No, your customer does not want to "fetch the data from the database". This is the worst possible way to report on Jira data and you should be telling them that their solution is utterly wrong.
Jira's database is not a proper database. It uses a database as a data store and it is not built for anything other than being a data store.
Simple example - look at an issue. You get a load of data from fields which looks simple in itself. That was probably 40 joins in the database (it's a minimum of 17 last time I looked, and can easily be hundreds). Simple JQL queries "translated" into SQL can go from one intuitive clause to SQL that needs multiple joins.
Get your customer to define their reporting properly and see if you can do it in Jira or with an App. If you can't, then look at getting the reporting add-ons that understand the Jira database in full (i.e. EasyBI or Dataplane) or add-ons that connect directly to other reporting tools. If they can't do the reports, then you will need to find or write something that uses the REST interface.
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