(maint) workaround postgresql bug #15865
(maint) workaround postgresql bug #15865

PostgreSQL 9.6.14 introduced a bug:

BUG #15865: ALTER TABLE statements causing "relation already exists" errors when some indexes exist

See: https://www.postgresql.org/message-id/15865-17940eacc8f8b081%40postgresql.org

This will make any ALTER TABLE statement only affect a single column per
statement.

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