Does liquibase recognize if table is already up to date before executing changeSet?

Question

In our case liquibase is used to update databses for existing installation. New installations are already up to date.

Assuming we have got a new installation. Starting the application will force to execute liquibase changesets (e.g. change type of a column) but as I mentioned before there is nothing to update as the column already was created with the correct type.

Does liquibase recognize that the table column is already up to date or does it try to execute the changeset as there is no entry within the databasechangelog table for it?

Answer 1

Liquibase uses an alternative approach that avoids a need to analyze the target database's data dictionary. This makes DB operations simpler and more cross platform.

A special table "DATABASECHANGELOG" keeps a record of the changesets applied to the target database instance. This table also contains a checksum (calculated at runtime) to determine if changsets are altered between runs of liquibase.

So if you altered the type of a table column, liquibase can detect this and can throw an error, when run against an existing database. (Obviously, on a new DB, the table would be created as expected).

Finally, the changeset documentation describes two optional attributes ("runAlways" and "runOnChange") which could tell lqiuibase to reapply a changeset more than once to a database. There is also a "clearCheckSums" command that can be used to reset the checksums on an existing database. Obviously you need to know what you're doing when using such an option :-)