[Bug] New `onSchemaChange` statements get rate limited after 5 new columns

[HHammond]

When using the new onSchemaChange feature the generated SQL adds each new column with a separate ALTER TABLE statement, which hits BigQuery's 5 alterations per 10 seconds rate limit.

Feature Announcement

Error:

Exceeded rate limits: too many table update operations for this table. For more information, see https://cloud.google.com/bigquery/docs/troubleshoot-quotas

Minimal case to reproduce

config {
  schema: 'scratch',
  type: "incremental",
  onSchemaChange: "EXTEND",
  uniqueKey: "a"
}

${when(incremental(), 
  `SELECT 1 as a, 2 as b, 3 as c, 4 as d, 5 as e, 6 as f, 7 as g`,
  `SELECT 1 as a`
)}

Current Behaviour

Dataform maps all new column additions to separate ALTER TABLE statements:

  IF ARRAY_LENGTH(columns_added) > 0 THEN
    FOR new_column IN (SELECT * FROM UNNEST(columns_added) AS column_info)
    DO
      EXECUTE IMMEDIATE FORMAT(
            'ALTER TABLE `project.schema.table` ADD COLUMN %s %s',
              new_column.column_name, new_column.data_type
        );
    END FOR;
  END IF;

Expected Behaviour

Dataform should use a single ALTER TABLE ADD COLUMN statement to add multiple columns at once:

A single statement has the following benefits:

• Atomic changes — either all fields are added or none are. Currently a single incorrect column configuration can leave a table in a partially updated state.
• Better resource use — avoid hitting the 5 updates/10s rate limit

Concerns

It's not clear to me if this is a dataform core issue -- there is no logic in dataform core for the actual SQL generation, if this is the case let me know.