How to calculate efficient tables like this

How can we make statistics on such tables more efficient? The content in the table is as follows: idbid11382150313941395139613871388150 as long as I count the values in the bid field and do not repeat them. how can I calculate the efficiency? The statistical result is as follows: 138150139. how can a group table be more efficient?
The table content is as follows:

Id bid
1 138
2 150
3 139
4 139
5 139
6 138
7 138
8 150


I only need to count the values in the bid field. do not repeat them.

How can we make statistics more efficient?

The statistical results are as follows:
138
150
139

------ Solution --------------------
Group by bid;
------ Solution --------------------
Select distinct bid from table_name;
------ Solution --------------------
Select distinct bid from table_name;
------ Solution --------------------
Select distinct bid FROM t;