how to merge on one column while keeping index?
Question:
In older version of pandas, I got this:

df1
         value
index
10        100
20        200
30        300

df2
         value
index
40        100
50        300
60        600

by merge on "value" and set "left_index=True" I got
df3
         value
index
10        100
30        300

but after update the library, it's illegal to do this, 
if I set left_index=True, there won't be any intersection, 
if I use "on" key word, I lost the original index
what is the solution?
