PostgreSQL Lag Function between of Two Tables - postgresql

Question

I am new to Postgresql/Python, so please bear with me!
Assuming we have two tables:
item table having a itemid, price, time.
user table having colums userid, itemid, timecreated, quantity, firstprice, lastprice, difference.
Table examples like :
item table:
itemid price time
RBK 92 1546408800
LBV 51 1546408800
ZBT 49 1546408800
GLS 22 1546408800
DBC 17 1546408800
RBK 91 1546495200
LBV 55 1546495200
ZBT 51 1546495200
GLS 24 1546495200
DBC 28 1546581600
RBK 108 1546581600
LBV 46 1546581600
ZBT 49 1546581600
GLS 21 1546581600
DBC 107 1546581600
In item table all those values comes up with api.
and user table:
userid itemid timecreated quantitty firstprice currentprice difference
1 RBK 1546408800 20
2 RBK 1546408800 15
3 RBK 1546408800 35
3 GLS 1546408800 101
3 DBC 1546495200 140
1 RBV 1546495200 141
2 RBK 1546495200 25
2 RBV 1546581600 31
User table is djangobased table which is user can register\add new items to follow prices.
My struggle access the item table to fetch first price which is having a same timestamp. In that example userid 1 RBK First price (1546408800) must be filling with 92
I did some trick with postgresql with (lag) But this does not seems to be working:
update user
set firstprice = tt.prev_price
from (select item.*,
lag(price) over (partition by itemid order by time) as prev_price
from item
) tt
where tt.id = item.id and
tt.prev_close is distinct from item.price;
I can call current price from the api but didnt find out the way to filling firstprice from the item table. I will be making for a trigger for this query. I searched on google and on stackoverflow but I couldn't find anything that could help me. Thanks in advance.

Answer 1

I can advice next approach (may be not fastest):
update "user" set firstprice = (
select price from "item" i
where i.itemid = "user".itemid and i.time >= "user".timecreated order by i.time limit 1
);
It calculate firstprice using sub-query. Test this SQL here