PostGIS SQL: Internal buffer of unioned polygons

Question

I have a polygon layer containing titles which are grouped by a property ID (i.e. there may be one or more title polygon per property)

I need to generate an internal buffer of those unioned titles/of the property but can't quite get it working- with the code below, the internal buffer is generated but for all titles individually (i.e. there are buffers on the internal divisions)

How can I apply a union to the 'titles' before the difference?

Alternatively, is there a better way of generating an internal buffer of unioned/merged polygons grouped by an attribute?

SELECT
  row_number() OVER () AS fid,
  g.geom::geometry(Polygon,2193) AS geom,
  g.forest_id
FROM (
  SELECT
    (ST_Dump(
       ST_Union(
         ST_Difference(
           titles.geom,
           ST_Buffer(titles.geom, - 10::double precision)
         )
       )
    )).geom AS geom,
    titles.forest_id
  FROM public.titles
  GROUP BY titles.forest_id) g;

Answer 1

To create the residue area of a subtraction of unioned titles.geom and their negative buffer based on common titles.forest_id, run:

SELECT
  ROW_NUMBER() OVER() AS fid,
  u.forest_id,
  dmp.path[1] AS seq,
  dmp.geom
FROM (
  SELECT
    forest_id,
    ST_Union(geom) AS geom
  FROM
    titles
  GROUP BY
    1
) AS u,
  LATERAL ST_Dump(
    ST_Difference(
      u.geom,
      ST_Buffer(u.geom, -10)
    )
  ) AS dmp
;

---

Update:

If your geometric conditions are simple enough, it is more performant to create a side-specific buffer along the ST_Boundary of the unioned geometries; see the example section in the docs for ST_Buffer.

Since I'm at it, here's a possible alternative to the above; on larger datasets, this should definitely be a lot faster.

SELECT
  ROW_NUMBER() OVER() AS fid,
  u.forest_id,
  buf AS geom
FROM (
  SELECT
    forest_id,
    ST_Union(geom) AS geom
  FROM
    titles
  GROUP BY
    1
) AS u,
  LATERAL ST_Buffer(
    ST_Boundary(u.geom),
    10,
    'side=right'
  ) AS buf
;

Note that you need to make sure all your Polygons are oriented the same (i.e. via ST_ForceRHR, either on-the-fly or as an UPDATE to the table) for the side=left|right parameter working as expected.