According to JMeter Test Elements execution order listeners are being executed after post processors and your prev.setIgnore() instruction prevents passing the sample result to the listeners.
One of possible options is writing the response data into a file in the JSR223 Post Processor itself, the relevant code would be something like:
Are you asking about integration/e2e tests or unit tests? For unit tests, the precision returned shouldn't matter, since you're providing those values directly to the test.
If you're talking about integration or e2e tests, ideally you'd run against all your supported DBs (maybe in ephemeral environments against containerized persistence layers), then ...
should I write down test cases that coverage all data types like int,
float, boolean etc. or just those visible on this screen?
It depends on the risks and components you want to investigate.
If you are testing the UI in isolation, most probably you need to deal with what is visible (or more precisely, what can be visible).
However, if you are investigating ...
I would start from analyzing the DDL statement of creating a table in your particular rdbms. For example below is the syntax for plsql:
So using this grammar and digging into the grammar components you can learn what are the mandatory data for creating a table and what is optional.
Then you start applying standard test design practices like equivalence ...
Given you're talking about "millions" it makes sense to use a load testing tool which can do multiple actions in parallel. If you don't have a better candidate yet you can consider Apache JMeter which provides:
CSV Data Set Config - so you can read "expected" data from file(s) in your local file system
JDBC Request sampler - to execute ...
In my experience, querying the DB to assert content is correct in the front-end is rarely done.
The main idea behind the E2E automation tests is to ensure the functionality of the web elements are working and present. Things like:
Links are clickable and navigate to the correct page
Forms can be filled out
User sign up, user login, user modification (...
I don't think querying the DB for verification is necessary. Since your app FE is connected to API, you just need to query through API to verify things. The data verification should be done through testing of the APIs themselves.