OutOfMemory error when trying to display large tables

Question

We use JDeveloper 10.1.3. Our project uses ADF Faces + EJB3 Session Facade + TopLink.
We have a large table (over 100K rows) which we try to show to the user via an ADF Read-only Table. We build the page by dragging the facade findAllXXX method's result onto the page and choosing "ADF Read-only Table".
The problem is that during execution we get an OutOfMemory error. The Facade method attempts to extract the whole result set and to transfer it to a List. But the result set is simply too large. There's not enough memory.
Initially, I was under the impression that the table iterator would be running queries that automatically fetch just a chunk of the db table data at a time. Sadly, this is not the case. Apparently, all the data gets fetched. And then the iterator simply iterates through a List in memory. This is not what we needed.
So, I'd like to ask: is there a way for us to show a very large database table inside an ADF Table? And when the user clicks on "Next", to have the iterator automatically execute queries against the database and fetch the next chunk of data, if necessary?
If that is not possible with ADF components, it looks like we'll have to either write our own component or simply use the old code that we have which supports paging for huge tables by simply running new queries whenever necessary. Alternatively, each time the user clicks on "Next" or "Previous", we might have to intercept the event and manually send range information to a facade method which would then fetch the appropriate data from the database. I don't know how easy or difficult that would be to implement.
Naturally, I'd prefer to have that functionality available in ADF Faces. I hope there's a way to do this. But I'm still a novice and I would appreciate any advice.

Answer 1

Hi Shay,
We do use search pages and we do give the users the opportunity to specify search criteria.
The trouble comes when the search criteria are not specific enough and the result set is huge. Transferring the whole result set into memory will be disastrous, especially for servers used by hundreds of users simultaneously. So, we'll have to limit the number of rows fetched at a time. We should do this either by setting the Maximum Rows option for the TopLink query (or using rownum<=XXX inside the SQL), or through using a data provider that supports paging.
I don't like the first approach very much because I don't have a good recipe for calculating the optimum number of Maximum Rows for each query. By specifying some average number of, say, 500 rows, I risk fetching too many rows at once and I also risk filling the TopLink cache with objects that are not necessary. I can use methods like query.dontMaintainCache() but in my case this is a workaround, not a solution.
I would prefer fetching relatively small chunks of data at a time and not limiting the user to a certain number of maximum rows. Furthermore, this way I won't fetch large amounts of data at the very beginning and I won't be forced to turn off the caching for the query.
Regarding the "ADF Developer's Guide", I read there that "To create a table using a data control, you must bind to a method on the data control that returns a collection. JDeveloper allows you to do this declaratively by dragging and dropping a collection from the Data Control Palette."
So, it looks like I'll have to implement a collection which, in turn, implements the paging functionality that I need. Is the TopLink object you are referring to some type of collection? I know that I can specify a collection class that TopLink should use for queries through the query.useCollectionClass(...) method. But if TopLink doesn't provide the collection I need, I will have to write that collection myself. I still haven't found the section in the TopLink documentation that says what types of Collections are natively provided by TopLink. I can see other collections like oracle.toplink.indirection.IndirectList, for example. But I have not found a specific discussion on large result sets with the exception of Streams and Cursors and I feel uneasy about maintaining cursors between client requests.
And I completely agree with you about reading the docs first and doing the programming afterwards. Whenever time permits, I always do that. I have already read the "ADF Developer's Guide" with the exception of chapters 20 and 21. And I switched to the "TopLink Developer's Guide" because it seems that we must focus on the model. Unfortunately, because of the circumstances, I've spent a lot of time reading and not enough time practicing what I read. So, my knowledge is kind of shaky at the moment and perhaps I'm not seeing things that are obvious to you. That's why I tried using this forum -- to ask the experts for advice on the best method for implementing paging. And I'm thankful to everyone who replied to my post so far.