How can I call PostGIS's ST_Simplify function?

postgres=# SELECT ST_Simplify( geom, 0.1 ) FROM mains;

ERROR:  function st_simplify(geography, numeric) does not exist
LINE 1: SELECT ST_Simplify( geom, 0.1 ) FROM mains;
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

The function does exist:

postgres=# \df st_simplify
                                  List of functions
 Schema |    Name     | Result data type |         Argument data types         | Type
 public | st_simplify | geometry         | geometry, double precision          | func
 public | st_simplify | geometry         | geometry, double precision, boolean | func
(2 rows)

And my column type seems correct:

postgres=# \d my_table;

geom | geography(MultiLineString,4326) 

My PostGIS version:

postgres=# SELECT PostGIS_full_version();

POSTGIS="3.1.4 3.1.4" [EXTENSION] PGSQL="140" GEOS="3.9.1-CAPI-1.14.1" PROJ="7.1.1" LIBXML="2.9.9" LIBJSON="0.12" LIBPROTOBUF="1.2.1" WAGYU="0.5.0 (Internal)" (core procs from "3.2.1 3.2.1" need upgrade)

Could the problem be the final message, core procs from "3.2.1 3.2.1" need upgrade? If so, where would be the best source of a Windows binary?

  • 2
    It seems like ST_Simplify allows only geometry type, not geography?
    – RainForest
    Jul 15 at 7:23
  • Many thanks - hadn't noticed that geom had changed type in my many varied efforts to import data! Jul 15 at 9:35

1 Answer 1


As S.Chernyshov comments and you note in your question ST_Simplify only works on geometry objects not geography ones.

You will need to cast your geom to a geometry

SELECT ST_Simplify( geom::geometry, 0.1 ) FROM mains;

will be syntactically correct but probably not really a good idea as this sort of operation should really be carried out in a planar coordinate system not a geographic one.

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