Working with Microsoft SQL Server databases

This section provides basic information about working with an external Microsoft SQL Server databases.

To work with an external Microsoft SQL Server database, you need to follow these steps:

1. Create a secret containing the password for connecting to the database.

   CREATE OBJECT ms_sql_server_datasource_user_password (TYPE SECRET) WITH (value = "<password>");
   

2. Create an external data source that describes a specific Microsoft SQL Server database. The LOCATION parameter contains the network address of the Microsoft SQL Server instance to connect to. The DATABASE_NAME specifies the database name (for example, master). The LOGIN and PASSWORD_SECRET_NAME parameters are used for authentication to the external database. You can enable encryption for connections to the external database using the USE_TLS="TRUE" parameter.

   CREATE EXTERNAL DATA SOURCE ms_sql_server_datasource WITH (
       SOURCE_TYPE="Microsoft SQL Server",
       LOCATION="<host>:<port>",
       DATABASE_NAME="<database>",
       AUTH_METHOD="BASIC",
       LOGIN="user",
       PASSWORD_SECRET_NAME="ms_sql_server_datasource_user_password",
       USE_TLS="TRUE"
   );
   

3. Deploy the connector and configure the YDB dynamic nodes to interact with it. Additionally, ensure network access from the YDB dynamic nodes to the external data source (at the address specified in the LOCATION parameter of the CREATE EXTERNAL DATA SOURCE request). If network connection encryption to the external source was enabled in the previous step, the connector will use the system's root certificates (more details on TLS configuration can be found in the guide on deploying the connector).

4. Execute a query to the database.

Query syntax

The following SQL query format is used to work with Microsoft SQL Server:

SELECT * FROM ms_sql_server_datasource.<table_name>

where:

• ms_sql_server_datasource - the external data source identifier;
• <table_name> - the table name within the external data source.

Limitations

When working with Microsoft SQL Server clusters, there are a number of limitations:

1. External sources are available only for reading data through SELECT queries. The federated query processing engine currently does not support queries that modify tables in external sources.

2. If the date value stored in the external data source is outside the allowed range for YDB (all dates used must be later than 1970-01-01 but earlier than 2105-12-31), such a value in YDB will be converted to NULL.

3. The YDB federated query processing system is capable of delegating the execution of certain parts of a query to the system acting as the data source. Query fragments are passed through YDB directly to the external system and processed within it. This optimization, known as "predicate pushdown", significantly reduces the volume of data transferred from the source to the federated query processing engine. This reduces network load and saves computational resources for YDB.

   A specific case of predicate pushdown, where filtering expressions specified after the WHERE keyword are passed down, is called "filter pushdown". Filter pushdown is possible when using:

   Description	Example
   Filters like IS NULL/IS NOT NULL	WHERE column1 IS NULL or WHERE column1 IS NOT NULL
   Logical conditions OR, NOT, AND	WHERE column IS NULL OR column2 is NOT NULL
   Comparison conditions =, <>, <, <=, >, >= with other columns or constants	WHERE column3 > column4 OR column5 <= 10

   Supported data types for filter pushdown:

   YDB Data Type
   Bool
   Int8
   Int16
   Int32
   Int64
   Float
   Double

Supported data types

In the Microsoft SQL Server database, the optionality of column values (whether the column can contain NULL values or not) is not a part of the data type system. The NOT NULL constraint for any column of any table is stored within the IS_NULLABLE column the INFORMATION_SCHEMA.COLUMNS system table, i.e., at the table metadata level. Therefore, all basic Microsoft SQL Server types can contain NULL values by default, and in the YDB type system, they should be mapped to optional.

Below is a correspondence table between Microsoft SQL Server types and YDB types. All other data types, except those listed, are not supported.