[APEX 3] Requested source data of the report has been modified

Question

Hello APEX-Friends,
I have a common problem but the situation is a bit different here. Many of you might know the "invalid set of rows requested the source data of the report has been modified" problem. Often it occurs on submit. That means, you have a report, you select rows, you do things, you submit the page and everything blews up.
This is because you enter some values into fields the report depends on and so you modify your report parameters and the source data changes.
But:
In my case I have a dynamically created report that blews up before any submits occur or values change.
My query is a union of two selects. Both query different views. Those views use a date field as parameter and some compare functions.
I read the field with a V-Function i wrapped arround the apex V Function - declared as deterministic. My date compare function is also declared deterministic (I doubt this makes any differences as it might be only important for the optimizer, but as long as I don't know exactly what APEX evaluates, I go for sure).
I ensured, that the date field is set by default with the current date (and that works, because my interactive report initially displays correct data from the current date).
So everything is deterministic and the query must return same results on subsequent calls, but APEX still throws this "source data has changed" error and I am to 99.99% sure, that this cannot be true.
And now the awesome thing about this:
If I change the value of the date field, an javascript performs a submit. The page is reloaded (without resetting pagination!) and everything works fine. I can leave the page, reenter, do things - everything works well.
But if I log into the application and directly move to the corrupted report and try to use the pagination without editing fields or submitting the page the error occurs.
Do you have any Idea what's happing there? I could try to workaround this by submitting the page the first time it's entered to trigger this "mystery submit" that gets everything working. But I would like to understand this issue and have a clean solution.
Thanks in advance,
Mike aka UniversE

Answer 1

Okay, I found a solution, but I do not understand it - it might be a design flaw in APEX.
I mentioned the date field that is used in the query. I also mentioned that it is set with the current date by default. I did not mention how.
There are some possibilities in APEX to do so.
1. Default-Setting in the element properties
2. Static assignment if no value is in session cache
3. Computation before header
I did the first and second.
BUT:
An interactive report seems to work as follows. A query is executed to get all rows of the report. Then a second query is executed to get the rows that shall be displayed. And the order is screwed up, I think.
1. The first report query to get all rows
2. The elements are loaded and set to default values
3. The second report query to get the display rows
And that's the reason why nothing worked. Scince I added a computation before header the date field is set before the report queries are executed and everything works all fine now.
But I think it's a design flaw. Either both queries shall be executed before Regions or afterwards but not split as field values might change when elements are loaded.
Greetings,
UniversE