SUM, AVERAGE, COUNT, STDEV across columns?

Please consider the following example in pure Vertica SQL.

CREATE TABLE my_table (userid int, a1 int, a2 int, a3 int, a4 int, a5 int, a6 int) order by userid;

COPY my_table FROM STDIN DELIMITER ',' NULL AS 'NULL' ABORT ON ERROR;
1,1,2,3,4,5,6,
2,7,8,9,10,11,NULL,
3,12,13,14,15,NULL,16,
\.

SELECt * FROM my_table ORDER BY 1;
--  userid | a1 | a2 | a3 | a4 | a5 | a6
-- --------+----+----+----+----+----+----
--       1 |  1 |  2 |  3 |  4 |  5 |  6
--       2 |  7 |  8 |  9 | 10 | 11 |
--       3 | 12 | 13 | 14 | 15 |    | 16
-- (3 rows)

SELECT
       userid,
       APPLY_COUNT(ARRAY[a1,a2,a3,a4,a5,a6]) AS COUNT,
       APPLY_AVG(ARRAY[a1,a2,a3,a4,a5,a6]) AS AVG
FROM my_table
ORDER BY 1;
--  userid | COUNT | AVG
-- --------+-------+-----
--       1 |     6 | 3.5
--       2 |     5 |   9
--       3 |     5 |  14
-- (3 rows)

SELECT EXPLODE(userid,ARRAY[a1,a2,a3,a4,a5,a6]) OVER(PARTITION BEST)
FROM my_table
ORDER BY 1;
--  userid | position | value
-- --------+----------+-------
--       1 |        0 |     1
--       1 |        1 |     2
--       1 |        2 |     3
--       1 |        3 |     4
--       1 |        4 |     5
--       1 |        5 |     6
--       2 |        0 |     7
--       2 |        1 |     8
--       2 |        2 |     9
--       2 |        3 |    10
--       2 |        4 |    11
--       2 |        5 |
--       3 |        0 |    12
--       3 |        1 |    13
--       3 |        2 |    14
--       3 |        3 |    15
--       3 |        4 |
--       3 |        5 |    16
-- (18 rows)

WITH
list1 AS (SELECT EXPLODE(userid,ARRAY[a1,a2,a3,a4,a5,a6]) OVER(PARTITION BEST) FROM my_table)
SELECT userid, SUM(value), COUNT(value), AVG(value), VARIANCE(value), STDDEV(value)
FROM list1
GROUP BY userid
ORDER BY 1;
--  userid | SUM | COUNT | AVG | VARIANCE |      STDDEV
-- --------+-----+-------+-----+----------+------------------
--       1 |  21 |     6 | 3.5 |      3.5 | 1.87082869338697
--       2 |  45 |     5 |   9 |      2.5 | 1.58113883008419
--       3 |  70 |     5 |  14 |      2.5 | 1.58113883008419
-- (3 rows)

This is perfect. Thank you!

Aha, the Collection functions are exactly the concept I was seeking. Thank you!
These work perfectly for this purpose.

Do you know if there is an APPLY_VARIANCE or APPLY_STDDEV or equivalent?

Q - Do you know if there is an APPLY_VARIANCE or APPLY_STDDEV or equivalent?
A - No. The existing Collection Functions are shown here
https://www.vertica.com/docs/12.0.x/HTML/Content/Authoring/SQLReferenceManual/Functions/Array/ArrayFunctions.htm

For VARIANCE and STDDEV you can use the last query example or create a temporary table or create a stored procedure.
You can also open a support case, explain the business need and suggest it as a NFR.

@mosheg The function to iterate over arrays via EXPLODE(x) OVER (PARTITION BY ..) is amazing