I'm attempting to an efficient way to find "overlapping" polygons. Specifically we have a bunch of postal code/city/neighborhood data where I specifically don't want the default behavior of st_intersects that returns polygons that touch. We have an implementation using st_intersects that is quite performant but has the drawbacks of returning the polygons that touch as well. We also tried using st_buffer on the region with a negative value to "shrink" it before calling st_intersects. This functionality was what we want, but it was incredibly slow. Finally, we tried to run st_intersects and not st_touches which gives mostly the desired behavior (tiny overlaps would be nice to omit, but is acceptable). I tried st_overlaps based on the suggestion of JGH in the comments, but I do actually want the polygon to be returned when they are the same shape. Many postal cities consist of exactly 1 postal code. When running the query to see which zip codes intersect with those postal cities, I want that 1 and only 1 zip code (and not the neighboring zips)

I'm struggling to find other alternatives. Is there a way to combine the two underlying DE-9IM matrixes in a way to perform the 2 operations in a single step?

st_intersects T******** *T*******   ***T*****   ****T****
st_touches FT*******    F**T*****   F***T****

I'm not entirely sure what that looks like though? Any thoughts on a performant way to run this query (at scale), this service gets hit a lot.

  • 2
    ST_Relate is the direct DE-9IM applicaiton in PostGIS - but it is an expensive function, and not indexable; with ST_Intersects AND NOT ST_Touches, you will benefit from the index lookup for ST_Intersects, and then only run ST_Touches on the small hit set. Alternatively, for time critical scenarios, TRIGGER a negative ST_Buffer on INSERT - then run ST_Intersection only. You could create an index on ST_Buffer, or precede a && test before ST_Intersects to force bbox exclusion, but, depending on the complexity of your geometries, an on-the-fly buffer is very costly.
    – geozelot
    Commented Feb 3, 2022 at 9:09
  • could you use st_contains on the centroids of the polygons on one side of the test
    – Ian Turton
    Commented Feb 3, 2022 at 9:29
  • 1
    To be clear, ST_Relate can work with a spatial index, but the index needs to be queried explicitly with &&. It just doesn't include an automatic index check like most of the other predicate functions (this is because it may be testing for the Disjoint condition).
    – dr_jts
    Commented Feb 3, 2022 at 16:00

2 Answers 2


I believe you want to use st_overlaps, which should return true if 1) the intersection geometry has the same dimension as the input and 2) the two geometries are not equal.

The first point is the important one here, as if two polygons only touch each others, the intersection would be a line and the function would return false. This is however poorly documented on the postgis doc so use with caution.

with src (geom1,geom2,cmt) as (values 
    ('polygon((0 0, 0 1, 1 1, 1 0, 0 0))' , 'polygon((0 0, 0 1, 1 1, 1 0, 0 0))','same'),
    ('polygon((0 0, 0 1, 1 1, 1 0, 0 0))' , 'polygon((0 0, 0 1, -1 1, -1 0, 0 0))','touch'),
    ('polygon((0 0, 0 1, 1 1, 1 0, 0 0))' , 'polygon((-.5 -.5, -.5 .5, .5 .5, .5 -.5, -.5 -.5))','overlap'))
select  cmt, 
    st_overlaps(geom1,geom2) , 
from src;

   cmt   | st_overlaps | st_intersects | st_touches
 same    | f           | t             | f
 touch   | f           | t             | t
 overlap | t           | t             | f
  • 1
    The PostGIS manual page for ST_Overlaps has been improved recently: postgis.net/docs/manual-dev/ST_Overlaps.html
    – dr_jts
    Commented Feb 3, 2022 at 15:56
  • Thanks, the new doc confirms the behavior!
    – JGH
    Commented Feb 3, 2022 at 16:25
  • Ah, this is a great suggestion but I'm not sure it covers 1 use case I need and forgot to mention in the original question. I do actually want the behavior to return true when they are the same shape. For example, a number of postal cities are equivalent to their 1 and only zipcode. Commented Feb 3, 2022 at 18:12

The DE9IM pattern for "shares some area" is the one where the top-left cell is 2, and you don't really care about any of the others.

SELECT zip.id, city.id
FROM zip, city
WHERE zip.geom && city.geom
  AND ST_Relate(zip.geom, city.geom, '2********');

Throw in the index operator to speed up the query.

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