Materialize supports schema changes in the upstream database as follows:

#### Compatible schema changes

- Adding columns to tables. Materialize will **not ingest** new columns added
  upstream unless you use [`DROP SOURCE`](/sql/alter-source/#context) to first
  drop the affected subsource, and then add the table back to the source using
  [`ALTER SOURCE...ADD SUBSOURCE`](/sql/alter-source/).

- Dropping columns that were added after the source was created. These columns
  are never ingested, so you can drop them without issue.

- Adding or removing `NOT NULL` constraints to tables that were nullable when
  the source was created.

#### Incompatible schema changes

All other schema changes to upstream tables will set the corresponding subsource
into an error state, which prevents you from reading from the source.

To handle incompatible [schema changes](#schema-changes), use [`DROP SOURCE`](/sql/alter-source/#context)
and [`ALTER SOURCE...ADD SUBSOURCE`](/sql/alter-source/) to first drop the
affected subsource, and then add the table back to the source. When you add the
subsource, it will have the updated schema from the corresponding upstream
table.