4

I have a polygon column for which I would like to add a unique constraint that tests for spatial equality akin to ST_Equals so that it ignores vertex order. For instance, I would like these polygons to count as duplicates:

ST_GeomFromText('POLYGONZ((0 0 1, 1 0 1, 1 1 1, 0 1 1, 0 0 1))')
ST_GeomFromText('POLYGONZ((1 0 1, 1 1 1, 0 1 1, 0 0 1, 1 0 1))')

Is there a standard way to implement this other than a GIST index, which the docs say only indexes the bounding box?

One solution I'm thinking of is to index on a function that returns a sorted array of polygon vertices, but it seems like a hack.

  • I just realized that you are using POLYGONZ; ST_Equals does not account for Z values. Might be why you suggests a vertice array. – ThingumaBob Mar 7 at 16:20
4

You can run a CHECK constraint against a custom function.

Create the function:

CREATE FUNCTION equality_constraint_func(
  id INT,
  gm GEOMETRY
)
RETURNS boolean AS
  $$
    SELECT NOT EXISTS (
      SELECT 1
      FROM   <table> AS a
      WHERE  a.<id> <> id
        AND  a.<geom> && gm
        AND  ST_Equals(a.<geom>, gm)
    );
  $$
  LANGUAGE sql
;

and add the actual constraint:

ALTER TABLE <table>
  ADD CONSTRAINT equality_constraint
  CHECK (equality_constraint_func(<id>, <geom>))
;

Inspired by and adapted from this excellent answer, to adress this rather common use case directly.

Read there why e.g. the <id> is passed.

  • I'm accepting this because it's a good answer to the question I asked. Unfortunately, I didn't realize ST_Equals doesn't work with 3D geometry as you pointed out. In light of that, I think the easiest and most performant solution may in fact be a vertex array. – user3747260 Mar 7 at 20:07

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