Known limitations on Identity Columns

This is a list of known limitations related to identity column support on distributed tables, which should be tracked for future releases:

- [ ] Metadata resync causes a loss of state for the sequence used by identity columns. https://github.com/citusdata/citus/pull/6738#pullrequestreview-1363223440
- [ ] Identity columns can only be used with the `bigint` data type on distributed tables. https://github.com/citusdata/citus/pull/6738#issue-1602985110
- [ ] alter_distributed_table and undistribute_table UDFs currently do not support identity columns. https://github.com/citusdata/citus/pull/6738#issue-1602985110
- [ ] Adding an identity constraint to an existing column using an ALTER TABLE statement is not supported.
- [ ] ALTER TABLE statements that involve changing the identity column are not supported.
- [ ] Adding a new identity column to a table with existing data is not supported.