To be able to defer the re-encoding of rows (e.g. when ALTER changes column order) the row header must store the version/identifier of the format used to encode

To be able to defer the re-encoding of rows (e.g. when ALTER changes column order) the row header must store the version/identifier of the format used to encode it. You can also do it block-at-a-time instead of row-at-a-time, so only each block would store a version/identifier of the format. But any way, deferrable re-encoding means some storage overhead and inconsistent UPDATE performance after ALTER until all rows (or all blocks) have been re-encoded.