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Fragment table includes dataspace_id and anchor_id columns. Anchor table also has dataspace_id column. Since an Anchor belongs to only one Dataspace, and a Fragment belongs to only one Anchor, the dataspace_id column in Fragment is redundant (since the fragment's dataspace_id is a duplicate of the anchor's dataspace_id). It is at best redundant to have the same data duplicated in multiple database tables, and at worst could lead to inconsistencies.This task is to remove dataspace_id column from the Fragment table.

In addition to wasting space on duplicate data, the Fragment table also includes an index for dataspace_id, a foreign key constraint, and dataspace_id is included in the uniqueness constraint for Fragment. Removing all these may improve write performance (albeit a small improvement). There are no negative performance impacts expected, as the only functionality relying on dataspace_id is query-across-anchors (relevant SQL queries can be changed to use an inexpensive table JOIN on Anchor table).

This task is to remove dataspace_id column from the Fragment table.

Implementation Proposal

It is proposed to remove the dataspace_id column from the Fragment table in the Liquibase changelog. This also involves removing associated indexes and constraints.

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Fragment table before update, after update, and after rollback

As can be seen, the same data is restored after update and rollback (though the ordering of the columns has changed - testing has shown this to have no impact on functionality).

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