A common question that is frequently asked when performing maintenance on Greenplum tables is “Why does my ALTER TABLE add column DDL statement take so long to run?” Although it appears to be a simple command and the expectations are that it will execute in minutes this is not true depending on the table organization (heap, AO columnar compressed, AO row compressed), number of range partitions and the options used in the alter table command.
Depending on the size of the table a rewrite table operation triggered by an alter table/column DDL command could take from minutes to multiple hours. During this time the table will hold the access exclusive locks and may cause cascading effects on other ETL processing. While this rewrite operation is occurring there is no easy way to predict its completion time. Please note that since Greenplum supports polymorphic tables a range partitioned table can contain all three table organizations within a single parent table, this implies that some child partitions can trigger a rewrite while others may be altered quickly. However, all operations on a range partitioned table must complete before the DDL operation is completed.