Allow to use arbitrary queries (e.g. DESCRIBE) in a subquery

[ramazanpolat]

Use case
DESCRIBE output is something like this:

DESCRIBE TABLE ABC

┌─name────────┬─type──┬─default_type─┬─default_expression─┬─comment─┬─codec_expression─┬─ttl_expression─┐
│ ID          │ Int32 │              │                    │         │                  │                │
│ BORYIL      │ Int32 │              │                    │         │                  │                │
│ BORDONEM    │ Int32 │              │                    │         │                  │                │
│ BORYILDONEM │ Int32 │              │                    │         │                  │                │
└─────────────┴───────┴──────────────┴────────────────────┴─────────┴──────────────────┴────────────────┘

Unfortunately we cannot use this result in SQL since the output is not a table, althought it looks exactly like a table.

Therefore something like SELECT name FROM (DESCRIBE TABLE ABC) where type='Int32' is not possible.

Proposed solution
Make a table function called describe that can return DESCRIBE output as table. So users will be able to use this functions to get information about the a table and use it in SQL statements.

Somthing like these will be possible:

SELECT name FROM describe(ABC) where type='Int32

SELECT count() FROM describe(jdbc('datasource://abc', 'schema', 'table'))

[filimonov]

SELECT name FROM (DESCRIBE TABLE ABC) where type='Int32'

didn't check that, but most probably allowing that will be simpler/more logical (comparing to introducing table function which is able to accept another tables / table functions).

[alexey-milovidov]

We should allow to select from anything returning a table like DESCRIBE, SHOW TABLES, ...

[amosbird]

An SQL table funtion then, though the result schema needs to be inferred.