PostgreSQL lag() : should not sorted

[565ee]

--------------- tb
   num txt
0    3   c
1    1   a
2    2   b
3    1   a
4    3   c
5    1   a
------------------------------------------------------------
>>> tb.num.lag().execute()    Result is error : should not sorted
0    NaN
1    1.0
2    1.0
3    1.0
4    2.0
5    3.0
Name: tmp, dtype: float64
------------------------------------------------------------
>>> tb.num.lead().execute()    Result is error : should not sorted
0    1.0
1    1.0
2    2.0
3    3.0
4    3.0
5    NaN
Name: tmp, dtype: float64
------------------------------------------------------------
Backend=PostgreSQL_10    ibis=0.14    Python=3.6  Ubuntu=18.04    Anaconda=5.1