Time grain support for unix-timestamp columns (#1093)

* Add time grain support for time columnd in unix timestamp

* Fix datetime parsing for unix epoch

Since we've already converted unix epoch to datetime type,
we shouldn't specify 'unit' parameter in pandas.to_datetime

* Fix SQLite timestamp to datetime conversion

caravel/models.py

@@ -591,6 +591,22 @@ def grains(self):
     def grains_dict(self):
         return {grain.name: grain for grain in self.grains()}
 
+    def epoch_to_dttm(self, ms=False):
+        """Database-specific SQL to convert unix timestamp to datetime
+        """
+        ts2date_exprs = {
+            'sqlite': "datetime({col}, 'unixepoch')",
+            'postgresql': "(timestamp 'epoch' + {col} * interval '1 second')",
+            'mysql': "from_unixtime({col})",
+            'mssql': "dateadd(S, {col}, '1970-01-01')"
+        }
+        ts2date_exprs['redshift'] = ts2date_exprs['postgresql']
+        ts2date_exprs['vertica'] = ts2date_exprs['postgresql']
+        for db_type, expr in ts2date_exprs.items():
+            if self.sqlalchemy_uri.startswith(db_type):
+                return expr.replace('{col}', '({col}/1000.0)') if ms else expr
+        raise Exception(_("Unable to convert unix epoch to datetime"))
+
     def get_extra(self):
         extra = {}
         if self.extra:
@@ -795,6 +811,12 @@ def query(  # sqla
             # Transforming time grain into an expression based on configuration
             time_grain_sqla = extras.get('time_grain_sqla')
             if time_grain_sqla:
+                if dttm_col.python_date_format == 'epoch_s':
+                    dttm_expr = self.database.epoch_to_dttm().format(
+                        col=dttm_expr)
+                elif dttm_col.python_date_format == 'epoch_ms':
+                    dttm_expr = self.database.epoch_to_dttm(ms=True).format(
+                        col=dttm_expr)
                 udf = self.database.grains_dict().get(time_grain_sqla, '{col}')
                 timestamp_grain = literal_column(
                     udf.function.format(col=dttm_expr)).label('timestamp')

caravel/viz.py

@@ -172,12 +172,9 @@ def get_df(self, query_obj=None):
             raise Exception("No data, review your incantations!")
         else:
             if 'timestamp' in df.columns:
-                if timestamp_format == "epoch_s":
+                if timestamp_format in ("epoch_s", "epoch_ms"):
                     df.timestamp = pd.to_datetime(
-                        df.timestamp, utc=False, unit="s")
-                elif timestamp_format == "epoch_ms":
-                    df.timestamp = pd.to_datetime(
-                        df.timestamp, utc=False, unit="ms")
+                        df.timestamp, utc=False)
                 else:
                     df.timestamp = pd.to_datetime(
                         df.timestamp, utc=False, format=timestamp_format)