Limit the maximum overview level to 23

Fixes #257

23 = 32 bits per integer - 1 sign bit - 8 bits px/tile

scripts-available/CDB_Overviews.sql

@@ -1,3 +1,11 @@
+CREATE OR REPLACE FUNCTION _CDB_MaxOverviewLevel()
+RETURNS INTEGER
+AS $$
+  BEGIN
+    RETURN 23;
+  END;
+$$ LANGUAGE PLPGSQL IMMUTABLE;
+
 -- Information about tables in a schema.
 -- If the schema name parameter is NULL, then tables from all schemas
 -- that may contain user tables are returned.
@@ -322,7 +330,7 @@ AS $$
           WHERE the_geom_webmercator && CDB_XYZ_Extent(x*2 + xx, y*2 + yy, t.z+1)
       )
       FROM t, base, (VALUES (0, 0), (0, 1), (1, 1), (1, 0)) AS c(xx, yy)
-      WHERE t.e > %2$s AND t.z < (base.z + %3$s)
+      WHERE t.e > %2$s AND t.z < (base.z + %3$s) AND t.z <= _CDB_MaxOverviewLevel()
     )
     SELECT MAX(e/ST_Area(CDB_XYZ_Extent(x,y,z))) FROM t where e > 0;
   ', reloid::text, min_features, nz, n, c, reloid::oid)
@@ -363,7 +371,7 @@ AS $$
     -- find minimum z so that fd*ta(z) <= lim
     -- compute a rough 'feature density' value
     SELECT CDB_XYZ_Resolution(-8) INTO c;
-    RETURN ceil(log(2.0, (c*c*fd/lim)::numeric)/2);
+    RETURN least(_CDB_MaxOverviewLevel(), ceil(log(2.0, (c*c*fd/lim)::numeric)/2));
   END;
 $$ LANGUAGE PLPGSQL STABLE;