Using ALTER TABLE to eliminate extent interleaving

If you use the ALTER TABLE statement to add or drop a column or to change the data type of a column, the database server copies and reconstructs the table. When the database server reconstructs the entire table, it rewrites the table to other areas of the dbspace. However, if other tables are in the dbspace, no guarantee exists that the new extents will be adjacent to each other.

Important: For certain types of operations that you specify in the ADD, DROP, and MODIFY clauses, the database server does not copy and reconstruct the table during the ALTER TABLE operation. In these cases, the database server uses an in-place alter algorithm to modify each row when it is updated (rather than during the ALTER TABLE operation). For more information about the conditions for this in-place alter algorithm, see In-place alter.

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