shopkeeper table has following fields:
id (bigint),amount (numeric(19,2)),createddate (timestamp)
Let’s say, I have the above table. I want to get the records for yesterday and generate a report by having the amount printed to cents.
One way of doing is to perform calculations in my java application and execute a simple query
Date previousDate ;// $1 calculate in application Date todayDate;// $2 calculate in application select amount where createddate between $1 and $2
and then loop through the records and convert amount to cents in my java application and generate the report
Another way is like performing calculations in sql query itself:
select cast(amount * 100 as int) as "Cents"
from shopkeeper where createddate between date_trunc('day', now()) - interval '1 day' and date_trunc('day', now())
and then loop through the records and generate the report
In one way , all my processing is done in java application and a simple query is fired. In other case all the conversions and calculations is done in Sql query.
The above use case is just an example, in a real scenario a table can have many columns that require processing of the similar kind.
Can you please tell me which approach is better in terms of performance and other aspects and why?
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Answer
It depends on a lot of factors – but most crucially:
- complexity of calculations (prefer doing complex crunching on an app-server, since that scales out; rather than a db server, which scales up)
- volume of data (if you need to access/aggregate a lot of data, doing it at the db server will save bandwidth, and disk io if the aggregates can be done inside indexes)
- convenience (sql is not the best language for complex work – especially not great for procedural work, but very good for set-based work; lousy error-handling, though)
As always, if you do bring the data back to the app-server, minimising the columns and rows will be to your advantage. Making sure the query is tuned and appropriately indexed will help either scenario.
Re your note:
and then loop through the records
Looping through records is almost always the wrong thing to do in sql – writing a set-based operation is preferred.
As a general rule, I prefer to keep the database’s job to a minimum “store this data, fetch this data” – however, there are always examples of scenarios where an elegant query at the server can save a lot of bandwidth.
Also consider: if this is computationally expensive, can it be cached somewhere?
If you want an accurate “which is better”; code it both ways and compare it (noting that a first draft of either is likely not 100% tuned). But factor in typical usage to that: if, in reality, it is being called 5 times (separately) at once, then simulate that: don’t compare just a single “1 of these vs 1 of those”.