pycsw has the ability to perform server side repository / database filters as a means to mask all CSW requests to query against a specific subset of the metadata repository, thus providing the ability to deploy multiple pycsw instances pointing to the same database in different ways via the repository.filter configuration option.
Repository filters are a convenient way to subset your repository at the server level without the hassle of creating proper database views. For large repositories, it may be better to subset at the database level for performance.
The repository.filter option accepts all core queryables set in the pycsw core model (see pycsw.config.StaticContext.md_core_model for the complete list).