The talk enumerates different classes of ALTER TABLE operations:
operations not involving other than metadata
operations that can be performed instantly by introducing a backward-compatible data file format change and 'faking' the operation (ADD or DROP COLUMN)
operations that can avoid rebuilding a table
operations that must rebuild the table
variations of the previous 2 classes that allow concurrent modifications to the table
We also show how ALTER TABLE can be executed concurrently on multiple nodes in statement-based replication. Finally, we cover some theoretical limits of what kind of ALTER TABLE operations can be supported without rebuilding the table, by introducing an optional validation step and on-demand conversion of records in previous schema versions of the table.