oracle merge query in postgres - postgresql

Question

I have this merge query in oracle and it was working fine. Now we are migrating to postgres 10 and trying to find equivalent for this in postgres.
MERGE INTO s.act_pack C USING((SELECT A.jid, A.pid, B.pcode,
B.mc, A.md, A.hd FROM s.act_pack A INNER JOIN s.act_pack B
ON A.pid = B.pid AND A.pcode = B.mc AND (A.hd <> B.hd
OR A.md<> B.md)) order by A.upd_ts desc) D ON(C.pid = D.pid AND
C.pcode = D.pcode AND C.jid = D.jid) WHEN MATCHED THEN UPDATE SET C.md =
D.md, C.hd= D.hd;
I see some forums on web says postgres doesnt support merge, and use INSERT ... ON CONFLICT
but with no background in postgres, I am not able to understand how this complex query can be written using that.
And some says postgres9.5 and above support merge statement. since we are using postgres 10 tried to use same oracle query in postgres but recieved ERROR: syntax error at or near "MERGE"
Any help is highly appreciated.

Answer 1

You don't need an "UPSERT" as you are not doing an INSERT, so a regular UPDATE is enough:
update act_pack C
SET C.md = D.md,
C.hd = D.h
from (
SELECT A.jid, A.pid, B.pcode, B.mc, A.md, A.hd
FROM s.act_pack A
INNER JOIN s.act_pack B
ON A.pid = B.pid
AND A.pcode = B.mc
AND (A.hd <> B.hd OR A.md<> B.md)
) d
where C.pid = D.pid
AND C.pcode = D.pcode
AND C.jid = D.jid
This is a direct "translation" of your code. But the fact that the same table is used three times is a bit strange. But without more information it's hard to know where exactly this could be made more efficient.