Compute aggregation cell size accurately

This fixes #1001

lib/cartodb/models/aggregation/aggregation-query.js

@@ -237,14 +237,15 @@ const havingClause = ctx => {
 };
 
 // SQL expression to compute the aggregation resolution (grid cell size).
-// This is equivalent to `${256/ctx.res}*CDB_XYZ_Resolution(CDB_ZoomFromScale(!scale_denominator!))`
 // This is defined by the ctx.res parameter, which is the number of grid cells per tile linear dimension
 // (i.e. each tile is divided into ctx.res*ctx.res cells).
 // We limit the the minimum resolution to avoid division by zero problems. The limit used is
 // the pixel size of zoom level 30 (i.e. 1/2*(30+8) of the full earth web-mercator extent), which is about 0.15 mm.
+// Computing this using !scale_denominator!, !pixel_width! or !pixel_height! produces
+// inaccurate results due to rounding present in those values.
 const gridResolution = ctx => {
     const minimumResolution = 2*Math.PI*6378137/Math.pow(2,38);
-    return `GREATEST(${256*0.00028/ctx.res}*!scale_denominator!, ${minimumResolution})::double precision`;
+    return `GREATEST(${256/ctx.res}*CDB_XYZ_Resolution(CDB_ZoomFromScale(!scale_denominator!)), ${minimumResolution})::double precision`;
 };
 
 // Notes: