Avoiding roundtrips in the database caused by looping - sql

Question

I am using postgres and, I recently encountered that the code I am using has too many roundtrips.
What I am doing is basically getting data from a table on a daily basis because I have to look for changes on a daily basis, but the whole function that does this job is called once a month.
An example of my table
Amount
Id | Itemid | Amount | Date
1 | 2 | 50 | 20-5-20
Now this table can be updated to add items at any point in time and I have to see the total amount that is SUM(Amount) every day.
But here's the catch, I have to add interest to the amount of each day at the rate of 5%.
So I can't just once call the function, I have to look at its value every day.
For example if I add an item of 50$ on the 1st of may then the interest on that day is 5/100*50
I add another item on the 5th of may worth 50$ and now the interest on the 5th day is 5/100*50.
But prior to 5th, the interest was on only 50$ so If I just simply use SUM(Amount)*5/100. It is wrong.
Also, another issue is the fact that dates are stored as timestamps and I need to group it by date of the timestamp because if I group it on the basis of timestamp then it will create multiple rows for the same date which I want to avoid while taking the sum.
So if there are two entries on the same date but different hours ideally the query should sum it up as one single date.
Example
Amount Table
Date | Amount
2020-5-5 20:8:8 100
2020-5-5 7:8:8 | 100
Result should be
Amount Table
Date | Amount
2020-5-5 200
My current code.
for i in numberofdaysinthemonth:
amount = amount + session.query(func.sum(Amount.Amount)).filter(Amount.date<current_date).scalar() * 5/100
I want a query that gets all these values according to dates, for example
date | Sum of amount till that date
20-5-20 | 50
20-6-20 | 100
Any ideas about what I should do to avoid a loop that runs 30 times since the function is called once in a month.

Answer 1

I am supposed to get all this data in a table daywise and aggregated as the sum of amount for each day
That is a simple "running total"
select "date",
sum(amount) over (order by "date") as amount_til_date
from the_table
order by "date";
If you need the amount per itemid
select "date",
sum(amount) over (partition by itemid order by "date") as amount_til_date
from the_table
order by "date";
If you also need to calculate the "compound interest rate" up to that day, you can do that as well:
select item_id,
"date",
sum(amount) over (partition by itemid order by "date") as amount_til_date,
sum(amount) over (partition by item_id order by "date") * power(1.05, count(*) over (partition by item_id order by "date")) as compound_interest
from the_table
order by "date";
To get that for a specific month, add a WHERE clause:
where "date" >= date '2020-06-01'
and "date" < date '2020-07-01'

Answer 2

In general to avoid round trips between application and database, application code must be moved from application to database in stored code (stored procedures an stored functions) using a procedural language. This approach is sometimes called "thick database" in commercial databases like Oracle Database.
PostgreSQL default procedural language is pl/pgsql but you can use Java, Perl, Python, Javascript using PostgreSQL extensions that you would need to install in PostgreSQL.