Liquibase add comment to table - liquibase

Question

I'm currently trying to add comment on tables in liquibase 3.1.1
What I want to do is ALTER TABLE t1 COMMENT = 'New table comment';
I didn't find any help in the documentation of liquibase about this case, there is only help for adding comments on columns.
I am currently thinking of creating a customChange or doing the change by myself with SQL statements but as we are going to migrate from MySQL to Oracle I would like to avoid this solution (or use it in last resort).
Has anyone found another solution for this problem ?

Answer 1

COMMENT ON TABLE is absolutely helpful in this case. See example I provided below along with some tips and pitfalls about editing tables using liqubase.
Continuing the response above, I'd like to pay attention on tag <preConditions> here. It's critical when dealing with tables to ask liqubase whether the certain table exists at the moment you try to edit one. Here's some example of this behavior:
<changeSet id="your_table_id" author="author_name" dbms="postgresql">
<preConditions onFail="MARK_RAN">
<tableExists tableName="table_name" schemaName="schema_name"/>
</preConditions>
<sql>
COMMENT ON TABLE schema_name.table_name is 'YOUR_COMMENT_ON_TABLE';
</sql>
</changeSet>
In case dealing with columns (editing, adding, deleting) ALSO consider asking liqubase the similar way of the existing or absence of the specific column(s):
<preConditions onFail="MARK_RAN">
<not>
<columnExists tableName="table_name" schemaName="schema_name"
columnName="column_name"/>
</not>
</preConditions>
<sql>
...
</sql>
The example above checks if column NOT present in table and then executes some SQL script.

Answer 2

I solved this problem by creating a custom SQL statement with a <sql> tag (or "sql" key if you are using JSON) in the changeSet (documentation can be found here).
Since I am using PostgreSQL, the SQL syntax for adding a comment to a table is COMMENT ON TABLE table_name IS 'text'. Below is the entire changeSet in XML format:
<changeSet author="gfigueroa" id="add-comment-to-table-xyz">
<sql dbms="postgresql"
endDelimiter=";"
splitStatements="true"
stripComments="true">
COMMENT ON TABLE xyz IS 'This is a test comment';
</sql>
</changeSet>