Re: it's not NULL, then what is it?

...
>
>
>   canon=# select count(maf) from gallo.sds_seq_reg_shw
>   canon-#  where maf ISNULL;
>    count
>   -------
>        0
>   (1 row)
>
I believe count will only count not-null anyway so this will always 
return zero. Try count(*) instead of count(maf). Here's an example:

steve@[local]=> select * from barr;
LOG:  duration: 0.226 msa | b
---+---a | bc | d  | e
(3 rows)

steve@[local]=> select coalesce(a, 'a is null'), coalesce(b, 'b is 
null') from barr;
LOG:  duration: 0.283 mscoalesce  | coalesce
-----------+----------a         | bc         | da is null | e
(3 rows)

steve@[local]=> select count(a) from barr;
LOG:  duration: 0.339 mscount
-------    2
(1 row)

steve@[local]=> select count(*) from barr where a isnull;
LOG:  duration: 0.350 mscount
-------    1
(1 row)