When working in a regular mode, the query processing spans the basic column storage and the data changed by transactions (inserts, updates, and deletes). For instance, the inserted data, kept in a separate delta structure, are added to the column before other operations in the plan. In a 'read-only mode' modifying transactions are not allowed and the query processing spans only over the basic column storage. One of the consequences is that query execution may become faster.
The readonly mode is administered with a boolean variable, which can be set by the system administrator using the monetdb tool:
shell> monetdb set readonly=yes <mydatabasename>
It ensures that all tables are accessed in read only mode. Moreover, the database is protected against any catalog change, the user can not even create a temporary table for keeping an intermediate result around.
The alternative route is to deploy SQL schemas in combination with user authentication and access control grants to selectively allow users access to the database. See the corresponding description in the SQL manual.