sql: cannot perform other schema changes concurrently with alter column type

[RichardJCai]

Currently, ALTER COLUMN TYPE cannot be performed concurrently with other schema changes.

For example you cannot add a column while an alter column type is occuring, you cannot alter primary key while alter column type is occuring.

Jira issue: CRDB-5036

Epic CRDB-25314

[RichardJCai]

#49351 covers this

[rafiss]

re-opening, since this is referenced here:

cockroach/pkg/sql/alter_column_type.go

Line 220 in 4b8a9c7

47137, "table %s is currently undergoing a schema change", tableDesc.Name)

[ajwerner]

I feel like this isn't really about wanting the operations to occur concurrently, it's about wanting them to be ordered.

[ddelange]

CockroachDB executes ALTER COLUMN queries asynchronously which is at
odds with Django's assumption that the database is altered before the next
migration operation begins. CockroachDB will give an error like
unimplemented: table <...> is currently undergoing a schema change if a
later operation tries to modify the table before the asynchronous query
finishes. A future version of CockroachDB may fix this.

ref https://github.com/cockroachdb/django-cockroachdb/blob/5.1/README.md cc @timgraham

is there a possibility to await the change / make the query block until the change finishes?

[ajwerner]

I thought the asynchronous part was fixed such that the statement doesn’t now return until the alter is complete. That comment may be stale. Is it something you see on later cockroach versions?

[ddelange]

we are in research phase, and this looked like a blocker if we want to move to cockroachdb. can this issue be closed and the django-cockroachdb README amended?

[timgraham]

I believe the stable CockroachDB docs still state this restriction:

Note: Most ALTER COLUMN TYPE changes are finalized asynchronously. Schema changes on the table with the altered column may be restricted, and writes to the altered column may be rejected until the schema change is finalized.

[ajwerner]

@rafiss is it true? My recollection is that the the alter job waits on the job it creates and has for years.

[timgraham]

For context, we saw an intermittent failure in Django with CockroachDB 21.1 (#65691). Since then the test has been on the skip list.

[ddelange]

hi all 👋

did you happen to discuss this internally in the meantime?

this would be a hard blocker to migrate our django project over from vanilla postgres