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" The view_info object, which is attached to the schema object of the view, contains a data structure called "base_non_pk_columns_in_view_pk". This data structure contains column ids of the base table so is valid only for a particular version of the base table schema. This data structure is used by materialized view code to interpret mutations of the base table, those coming from base table writes, or reads of the base table done as part of view updates or view building. The base table schema version of that data structure must match the schema version of the mutation fragments, otherwise we hit undefined behavior. This may include aborts, exceptions, segfaults, or data corruption (e.g. writes landing in the wrong column in the view). Before this patch, we could get schema version mismatch here after the base table was altered. That's because the view schema did not change when the base table was altered. Another problem was that view building was using the current table's schema to interpret the fragments and invoke view building. That's incorrect for two reasons. First, fragments generated by a reader must be accessed only using the reader's schema. Second, base_non_pk_columns_in_view_pk of the recorded view ptrs may not longer match the current base table schema, which is used to generate the view updates. Part of the fix is to extract base_non_pk_columns_in_view_pk into a third entity called base_dependent_view_info, which changes both on base table schema changes and view schema changes. It is managed by a shared pointer so that we can take immutable snapshots of it, just like with schema_ptr. When starting the view update, the base table schema_ptr and the corresponding base_dependent_view_info have to match. So we must obtain them atomically, and base_dependent_view_info cannot change during update. Also, whenever the base table schema changes, we must update base_dependent_view_infos of all attached views (atomically) so that it matches the base table schema. Fixes #7061. Tests: - unit (dev) - [v1] manual (reproduced using scylla binary and cqlsh) " * tag 'mv-schema-mismatch-fix-v2' of github.com:tgrabiec/scylla: db: view: Refactor view_info::initialize_base_dependent_fields() tests: mv: Test dropping columns from base table db: view: Fix incorrect schema access during view building after base table schema changes schema: Call on_internal_error() when out of range id is passed to column_at() db: views: Fix undefined behavior on base table schema changes db: views: Introduce has_base_non_pk_columns_in_view_pk()