Correctly Migrate Postgres least() Behavior to BigQuery - sql

Question

I'm attempting to migrate postgres scripts over to bigquery, with the end goal of both scripts returning the exact same tables (schema and values).
I'm running into an issue when trying to replicate the behavior of least() in postgres in my bigquery selects.
In postgres, if any parameters of the least() call are null, they are skipped and the least non-null value is returned. In bigquery, however, if any of the parameters of the least() call are null, the function automatically returns null.
I'm looking for an elegant solution to replicate the postgres least() behavior in bigquery. My current—clunky—solution is below:
Postgres (returns -1):
SELECT LEAST(1, 0, -1, null)
BigQuery (returns null):
SELECT LEAST(1, 0, -1, null)
Postgres (returns -1):
SELECT LEAST(COALESCE(1, 0, -1, null),
COALESCE(0, 1, -1, null),
COALESCE(-1, 0, 1, null),
COALESCE(null, 0, -1, 1))
BigQuery (returns -1):
SELECT LEAST(COALESCE(1, 0, -1, null),
COALESCE(0, 1, -1, null),
COALESCE(-1, 0, 1, null),
COALESCE(null, 0, -1, 1))
This works but is a less-than-ideal solution.
In the original postgres script I need to migrate, there is nested logic like least(w, x, least(y, z)) so that fix gets exponentially more unreadable as the number of values/complexity grows. That same issue applies when you try to do this as a massive CASE block.
If anyone has an obvious fix that I'm missing or a more elegant way to mirror the postgres behavior in bigquery, it is much appreciated!

Answer 1

There is a simple workaround for BigQuery Standard SQL
You just create your own function (let's say myLeast)
It works for "standalone" as well as in nested scenario
#standardSQL
CREATE TEMP FUNCTION myLeast(x ARRAY<INT64>) AS
((SELECT MIN(y) FROM UNNEST(x) AS y));
SELECT
LEAST(1, 0, -1, NULL) AS least_standard,
LEAST(COALESCE(1, 0, -1, NULL),
COALESCE(0, 1, -1, NULL),
COALESCE(-1, 0, 1, NULL),
COALESCE(NULL, 0, -1, 1)) AS least_less_than_ideal,
myLeast([1, 0, -1, NULL]) AS least_workaround,
myLeast([1, 0, -1, NULL, myLeast([2, 0, -2, NULL])]) AS least_with_nested
Output is
least_standard least_less_than_ideal least_workaround least_with_nested
null -1 -1 -2
first two is from your question - third and forth are "standalone" and nested workaround
Hope you can apply this approach to your specific case

Answer 2

Both Oracle and Vertica behave the same as BigQuery, following general rule of SQL functions - if one of the arguments is NULL - the result is NULL. PostgreSQL makes an exception to that rule, explicitly stating in documentation:
The result will be NULL only if all the expressions evaluate to NULL.
Note that GREATEST and LEAST are not in the SQL standard, but are a
common extension. Some other databases make them return NULL if any
argument is NULL, rather than only when all are NULL.
I would open Feature Request in BigQuery issue tracker to add IGNORE NULLS parameter to LEAST and GREATEST to get PostgreSQL compatible behavior. Even though normally IGNORE NULLS only applies to aggregate functions, LEAST and GREATEST are kind of similar to aggregate functions.

Answer 3

Without a function:
select
(select min(col) from unnest([a,b,c,d,e]) col) least,
(select max(col) from unnest([a,b,c,d,e]) col) greatest,
*
from
(
select 1 a, 2 b, 3 c, null d, 5 e
union all
select null a, null b, null c, null d, null e
) tbl

Answer 4

Maybe something like this could work?
WITH tbl AS(
SELECT 1 AS a, 2 AS b
UNION ALL SELECT NULL, 2
UNION ALL SELECT 1, NULL
UNION ALL SELECT NULL, NULL
)
SELECT
tbl.*
, COALESCE( LEAST(a, b), a , b)
FROM tbl

Answer 5

How about this? :) "The Postgres library" :)
DECLARE input STRING DEFAULT (
WITH t AS
(
SELECT 1 a, 0 b, -1 c, null d
UNION ALL
SELECT 0, 1, -1, null
UNION ALL
SELECT -1, 0, 1, null
UNION ALL
SELECT null, 0, -1, 1
)
SELECT '['||STRING_AGG("'"||TO_JSON_STRING(t)||"'")||']' FROM t
)
;
EXECUTE IMMEDIATE '''
SELECT * FROM EXTERNAL_QUERY("project.location.connection",
\'\'\'
SELECT
GREATEST (
(t::json->>'a')::INT,
(t::json->>'b')::INT,
(t::json->>'c')::INT,
(t::json->>'d')::INT
)
FROM
UNNEST (ARRAY '''||input||") AS t ''')"
Of course it may improve with dynamic "body" for GREATEST but I hope nobody will use it.
It's too sad to live in the world with no IGNORE NULLS for GREATEST and LEAST...as well as impossible to direct string variable into EXTERNAL_QUERY 😔 😔 😔
Questions to community
Does anyone know about limitations for this "method"?
How long string is allowed to execute in EXECUTE IMMEDIATE?
as argument for EXTERNAL_QUERY?