Support EXPLAIN for DDL

[morgo]

Feature Request

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

Describe the feature you'd like:

TiDB's DDL has two major algorithms, which (using MySQL terminology) I will call INSTANT and INPLACE. Currently, it is impossible to sight-check a DDL query and tell which one will be used.

While not supported in MySQL, the TiDB EXPLAIN format already differs from MySQL. I think it would be a useful extension to permit EXPLAIN for DDL. The information I am looking for is:

• Algorithm (INSTANT or INPLACE)
• LOCK info (it's always NONE in TiDB for now, but perhaps it should be included for context since it may change in future)
• Rows Affected (an estimate of rows in the table)

Describe alternatives you've considered:

This has some relation to #8428

Teachability, Documentation, Adoption, Migration Strategy:

Output is relatively simple, but would need to be documented.

[winkyao]

@morgo Thanks. This feature will be developed in the TiDB 3.0.