Extra "count" row with total of 0
Why is there an additional row with an empty ga and a count of zero?


Because there are NULL values in the ga column.


  Why the count is zero?


While COUNT(*) can never give 0 as a result (as it counts rows), COUNT(column) or COUNT(expression) can give 0 because it counts not null values in the column/expression.

Thus the only logical explanantion is that the ga column has some nulls.


  How many?


You can find out by running:

SELECT ga, 
       count(ga) AS count_non_nulls,
       count(*)  AS count_all
FROM gb 
GROUP BY ga 
ORDER BY ga ;


which will give you something like:

ga | count_non_nulls | count_all
---+-----------------+-----------
?  |  200            |  200
A  |  100            |  100
B  |   50            |   50
   |    0            |   17  -- some number &gt;= 1 
(4 rows)