var uploader = new NpgsqlBulkUploader(context);
var data = GetALotOfData();

// To create a lot of objects
uploader.Insert(data);

// To update a lot of objects
uploader.Update(data);

// We need DbContext first
var context = GetContext();                 

// Obtained a collection of 50k transactions
var transactions = GetListOfTransction();   

// Need to get prices from DB for all transactions, do it with 1 SQL call
var prices = context.Prices.BulkSelect(
    x => new { x.Ticker, x.TradedOn },                      // need to specify key for JOIN
    transactions.Select(x => new { x.Ticker, x.TradedOn })  // obtain key data from local objects
);

// Done, prices variable now contains List<Price> which matches the JOIN DB table to in-memory collection