I don't think we should use MySQL as a model. They are not very good at reading the SQL standard before they implement features.

And in fact recent versions of the standard have "DESCRIBE query". I don't want to clash with that.

I propose that we keep EXPLAIN as is, but add DESCRIBE SCHEMA name, DESCRIBE TABLE name, DESCRIBE query.

1. describe table statement usually doesn't use TABLE keyword:

DESCRIBE table_name:

2. What syntax is expected from DESCRIBE query?

Read the SQL standard at http://jtc1sc32.org/doc/N1951-2000/32N1964T-text_for_ballot-FCD_9075-2.pdf. Without the TABLE keyword we'd clash with DESCRIBE <SQL statement name>.

Maybe TABLE could be optional, and without it, we'd look for a table only if the named statement did not exist. (And it wouldn't because at present we don't support named statements.)

To confirm, you suggest the following syntax:
1. DESCRIBE SCHEMA | DATABASE name
2. DESCRIBE TABLE table_name [column_name | expression]
3. DESCRIBE sql_statement (select * from t)
4. DESCRIBE table_name [column_name | expression] (only if 3rd point wasn't met)
Correct?

Yes, and remember that name and table_name can be composite. E.g. DESCRIBE SCHEMA myDb.mySchema, DESCRIBE sales.emps, DESCRIBE myDb.mySchema.emps, DESCRIBE select * from t. And all names can be quoted the same way as identifiers in a query (double-quote by default).

Review comments:

1. I see that DESCRIBE DATABASE currently does the same as DESCRIBE SCHEMA. That's OK, I suppose, but misleading.
2. Since JDBC's DatabaseMetaData has database, catalog, schema, we should also have a DESCRIBE CATALOG command.
3. I don't understand why you want to allow a character literal for a column, e.g. DESCRIBE emp 'deptno'. An identifier is sufficient, e.g. DESCRIBE emp "deptno". Remove the former.
4. Need to allow the optional TABLE keyword, e.g. DESCRIBE TABLE emp as an alternative to DESCRIBE emp.
5. Need to add DESCRIBE statement and DESCRIBE STATEMENT statement.
6. There is no test for compound identifier
7. We should not allow compound column name

I have implemented 2, 3, 4, 5 (partially), 6, 7 in my branch https://github.com/julianhyde/calcite/tree/1168-describe. Arina Ielchiieva, Please review and if OK I will check in.

I logged CALCITE-1221 to complete 1 and 5.

Julian, I have closed my PR as it was outdated. I have implemented it before we agreed with you on final scope. Sorry for delay in implementing it.
Anyway, I have reviewed your changes and they look good.

Fixed in http://git-wip-us.apache.org/repos/asf/calcite/commit/a065200a with fix up in http://git-wip-us.apache.org/repos/asf/calcite/commit/da875a67.

Resolved in release 1.8.0 (2016-06-13).