Where does this index come from? Looking for some index in one schema I get to create, I found one index with prefix sys_ such as below:

SQL&gt; select index_name from dba_indexes where owner = 'DANILO';

INDEX_NAME
------------------------------
INDEX_SALES_ID
SYS_C0010892


Indeed, I didn't create this index, Why do I have it in my schema?
That is an index that was automatically generated for a constraint. Example:

SQL&gt; select index_name from user_indexes;

no rows selected

SQL&gt; create table t1(c1 number primary key);

Table created.

SQL&gt; select index_name from user_indexes;

INDEX_NAME
--------------------------------------------------------------------------------
SYS_C006380