Don't use overviews for date histograms

lib/cartodb/models/dataview/overviews/histogram.js

@@ -8,7 +8,6 @@ dot.templateSettings.strip = false;
 var columnTypeQueryTpl = dot.template(
     'SELECT pg_typeof({{=it.column}})::oid FROM ({{=it.query}}) _cdb_histogram_column_type limit 1'
 );
-var columnCastTpl = dot.template("date_part('epoch', {{=it.column}})");
 
 var BIN_MIN_NUMBER = 6;
 var BIN_MAX_NUMBER = 48;
@@ -149,7 +148,9 @@ Histogram.prototype.sql = function(psql, override, callback) {
     }
 
     if (this._columnType === 'date') {
-        _column = columnCastTpl({column: _column});
+        // overviews currently aggregate dates to NULL
+        // to avoid problem we don't use overviews for histograms of date columns
+        return this.defaultSql(psql, override, callback);
     }
 
     var _query = this.rewrittenQuery(this.query);