Making STATS_AGG return NULL if no values present?

[jdmwood]

So let's say I'm doing something like:

SELECT
x,
STATS_AGG(a),
STATS_AGG(b)
from some_table 
group by x

One minor annoyance with STATS_AGG is that it returns a non null, but empty StatsSummary1D like (version:1,n:0,sx:0,sx2:0,sx3:0,sx4:0) even if all results for that column are null.

If you have a query with a large number of columns which are often sparse (i.e. NULL), then this is a large amount of wasted storage (for things like Timescale continuous aggregates).

I can work around it like this:

SELECT
x,
STATS_AGG(a) FILTER (WHERE a is NOT NULL),
STATS_AGG(b) FILTER (WHERE b is NOT NULL),
from some_table 
group by x

but

a) I'm not totally sure this is mathematically correct. Is STATS_AGG with no matched rows = STATS_AGG with many values which are NULL? I do know that, for example, AVERAGE('(version:1,n:0,sx:0,sx2:0,sx3:0,sx4:0)'::StatsSummary1D) is NULL and so is AVERAGE(NULL::StatsSummary1D).
b) If it is OK, is there any reason to not have STATS_AGG store NULL in this case too?
c) If it's not desirable to change STATS_AGG is there any problem with the FILTER WHERE approach above?

[jdmwood]

I'm pretty sure this is at least safe to do.

To show some workings:

-- Base example
WITH vals(x) AS (
  VALUES (NULL::float8), (NULL::float8)
)
SELECT 
  stats_agg(x) from vals;

--> (version:1,n:0,sx:0,sx2:0,sx3:0,sx4:0)

-- With the filter
WITH vals(x) AS (
  VALUES (NULL::float8), (NULL::float8)
)
SELECT 
  stats_agg(x) FILTER (WHERE x IS NOT NULL) from vals;
--> NULL

-- Showing that STATS_AGG handles NULL and "empty" equivalently
SELECT AVERAGE('(version:1,n:0,sx:0,sx2:0,sx3:0,sx4:0)'::StatsSummary1D) 
--> NULL

SELECT AVERAGE(NULL::StatsSummary1D)
--> NULL

-- Showing that a mixture of values and NULLs doesn't break anything
WITH vals(x) AS (
  VALUES (42), (NULL::float8)
)
SELECT 
  stats_agg(x) from vals;

--> (version:1,n:1,sx:42,sx2:0,sx3:0,sx4:0)

WITH vals(x) AS (
  VALUES (42), (NULL::float8)
)
SELECT 
  stats_agg(x) FILTER (WHERE x IS NOT NULL) from vals;

--> (version:1,n:1,sx:42,sx2:0,sx3:0,sx4:0)