Postgres handle constraint conflict then insert excluded row - postgresql

Question

If I have a table something like:
MyTable
id date value replaced_by_id
1 2020-01-01 10
2 2020-01-02 20 3
3 2020-01-02 21
With a unique constraint on date and replaced_by
e.g.
CREATE UNIQUE INDEX my_table_null_test ON my_table (date, (replaced_by_id is null))
WHERE replaced_by_id IS NULL;
How can I create an insert statement that sets the conflicting row's replaced_by_id to the the new rows id and then inserts the new row afterwards?
Along the lines of
insert into my_table (id, date, value) values (gen_id(), '2020-01-02', 21)
on conflict (date, (replaced_by_id is null) ) where replaced_by_id is null
do update
set replaced_by_id = excluded.id
**now insert the new row (insert the excluded row)**
for say the file the values are coming from a file that had many values for the same date. e.g.
date value
2020-01-01 10
2020-01-02 20
2020-01-02 21
2020-01-02 22
2020-01-02 23
2020-01-02 24
2020-01-02 22
would result in
MyTable
id date value replaced_by_id
1 2020-01-01 10
2 2020-01-02 20 3
3 2020-01-02 21 4
4 2020-01-02 22 5
5 2020-01-02 23 6
6 2020-01-02 24 7
7 2020-01-02 22

Answer 1

I think you can not use UPSERT for this requirement because UPSERT will perform only update operation on same table or DO Nothing.
Reason is:
You have added unique index which will prevent the insertion with combination of existing date and null of replace_by_id field.
You are trying to get the ID to update the existing records before insertion which is not possible.
So you have to use some workaround for it:
Using Triggers - You have to use 2 triggers on your table one is before insert and one is after insert like below:
Before Insert Trigger
Trigger Function
CREATE FUNCTION trig_beforeinsert()
RETURNS trigger
LANGUAGE 'plpgsql'
AS $BODY$
declare
flg int :=0;
begin
flg=(select count(*) from my_table where date_=new.date_ and replaced_by_id is null);
if flg>0 then
new.replaced_by_id=0;
end if;
return new;
end;
$BODY$;
Trigger on Table
CREATE TRIGGER my_table_before_insert
BEFORE INSERT
ON my_table
FOR EACH ROW
EXECUTE PROCEDURE trig_beforeinsert();
After Insert Trigger
Trigger Function
CREATE FUNCTION trig_afterinsert()
RETURNS trigger
LANGUAGE 'plpgsql'
COST 100
VOLATILE NOT LEAKPROOF
AS $BODY$
declare
flg int :=0;
begin
if new.replaced_by_id = 0 then
UPDATE my_table set replaced_by_id=new.id where date_=new.date_ and replaced_by_id is null;
UPDATE my_table set replaced_by_id=null where id=new.id;
end if;
return new;
end;
$BODY$;
Trigger on Table
CREATE TRIGGER my_table_after_insert
AFTER INSERT
ON public.my_table
FOR EACH ROW
EXECUTE PROCEDURE public.trig_afterinsert();
DEMO