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When a view's partition key contains only columns from the base's partition key (and not an additional one), the liveness (existance or disappearance) of a view-table row is tied to the liveness of the base table row - and that depends not only on selected columns (base-table columns SELECTed to also appear in the view) but also on unselected columns. This means that we may need to keep a view row alive even without data, just because some unselected column is alive in the base table. Before this patch we tried to build a single "row marker" in the view column which summarizes the liveness information in all unselected columns, but this proved unworkable, as explained in issue #3362 and as will be demonstrated in unit tests in a later patch. Because we can't replace several unselected cells by one row marker, what we do in this patch is to add for each for the unselected cell a "virtual cell" which contains the cell's liveness information (timestamp, deletion, ttl) but not its value. For collections, we can't represent the entire collection by one virtual cell, and rather need a collection of virtual cells. This patch just adds the virtual columns to the view schema. Code in the previous patch, when it notices the virtual columns in the view's schema, added the appropriate content into these columns. We may need to add virtual columns to a view when first created, but also when an unselected column is added to the base table with "ALTER TABLE", so both are supported in this patch. Fixes #3362. Signed-off-by: Nadav Har'El <nyh@scylladb.com>