SqlDataReader 和 SQL Server 2016 FOR JSON 将 json 分成 2k 字节的块

Question

最近我试用了 Azure SQL 数据库的新 for json auto 功能。

当我 select 很多记录时，例如这个查询：

Select
    Wiki.WikiId
    , Wiki.WikiText
    , Wiki.Title
    , Wiki.CreatedOn
    , Tags.TagId
    , Tags.TagText
    , Tags.CreatedOn
From
    Wiki
Left Join
    (WikiTag
Inner Join 
    Tag as Tags on WikiTag.TagId = Tags.TagId) on Wiki.WikiId = WikiTag.WikiId
For Json Auto

然后使用 C# SqlDataReader:

执行 select

var connectionString = ""; // connection string
var sql = "";  // query from above
var chunks = new List<string>();

using (var connection = new SqlConnection(connectionString)) 
using (var command = connection.CreateCommand()) {
    command.CommandText = sql;
    connection.Open();

    var reader = command.ExecuteReader();

    while (reader.Read()) {
            chunks.Add(reader.GetString(0)); // Reads in chunks of ~2K Bytes
    }
}

var json = string.Concat(chunks);

我得到了很多数据块。

为什么我们有这个限制？为什么我们不把所有东西都集中在一起？

当我阅读 nvarchar(max) 专栏时，我会把所有内容都集中在一起。

感谢您的解释

Answer 1

来自Format Query Results as JSON with FOR JSON：

Output of the FOR JSON clause

The result set contains a single column.

A small result set may contain a single row.

A large result set splits the long JSON string across multiple rows. By default, SQL Server Management Studio (SSMS) concatenates the results into a single row when the output setting is Results to Grid. The SSMS status bar displays the actual row count.

Other client applications may require code to recombine lengthy results into a single, valid JSON string by concatenating the contents of multiple rows. For an example of this code in a C# application, see Use FOR JSON output in a C# client app.

我会说这完全是出于性能原因，类似于 XML。更多 and What does server side FOR XML return?

In SQL Server 2000 the server side XML publishing - FOR XML (see http://msdn2.microsoft.com/en-us/library/ms178107(SQL.90).aspx) - was implemented in the layer of code between the query processor and the data transport layer. Without FOR XML a SELECT query is executed by the query processor and the resulting rowset is sent to the client side by the server side TDS code. When a SELECT statement contains FOR XML the query processor produces the result the same way as without FOR XML and then FOR XML code formats the rowset as XML. For maximum XML publishing performance FOR XML does steaming XML formatting of the resulting rowset and directly sends its output to the server side TDS code in small chunks without buffering whole XML in the server space. The chunk size is 2033 UCS-2 characters. Thus, XML larger than 2033 UCS-2 characters is sent to the client side in multiple rows each containing a chunk of the XML. SQL Server uses a predefined column name for this rowset with one column of type NTEXT - “XML_F52E2B61-18A1-11d1-B105-00805F49916B” – to indicate chunked XML rowset in UTF-16 encoding. This requires special handling of the XML chunk rowset by the APIs to expose it as a single XML instance on the client side. In ADO.Net, one needs to use ExecuteXmlReader, and in ADO/OLEDB one should use the ICommandStream interface.

Answer 2

作为 SQL 代码中的解决方法（即，如果您不想更改查询代码以将块放在一起），我发现将查询包装在 CTE 中，然后选择给我预期的结果：

--Note that I query from information_schema to just get a lot of data to replicate the problem.

--doing this query results in multiple rows (chunks) returned
SELECT * FROM information_schema.columns FOR JSON PATH, include_null_values

--doing this query results in a single row returned
;WITH SomeCTE(JsonDataColumn) AS
(
    SELECT * FROM information_schema.columns FOR JSON PATH, INCLUDE_NULL_VALUES
) 
SELECT JsonDataColumn FROM SomeCTE

第一个查询为我重现了问题（returns 多行，每一行都是总数据的一部分），第二个查询给出了包含所有数据的一行。 SSMS 无法重现问题，您必须使用其他客户端代码进行尝试。

SqlDataReader 和 SQL Server 2016 FOR JSON 将 json 分成 2k 字节的块

SqlDataReader and SQL Server 2016 FOR JSON splits json in chunks of 2k bytes

c#

tsql

sql-server

sql-server-2016

azure-sql-database