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Batched Queries


Batched query support was optimized quite a bit in the Marten V4 release. It's now possible to work with the results of the earliest queries in the batch before the later queries are completely processed in a background thread to enable you to optimize your code that uses batch querying.

For the sake of performance, if you have a case where you may need to fetch several sets of document data from Marten at one time, you can opt to batch those queries into a single request to the underlying database to reduce network round trips.

The mechanism for doing this is the IBatchedQuery object that you can create with the IQuerySession.CreateBatchQuery() method. The batched queries in Marten work by allowing a user to define the queries they want to run through the batch and getting back a .Net Task that can be used to retrieve the actual results later after the batch is finished. When the batch is executed, Marten combines all the queries into a single command sent to the underlying Postgresql database, then reads through all the data returned and sets the results of the Task objects handed out earlier.

This functionality is demonstrated below:

// Start a new IBatchQuery from an active session
var batch = theSession.CreateBatchQuery();

// Fetch a single document by its Id
var user1 = batch.Load<User>("username");

// Fetch multiple documents by their id's
var admins = batch.LoadMany<User>().ById("user2", "user3");

// User-supplied sql
var toms = batch.Query<User>("where first_name == ?", "Tom");

// Where with Linq
var jills = batch.Query<User>().Where(x => x.FirstName == "Jill").ToList();

// Any() queries
var anyBills = batch.Query<User>().Any(x => x.FirstName == "Bill");

// Count() queries
var countJims = batch.Query<User>().Count(x => x.FirstName == "Jim");

// The Batch querying supports First/FirstOrDefault/Single/SingleOrDefault() selectors:
var firstInternal = batch.Query<User>().OrderBy(x => x.LastName).First(x => x.Internal);

// Kick off the batch query
await batch.Execute();

// All of the query mechanisms of the BatchQuery return
// Task's that are completed by the Execute() method above
var internalUser = await firstInternal;
Debug.WriteLine($"The first internal user is {internalUser.FirstName} {internalUser.LastName}");

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Combining Compiled Queries and Batch Queries

As of v0.8.10, Marten allows you to incorporate compiled queries as part of a batch query. The Marten team is hoping that this combination will make it easier to create performant applications where you may need to aggregate many documents in a single HTTP request or other operation.

Say you have a compiled query that finds the first user with a given first name:

public class FindByFirstName : ICompiledQuery<User, User>
    public string FirstName { get; set; }

    public Expression<Func<IMartenQueryable<User>, User>> QueryIs()
        return q => q.FirstOrDefault(x => x.FirstName == FirstName);

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To use that compiled query class in a batch query, you simply use the IBatchedQuery.Query(ICompiledQuery) syntax shown below:

var batch = theSession.CreateBatchQuery();

var justin = batch.Query(new FindByFirstName {FirstName = "Justin"});
var tamba = batch.Query(new FindByFirstName {FirstName = "Tamba"});

await batch.Execute();

(await justin).Id.ShouldBe(user1.Id);
(await tamba).Id.ShouldBe(user2.Id);

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Running Synchronously

As of v0.9.1, Marten also exposes the batch querying functionality with a synchronous option:

var batch = theSession.CreateBatchQuery();

var justin = batch.Query(new FindByFirstName { FirstName = "Justin" });
var tamba = batch.Query(new FindByFirstName { FirstName = "Tamba" });



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The mechanics of running synchronously are identical except for calling IBatchedQuery.ExecuteSynchronously().

Released under the MIT License.