGETABLE(VERSION ...) at the moment.)

Refer to: https://technet.microsoft.com/en-us/library/cc280358%28v=sql.105%29.aspx

A few questions:

1. Is anybody interested in change tracking for other databases?
2. Perhaps a unified change tracking API needs to be added to D4? For instance, a new tag Storage.ChangeTrackingType could be used to emit additional DDL statements during reconciliation (for SQL Server change tracking, it is necessary to (a) enable database-wide change tracking, (b) enable change tracking for tables; in Postgres, for every tracked table, have to create insert/update/delete triggers and a tombstone)
3. Not sure what it means to use change tracking for different devices? Wouldn't that wreak havoc? The timestamps/versions would be incompatible, wouldn't they?