Keep track of reltuples at the time of creating a migration
In order to report progress later, we'd like to keep track of pg_class.reltuples
for the table an question when we create the migration record. This is necessary because over time, pg_class.reltuples
is going to deviate and may turn not representative for the total number of tuples we need to update. This is because at the same time as creating the migration, we install sync triggers to keep new data automatically in sync - hence we don't need to touch those tuples with the migration.