Online DDL Operations clustered index is renamed with the name of the original table, and the original table is dropped from the database. The online performance enhancements that apply to operations on secondary indexes do not apply to the primary key index. The rows of an InnoDB table are stored in a clustered index organized based on the primary key, forming what some database systems call an “index-organized table” . Because the table structure is closely tied to the primary key, redefining the primary key still requires copying the data. When an operation on the primary key uses  ALGORITHM=INPLACE , even though the data is still copied, it is more efficient than using  ALGORITHM=COPY  because: •No undo logging or associated redo logging is required for  ALGORITHM=INPLACE . These operations add overhead to DDL statements that use  ALGORITHM=COPY . •The secondary index entries are pre-sorted, and so can be loaded in order. •The change buffer is not used, because there are no random-access inserts into the secondary indexes. •Dropping a primary key •Dropping a primary key and adding another ALTER TABLE  tbl_name  DROP PRIMARY KEY, ALGORITHM=COPY; ALTER TABLE  tbl_name  DROP PRIMARY KEY, ADD PRIMARY KEY ( column ), ALGORITHM=INPLACE, LOCK=NONE; Only  ALGORITHM=COPY  supports dropping a primary key without adding a new one in the same  ALTER TABLE  statement. Data is reorganized substantially, making it an expensive operation. The following table provides an overview of online DDL support for column operations. An asterisk indicates additional information, an exception, or a dependency. For details, see Syntax and Usage Notes. Table 15.18 Online DDL Support for Column Operations Column Operations Operation Instant In Place Rebuilds Table Permits Concurrent DML Only Modifies Metadata Adding a column Yes* Yes No* Yes* No Dropping a column No Yes Yes Yes No Renaming a column No Yes No Yes* Yes Reordering columns No Yes Yes Yes No Setting a column default value Yes Yes No Yes Yes Changing the column data type No No Yes No No 3179