Describe in SQL*Plus - does it call a package or run SQL? (Or something else?) - oracle

Question

When I describe an Oracle TYPE in SQL*Plus, what does SQL*Plus do "under the bonnet" - Is it calling DBMS_METADATA (or similar)? Or is it querying the data dictionary directly?
Or something else entirely?

Answer 1

You can obviously use alter session set sql_trace=true to answer such questions.
For
create type desc_type as object (
dummy varchar2(1),
constructor function desc_type (a number) return self as result,
final member procedure m
);
/
alter session set sql_trace=true;
desc desc_type
alter session set sql_trace=false;
drop type desc_type
/
I find the following relevant SQL Statements in my trace file:
select procedure#,procedurename,properties,itypeobj#
from procedureinfo$ where obj#=:1
order by procedurename desc, overload# desc
select position#,
sequence#,
level#,
argument,
type#,
charsetid,
charsetform,
properties,
nvl(length, 0),
nvl(precision#, 0),
nvl(scale, 0),
nvl(radix, 0),
type_owner,
type_name,
type_subname,
type_linkname,
pls_type
from argument$
where obj#=:1 and procedure#=:2 order by sequence# desc
select max(procedure#) from procedurec$ where obj#=:1
select max(procedure#) from procedureplsql$ where obj#=:1
select max(procedure#) from procedurejava$ where obj#=:1
select procedure#,entrypoint# from procedurec$
where obj#=:1 order by procedure#
select procedure#,entrypoint#
from procedureplsql$ where obj#=:1 order by procedure#
select procedure#,ownerlength,classlength,methodlength,siglength, flagslength,cookiesize
from procedurejava$ where obj#=:1 order by procedure#
select ownername,classname,methodname,signature,flags
from procedurejava$ where obj#=:1 and procedure#=:2 order by procedure#
select count(*)
from vtable$ where obj#=:1
select vindex,itypetoid,imethod#,iflags,itypeowner,itypename
from vtable$ where obj#=:1 order by vindex
select externtype, externname
from type$ where tvoid=:1
So, under the bonnet, SQL*Plus uses plain SQL.