MongoDB cursor contains retrieved documents before iterating over it? - mongodb

Question

When I executed find method and I have a cursor, the cursor contains retrieved documents before iterating over it?
I have this method:
public async Task<Post> GetPostByIdAsync(ObjectId id, CancellationToken cancellationToken)
{
using var cursor = await _Collection.FindAsync(post => post.Id == id, cancellationToken: cancellationToken);
return await cursor.SingleAsync(cancellationToken);
}
First, I'm calling FindAsync that returns a cursor, and then, I'm calling SingleAsync that returns the document. I have some questions about it.
In what point the query is executed? When I call FindAsync or when I call SingleAsync?
When I executed find method and I have a cursor, the cursor contains retrieved documents before iterating over it?
If I have a cursor with 100 documents, but I only iterate over first 20, the others 80 documents are queried and retrieved from server?
Why getting the cursor and iterating over it are the two operations async?
public async Task<Post> GetPostByIdAsync(ObjectId id, CancellationToken cancellationToken)
{
var query = _Collection.AsQueryable().Where(post => post.Id == id);
return await query.SingleAsync(cancellationToken);
}
If I call AsQueryable because I want to use LINQ, I have only one async method. There are any server operation running sync and blocking the thread?
I see that IMongoQueryable<T> is a subtype of IAsyncCursorSource<T>. What is the difference between Cursor and Cursor Source?

Answer 1

When a find is issued and the result set exceeds default (or specified) batch size, the response to the find includes:
The first batch of documents
Cursor id to retrieve the next batch
If you know you will only be processing a certain number of documents, use limit to restrict the result set to that many documents.