Avoid duplicate storage of info in serialized columns.

As is, info is store on any primary data column, but also in
the serialized column, which makes the header unnecessarily long.

astropy/table/serialize.py

@@ -134,7 +134,8 @@ def _represent_mixin_as_column(col, name, new_cols, mixin_cols,
         # MaskedColumn).  For primary data, we attempt to store any info on
         # the format, etc., on the column, but not for ancillary data (e.g.,
         # no sense to use a float format for a mask).
-        if data_attr == col.info._represent_as_dict_primary_data:
+        is_primary = data_attr == col.info._represent_as_dict_primary_data
+        if is_primary:
             new_name = name
             new_info = info
         else:
@@ -146,6 +147,10 @@ def _represent_mixin_as_column(col, name, new_cols, mixin_cols,
                                        and np.any(data.mask)) else Column
             new_cols.append(col_cls(data, name=new_name, **new_info))
             obj_attrs[data_attr] = SerializedColumn({'name': new_name})
+            if is_primary:
+                # Don't store info in the __serialized_columns__ dict for this column
+                # since this is redundant with info stored on the new column.
+                info = {}
         else:
             # recurse. This will define obj_attrs[new_name].
             _represent_mixin_as_column(data, new_name, new_cols, obj_attrs)

astropy/table/tests/test_mixin.py

@@ -817,6 +817,20 @@ def test_represent_mixins_as_columns_unit_fix():
     serialize.represent_mixins_as_columns(t)
 
 
+def test_primary_data_column_gets_description():
+    """
+    If the mixin defines a primary data column, that should get the
+    description, format, etc., so no __info__ should be needed.
+    """
+    t = QTable({'a': [1, 2] * u.m})
+    t['a'].info.description = 'parrot'
+    t['a'].info.format = '7.2f'
+    tser = serialize.represent_mixins_as_columns(t)
+    assert '__info__' not in tser.meta['__serialized_columns__']['a']
+    assert tser['a'].format == '7.2f'
+    assert tser['a'].description == 'parrot'
+
+
 def test_skycoord_with_velocity():
     # Regression test for gh-6447
     sc = SkyCoord([1], [2], unit='deg', galcen_v_sun=None)

docs/changes/table/12607.bugfix.rst

@@ -0,0 +1,2 @@
+Avoid duplicate storage of info in serialized columns if the column
+used to serialize already can hold that information.