Limit the maximum zoom level

Avoid returning zoom levels greater than the maximum
'safe' level. For a zero denominator (which would imply
and infinite zoom level) return the maximum level too.

scripts-available/CDB_ZoomFromScale.sql

@@ -1,3 +1,17 @@
+-- Maximum supported zoom level
+CREATE OR REPLACE FUNCTION _CDB_MaxSupportedZoom()
+RETURNS int
+LANGUAGE SQL
+IMMUTABLE
+AS $$
+  -- The maximum zoom level has to be limited for various reasons,
+  -- e.g. zoom levels greater than 31 would require tile coordinates
+  -- that would not fit in an INTEGER (which is signed, 32 bits long).
+  -- We'll choose 20 as a limit which is safe also when the JavaScript shift
+  -- operator (<<) is used for computing powers of two.
+  SELECT 29;
+$$;
+
 CREATE OR REPLACE FUNCTION cartodb.CDB_ZoomFromScale(scaleDenominator numeric)
 RETURNS int
 LANGUAGE SQL
@@ -5,8 +19,9 @@ IMMUTABLE
 AS $$
 SELECT
   CASE
-  -- Don't bother if the scale is larger than ~zoom level 0
+    -- Don't bother if the scale is larger than ~zoom level 0
     WHEN scaleDenominator > 600000000 OR scaleDenominator = 0 THEN NULL
-    ELSE CAST (ROUND(LOG(2, 559082264.028/scaleDenominator)) AS INTEGER)
+    WHEN scaleDenominator = 0 THEN _CDB_MaxSupportedZoom()
+    ELSE CAST (LEAST(ROUND(LOG(2, 559082264.028/scaleDenominator)), _CDB_MaxSupportedZoom()) AS INTEGER)
   END;
 $$;