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These changes mainly involve updating the tables' metadata (information about a table's structure, describing its columns and their types) and can be done quickly. However, the data in those tables must be updated to the format of the new type version as well. Updating this data can be time-consuming if there is a lot of it, so the ALTER TYPE command has options to let you choose whether to convert all dependent table data immediately or to leave it in the old format to be converted piecemeal as it is updated in the course of business.
The CASCADE option for ALTER TYPE propagates a type change to dependent types and tables. See "ALTER TYPE Statement for Type Evolution". CASCADE itself has options that let you choose whether to convert table data to the new type format as part of the propagation: the option INCLUDING TABLE DATA converts the data; the option NOT INCLUDING TABLE DATA does not convert it. By default, the CASCADE option converts the data. In any case, table data is always returned in the format of the latest type version. If the table data is stored in the format of an earlier type version, Oracle converts the data to the format of the latest version before returning it, even though the format in which the data is actually stored is not changed until the data is rewritten.
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