What happens when declaring an additional index on the primary key in Oracle SQL? -


i have multi-part question:

my journey began when trying figure out if can have descending/ascending primary keys in oracle sql.

there doesn't seem syntax along lines of create table mytab (id int primary key desc); or create table mytab (id int, constraint pk primary key(id desc));

later learned oracle implicitly creates index primary key enforce constraints. figured create index it, , make index descending.

create index myind on mytab (id desc);

this worked, when did query see indices existed got this: 

> select index_name user_indexes table_name = 'mytab';  index_name ------------------------------ sys_c0011939 myind

i see implicitly created index there own index... tried see if rid of implicitly created index creating index primary key during table create: 

 create table mytab (id int primary key using index (create index myind on mytab (id asc)));

this worked: 

> select index_name user_indexes table_name = 'mytab';  index_name ------------------------------ myind

interestingly, using desc index direction gives me following error:

specified index cannot used enforce constraint.

so questions are:

  1. does mean descending primary keys not possible in oracle sql?
  2. if so, why desc disallow primary key constraints being enforced?
  3. what behavior/effect of creating index separately in first example? desc worked there... happens when primary key has 2 indices on it? function same way db allows desc part of primary key specification?

oracle creates indexes double-linked lists, every index can traversed in ascending or descending order when needed.

for multi-column indexes, however, can add desc keyword have 1 or more columns stored in descending order respect other columns.

this has limited usefulness , not used - can useful when creating index support particular order by clause in particular query (but not all) columns sorted in descending order.


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