Support SQL/MED (Management of External Data) Extension

[zz-jason]

Feature Request

Is your feature request related to a problem? Please describe:

Not a problem, to make TiDB able to manage external data. From SQL/MED - Wikipedia:

The SQL/MED ("Management of External Data") extension to the SQL standard is defined by ISO/IEC 9075-9:2008 (originally defined for SQL:2003). SQL/MED provides extensions to SQL that define foreign-data wrappers and datalink types to allow SQL to manage external data. External data is data that is accessible to, but not managed by, an SQL-based DBMS. This standard can be used in the development of federated database systems.

This report can be useful: SQL/MED - A Status Report

Describe the feature you'd like:

The user interface can be discussed later.

Describe alternatives you've considered:

• PostgreSQL: SQL/MED - PostgreSQL and Forign Data Wrapper
• MariaDB has support for SQL/MED with the CONNECT storage engine. The implementation uses different syntax than the official standard.
• MySQL has supported The FEDERATED Storage Engine.

Teachability, Documentation, Adoption, Migration Strategy:

• N/A

[zz-jason]

SQL/MED specifications can be divided into:

• the wrapper interface: offers facilities to view external data managed by one or more external sources (formally known as foreign servers) simply as a set of SQL tables (formally known as foreign tables). External data may be stored in file systems, in HTML- formatted web pages, in XML documents, or in some other specialized repositories.

• the datalinks: offers facilities to let an SQL-server control the management of referential integrity, recovery, and authorization of data residing in one or more file systems.

SQL/MED - A Status Report describes the enhancements to the wrapper interface and datalinks.