The background update to remove duplicate rows naively deleted and
reinserted the duplicates. For large tables with a large number of
duplicates this causes a lot of bloat (with postgres), as the inserted
rows are appended to the table, since deleted rows will not be
overwritten until a VACUUM has happened.
This should hopefully also help ensure that the query in the last batch
uses the correct index, as inserting a large number of new rows without
analyzing will upset the query planner.
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