Phantom reads and data paging

Paging through the results is easy, right?

The client only needs to supply the number of rows to skip and the maximum number of rows it wants returned (aka the page number and the page size). The server then returns the data along with the information about the total number of results available. Et voila you have all the information you need. The number of rows to skip together with the page size give you the information about what page you’re showing and the page size with the total number of rows gives you the total number of pages available. Nothing too difficult or complex.

But there’s a catch. On the server, one needs to perform (at least) two queries – one query to get the data for the requested page and the second query to fetch the total number of rows. Now most of the databases set the default transaction isolation level to READ_COMMITTED and for very good reasons. But this transaction isolation level allows for phantom reads, i.e. 2 queries in the same transaction might “see” different number of rows of data, if another transaction committed and added or deleted rows that would be returned by the queries.
So, it may happen that you will:

  • return “rows 5 to 10 out of 2 total”,
  • say “there are no available results on the first page, while the total number of rows is 5”,
  • etc.

All that info acquired within one transaction.

What can you do about such situations? The obvious solution is to just admit that these things can happen 😉 Another option is to try and detect if such situation might have occured and re-try.

I’ve come up with the following rules for consistency of the results:

N is the actual number of elements on the page, P is the maximum number of elements on the page (i.e. the page size), I is the number of rows to skip and T is total number of results.

  • T < I && N == 0. This means we’re trying to show a page that is past the total number of results. We therefore expect the collection to be empty.
  • T - I > P && N == P. If we are are not showing the last page, the number of elements in the collection should be equal to the page size.
  • T - I <= P && N == T - I. If showing the last page, the collection should have all the remaining elements in it.

These are kind of obvious assumptions but phantom read can easily break them and therefore one should be checking them if one wants to be serious about returning meaningful results to the user.

So while paging is simple in principle, there are a couple of interesting corner cases that one needs to handle if one reads data out of a dynamic data set. It took us a good couple of years in RHQ to get to the bottom of this but hopefully now our paging is more robust than it was before.

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