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Currently, the base_info may or may not be set in view schemas. Even when it's set, it may be modified. This necessitates extra checks when handling view schemas, as well as potentially causing errors when we forget to set it at some point. Instead, we want to make the base info an immutable member of view schemas (inside view_info). The first step towards that is making sure that all newly created schemas have the base info set. We achieve that by requiring a base schema when constructing a view schema. Unfortunately, this adds complexity each time we're making a view schema - we need to get the base schema as well. In most cases, the base schema is already available. The most problematic scenario is when we create a schema from mutations: - when parsing system tables we can get the schema from the database, as regular tables are parsed before views - when loading a view schema using the schema loader tool, we need to load the base additionally to the view schema, effectively doubling the work - when pulling the schema from another node - in this case we can only get the current version of the base schema from the local database Additionally, we need to consider the base schema version - when we generate view updates the version of the base schema used for reads should match the version of the base schema in view's base info. This is achieved by selecting the correct (old or new) schema in `db::schema_tables::merge_tables_and_views` and using the stored base schema in the schema_registry.