Drying get_sql methods

Some updates to Nullable documentation

docs/field_types.md

@@ -121,8 +121,7 @@ db.select('SELECT * FROM $db.event', model_class=Event)
 
 Working with nullable fields
 ----------------------------
-From [some time](https://github.com/yandex/ClickHouse/pull/70) ClickHouse provides a NULL value support.
-Also see some information [here](https://github.com/yandex/ClickHouse/blob/master/dbms/tests/queries/0_stateless/00395_nullable.sql).
+[ClickHouse provides a NULL value support](https://clickhouse.yandex/docs/en/data_types/nullable).
 
 Wrapping another field in a `NullableField` makes it possible to assign `None` to that field. For example:
 
@@ -148,6 +147,8 @@ to `None`.
 
 NOTE: `ArrayField` of `NullableField` is not supported. Also `EnumField` cannot be nullable.
 
+NOTE: Using `Nullable` almost always negatively affects performance, keep this in mind when designing your databases.
+
 Working with field compression codecs
 -------------------------------------
 Besides default data compression, defined in server settings, per-field specification is also available.

src/infi/clickhouse_orm/fields.py

@@ -76,15 +76,20 @@ class Field(object):
         '''
         sql = self.db_type
         if with_default_expression:
-            if self.alias:
-                sql += ' ALIAS %s' % self.alias
-            elif self.materialized:
-                sql += ' MATERIALIZED %s' % self.materialized
-            else:
-                default = self.to_db_string(self.default)
-                sql += ' DEFAULT %s' % default
-            if self.codec and db and db.has_codec_support:
-                sql+= ' CODEC(%s)' % self.codec
+            sql += self._extra_params(db)
+        return sql
+
+    def _extra_params(self, db):
+        sql = ''
+        if self.alias:
+            sql += ' ALIAS %s' % self.alias
+        elif self.materialized:
+            sql += ' MATERIALIZED %s' % self.materialized
+        elif self.default:
+            default = self.to_db_string(self.default)
+            sql += ' DEFAULT %s' % default
+        if self.codec and db and db.has_codec_support:
+            sql += ' CODEC(%s)' % self.codec
         return sql
 
     def isinstance(self, types):
@@ -511,15 +516,7 @@ class NullableField(Field):
     def get_sql(self, with_default_expression=True, db=None):
         sql = 'Nullable(%s)' % self.inner_field.get_sql(with_default_expression=False, db=db)
         if with_default_expression:
-            if self.alias:
-                sql += ' ALIAS %s' % self.alias
-            elif self.materialized:
-                sql += ' MATERIALIZED %s' % self.materialized
-            elif self.default:
-                default = self.to_db_string(self.default)
-                sql += ' DEFAULT %s' % default
-            if self.codec and db and db.has_codec_support:
-                sql+= ' CODEC(%s)' % self.codec
+            sql += self._extra_params(db)
         return sql
 
 
@@ -549,13 +546,5 @@ class LowCardinalityField(Field):
             sql = self.inner_field.get_sql(with_default_expression=False)
             logger.warning('LowCardinalityField not supported on clickhouse-server version < 19.0 using {} as fallback'.format(self.inner_field.__class__.__name__))
         if with_default_expression:
-            if self.alias:
-                sql += ' ALIAS %s' % self.alias
-            elif self.materialized:
-                sql += ' MATERIALIZED %s' % self.materialized
-            elif self.default:
-                default = self.to_db_string(self.default)
-                sql += ' DEFAULT %s' % default
-            if self.codec and db and db.has_codec_support:
-                sql+= ' CODEC(%s)' % self.codec
+            sql += self._extra_params(db)
         return sql