diff --git a/layers/aerodrome_label/layer.sql b/layers/aerodrome_label/layer.sql
index 242a0d9..19b8b2a 100644
--- a/layers/aerodrome_label/layer.sql
+++ b/layers/aerodrome_label/layer.sql
@@ -38,5 +38,6 @@ SELECT
 FROM osm_aerodrome_label_point
 WHERE geometry && bbox
   AND zoom_level >= 10;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/aeroway/layer.sql b/layers/aeroway/layer.sql
index 6863010..93f03fc 100644
--- a/layers/aeroway/layer.sql
+++ b/layers/aeroway/layer.sql
@@ -56,5 +56,6 @@ FROM (
          WHERE zoom_level >= 14
      ) AS zoom_levels
 WHERE geometry && bbox;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/boundary/boundary.sql b/layers/boundary/boundary.sql
index bf75716..1100759 100644
--- a/layers/boundary/boundary.sql
+++ b/layers/boundary/boundary.sql
@@ -206,7 +206,10 @@ SELECT CASE
                THEN replace(SUBSTRING(name, POSITION(' at ' IN name) + 4), ' ', '')
            ELSE replace(replace(name, ' ', ''), 'Extentof', '')
            END;
-$$ LANGUAGE SQL IMMUTABLE;
+$$ LANGUAGE SQL IMMUTABLE
+                -- STRICT
+                PARALLEL SAFE
+                ;
 
 
 -- etldoc: ne_110m_admin_0_boundary_lines_land  -> boundary_z0
@@ -625,5 +628,6 @@ FROM (
          WHERE geometry && bbox
            AND zoom_level >= 13
      ) AS zoom_levels;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/building/building.sql b/layers/building/building.sql
index 85025d9..f08e863 100644
--- a/layers/building/building.sql
+++ b/layers/building/building.sql
@@ -112,6 +112,9 @@ FROM (
            AND geometry && bbox
      ) AS zoom_levels
 ORDER BY render_height ASC, ST_YMin(geometry) DESC;
-$$ LANGUAGE SQL IMMUTABLE;
+$$ LANGUAGE SQL STABLE
+                -- STRICT
+                PARALLEL SAFE
+                ;
 
 -- not handled: where a building outline covers building parts
diff --git a/layers/housenumber/layer.sql b/layers/housenumber/layer.sql
index 2cf0b55..84cb4d8 100644
--- a/layers/housenumber/layer.sql
+++ b/layers/housenumber/layer.sql
@@ -18,5 +18,6 @@ SELECT
 FROM osm_housenumber_point
 WHERE zoom_level >= 14
   AND geometry && bbox;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/landcover/landcover.sql b/layers/landcover/landcover.sql
index e2e183f..35e4d59 100644
--- a/layers/landcover/landcover.sql
+++ b/layers/landcover/landcover.sql
@@ -15,6 +15,7 @@ SELECT CASE
            %%FIELD_MAPPING: class %%
            END;
 $$ LANGUAGE SQL IMMUTABLE
+                -- STRICT
                 PARALLEL SAFE;
 
 -- etldoc: ne_110m_glaciated_areas ->  landcover_z0
@@ -185,5 +186,6 @@ FROM (
          WHERE zoom_level >= 14
            AND geometry && bbox
      ) AS zoom_levels;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/landuse/landuse.sql b/layers/landuse/landuse.sql
index 89233ae..a1cae13 100644
--- a/layers/landuse/landuse.sql
+++ b/layers/landuse/landuse.sql
@@ -214,5 +214,6 @@ FROM (
          WHERE zoom_level >= 14
      ) AS zoom_levels
 WHERE geometry && bbox;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/mountain_peak/layer.sql b/layers/mountain_peak/layer.sql
index a104fac..9a9385a 100644
--- a/layers/mountain_peak/layer.sql
+++ b/layers/mountain_peak/layer.sql
@@ -57,5 +57,6 @@ WHERE zoom_level >= 7
   AND (rank <= 5 OR zoom_level >= 14)
 ORDER BY "rank" ASC;
 
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
                 PARALLEL SAFE;
+-- TODO: Check if the above can be made STRICT -- i.e. if pixel_width could be NULL
diff --git a/layers/park/layer.sql b/layers/park/layer.sql
index bf2929a..e6547c8 100644
--- a/layers/park/layer.sql
+++ b/layers/park/layer.sql
@@ -336,5 +336,6 @@ FROM (
                     AND geometry_point && bbox
               ) AS park_point
      ) AS park_all;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
                 PARALLEL SAFE;
+-- TODO: Check if the above can be made STRICT -- i.e. if pixel_width could be NULL
diff --git a/layers/place/city.sql b/layers/place/city.sql
index 4cc0b84..9c44e4f 100644
--- a/layers/place/city.sql
+++ b/layers/place/city.sql
@@ -74,5 +74,6 @@ FROM (
             OR (zoom_level BETWEEN 11 AND 12 AND (gridrank <= 14 OR "rank" IS NOT NULL))
             OR (zoom_level >= 13)
      ) AS city_all;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/place/layer.sql b/layers/place/layer.sql
index 0dee27d..f4eae73 100644
--- a/layers/place/layer.sql
+++ b/layers/place/layer.sql
@@ -142,5 +142,6 @@ FROM (
          FROM layer_city(bbox, zoom_level, pixel_width)
          ORDER BY "rank" ASC
      ) AS place_all
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
                 PARALLEL SAFE;
+-- TODO: Check if the above can be made STRICT -- i.e. if pixel_width could be NULL
diff --git a/layers/poi/layer.sql b/layers/poi/layer.sql
index 6458aa4..a268bc6 100644
--- a/layers/poi/layer.sql
+++ b/layers/poi/layer.sql
@@ -94,5 +94,6 @@ FROM (
            AND zoom_level >= 14
      ) AS poi_union
 ORDER BY "rank"
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
                 PARALLEL SAFE;
+-- TODO: Check if the above can be made STRICT -- i.e. if pixel_width could be NULL
diff --git a/layers/transportation/layer.sql b/layers/transportation/layer.sql
index e008b33..31966b3 100644
--- a/layers/transportation/layer.sql
+++ b/layers/transportation/layer.sql
@@ -698,5 +698,6 @@ FROM (
      ) AS zoom_levels
 WHERE geometry && bbox
 ORDER BY z_order ASC;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/transportation_name/layer.sql b/layers/transportation_name/layer.sql
index 9b0ea35..1d148f4 100644
--- a/layers/transportation_name/layer.sql
+++ b/layers/transportation_name/layer.sql
@@ -147,5 +147,6 @@ FROM (
      ) AS zoom_levels
 WHERE geometry && bbox
 ORDER BY z_order ASC;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/water/water.sql b/layers/water/water.sql
index e59200b..f218dd9 100644
--- a/layers/water/water.sql
+++ b/layers/water/water.sql
@@ -385,5 +385,6 @@ FROM (
          WHERE zoom_level >= 14
      ) AS zoom_levels
 WHERE geometry && bbox;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/water_name/layer.sql b/layers/water_name/layer.sql
index 70a1d6b..2327a05 100644
--- a/layers/water_name/layer.sql
+++ b/layers/water_name/layer.sql
@@ -74,5 +74,6 @@ WHERE geometry && bbox
         OR (zoom_level >= "rank" AND "rank" IS NOT NULL)
         OR (zoom_level >= 8)
     );
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;
diff --git a/layers/waterway/waterway.sql b/layers/waterway/waterway.sql
index 57fb345..36e5ca2 100644
--- a/layers/waterway/waterway.sql
+++ b/layers/waterway/waterway.sql
@@ -220,5 +220,6 @@ FROM (
          WHERE zoom_level >= 14
      ) AS zoom_levels
 WHERE geometry && bbox;
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
+                -- STRICT
                 PARALLEL SAFE;