memory control on large results

Hello, I plan to use the embedded library (custom build) for a project that will likely address very large tables. While the control over statements is handled to the end user, I need an advice how to control memory usage on fetching huge amount of data. Specifically, I already skipped the default "monetdb_result_fetch" and implemented a compressed storage (which optional can be stored on disk), but I wasn't able to see a way of releasing the associated memory for already processed columns (persistent or transient bat). Unless the bat contents is shared between different res_col's, I would like to be able to release that memory to the system before proceeding to the next column. Any suggestion? Thank you, Dan