Paging, or pagination, is the process of splitting a dataset into pages, reading one page at a time. This is useful for optimizing bandwidth traffic, optimizing hardware usage, or just because no user can handle huge amounts of data at once anyway.
RavenDB makes it very easy to work with paging. In fact, with large data sets, it makes it mandatory (see: Safe By Default).
It is as simple as specifying a page size and passing a starting point. Using Linq from the Client API, it looks like this:
// Assuming a page size of 10, this is how will retrieve the 3rd page: var results = session.Query<BlogPost>() .Skip(20) // skip 2 pages worth of posts .Take(10) // Take posts in the page size .ToArray(); // execute the query
Finding the total results count when paging
While paging you sometimes need to know the exact number of results returned from the query. The Client API supports this explicitly:
RavenQueryStatistics stats; var results = session.Query<BlogPost>() .Statistics(out stats) .Where(x => x.Category == "RavenDB") .Take(10) .ToArray(); var totalResults = stats.TotalResults;
While the query will return with just 10 results,
totalResults will hold the total number of matching documents.
Paging through tampered results
For some queries, RavenDB will skip over some results internally, and by that invalidate the
TotalResults value. For example when executing a Distinct query,
TotalResults will contain the total count of matching documents found, but will not take into account results that were skipped as a result of the
SkippedResults is greater than 0 it implies that we skipped over some results in the index.
In order to do proper paging in those scenarios, you should use the
SkippedResults when telling RavenDB how many documents to skip. In other words, for each page the starting point should be
.Skip(currentPage * pageSize + SkippedResults).
For example, assuming a page size of 10:
RavenQueryStatistics stats; // get the first page var results = session.Query<BlogPost>() .Statistics(out stats) .Skip(0 * 10) // retrieve results for the first page .Take(10) // page size is 10 .Where(x => x.Category == "RavenDB") .Distinct() .ToArray(); var totalResults = stats.TotalResults; var skippedResults = stats.SkippedResults; // get the second page results = session.Query<BlogPost>() .Statistics(out stats) .Skip((1 * 10) + skippedResults) // retrieve results for the second page, taking into account skipped results .Take(10) // page size is 10 .Where(x => x.Category == "RavenDB") .Distinct() .ToArray(); // and so on...