Postgres 14

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CREATE TABLE tbl (a int, b text, c date); -- any data type

INSERT INTO tbl VALUES
  (1, null, null)              -- 2 nulls
, (null, null, null)           -- 3 nulls
, (7,'foo','2015-07-10')       -- 0 nulls
, (null, 'bar', '2015-07-10')  -- 1 nulls
;

CREATE OR REPLACE FUNCTION f_ct_nulls(_row anyelement)
  RETURNS int LANGUAGE sql IMMUTABLE AS
'SELECT (count(*) - count(v))::int FROM svals(hstore(_row)) v';
4 rows affected

SELECT *
    , (SELECT count(*) - count(v) FROM json_each_text(row_to_json(t)) x(k,v)) AS ct_nulls
FROM   tbl t;


a
b
c
ct_nulls


1
null
null
2

null
null
null
3

7
foo
2015-07-10
0

null
bar
2015-07-10
1


SELECT *, (SELECT count(*) - count(v) FROM svals(hstore(t)) v) AS ct_nulls
FROM   tbl t;


a
b
c
ct_nulls


1
null
null
2

null
null
null
3

7
foo
2015-07-10
0

null
bar
2015-07-10
1


SELECT *, f_ct_nulls(t) AS ct_nulls
FROM   tbl t;


a
b
c
ct_nulls


1
null
null
2

null
null
null
3

7
foo
2015-07-10
0

null
bar
2015-07-10
1

a	b	c	ct_nulls
1	null	null	2
null	null	null	3
7	foo	2015-07-10	0
null	bar	2015-07-10	1

a	b	c	ct_nulls
1	null	null	2
null	null	null	3
7	foo	2015-07-10	0
null	bar	2015-07-10	1

a	b	c	ct_nulls
1	null	null	2
null	null	null	3
7	foo	2015-07-10	0
null	bar	2015-07-10	1