Effecient searching of index of 2D array for rows present in another 2d array.

Question

I have two 2D arrays and I am looking to find the most efficient way to find the index in one array of rows present in the other array.

With the help of the following answer I came up with the following code that works but I fell that It can be done more efficiently (I am planning to use it on an array of size 4 million):

import numpy as np

a = np.array([[ 2, 3, 4],
            [ 6, 7, 8],
            [ 7, 10, 9],
            [ 3, 2, 1]])

b = np.array([[ 2, 3, 4],
            [ 7, 10, 9]])

index = []

for elem in b:
    np.where((elem == a).all(axis=1))
    index.append(np.where((elem == a).all(axis=1))[0][0])