group by MultiIndex

[vkrot-exos]

Most likely I missed it in docs, but how can I groupby on index column? Following code, which is valid for pandas, does not work

import databricks.koalas as ks
df = ks.DataFrame({'a': [1, 2, 1], 'b': ['a', 'b', 'c'], 'c': ['a', 'b', 'c']})
df = df.set_index(['a', 'b'])
df.groupby(by='a').head(1)

[HyukjinKwon]

Seems like a bug ..