In a partition why 1st time encounter NULL then call minvfunc

[jian he]

same question:
https://stackoverflow.com/questions/73476732/postgresql-in-a-partition-1st-time-null-then-call-minvfunc
dbfilddle
<https://dbfiddle.uk/?rdbms=postgres_14&fiddle=307e1fa8d83af57aeb7698d9d58056cf>

source
<https://git.postgresql.org/gitweb/?p=postgresql.git;f=src/test/regress/expected/window.out;hb=5c292e6b90433c760a3e15027646c7b94afd0cdd#l3319>

create or replace function logging_msfunc_strict(text,anyelement)returns text as
$$select $1 || '+' || quote_nullable($2)
$$LANGUAGE sql strict IMMUTABLE;
create or replace function logging_minvfunc_strict(text,
anyelement)returns text as
$$select $1 || '-' || quote_nullable($2)
$$LANGUAGE sql strict IMMUTABLE;
create aggregate logging_agg_strict(text)
(
    stype = text,
    sfunc = logging_sfunc_strict,
    mstype =  text,
    msfunc = logging_msfunc_strict,
    minvfunc = logging_minvfunc_strict
);

create aggregate logging_agg_strict_initcond(anyelement)
(
    stype = text,
    sfunc = logging_sfunc_strict,
    mstype = text,
    msfunc = logging_msfunc_strict,
    minvfunc = logging_minvfunc_strict,
    initcond = 'I',
    minitcond = 'MI'
);

execute following query:

SELECT
    p::text || ',' || i::text || ':' || COALESCE(v::text, 'NULL') AS _row,
    logging_agg_strict (v) OVER w AS nstrict,
    logging_agg_strict_initcond (v) OVER w AS nstrictFROM (
    VALUES (1, 1, NULL),
        (1, 2, 'a'),
        (1, 3, 'b'),
        (1, 4, NULL),
        (1, 5, NULL),
        (1, 6, 'c'),
        (2, 1, NULL),
        (2, 2, 'x'),
        (3, 1, 'z')) AS t (p, i, v)
    WINDOW w AS (PARTITION BY p ORDER BY i ROWS BETWEEN 1 PRECEDING
AND CURRENT ROW);

return following result:

_row   |  nstrict  |    nstrict----------+-----------+----------------
 1,1:NULL | [[null]]  | MI
 1,2:a    | a         | MI+'a'
 1,3:b    | a+'b'     | MI+'a'+'b'
 1,4:NULL | a+'b'-'a' | MI+'a'+'b'-'a'
 1,5:NULL | [[null]]  | MI
 1,6:c    | c         | MI+'c'
 2,1:NULL | [[null]]  | MI
 2,2:x    | x         | MI+'x'
 3,1:z    | z         | MI+'z'
(9 rows)


For now I don't understand row 1,4:NULL | a+'b'-'a' | MI+'a'+'b'-'a'. I am
not sure why the 1st time you encounter NULL then it will call inverse
transition function Overall, not sure about the idea of inverse transition
function.