Mohamed Houri’s Oracle Notes

March 17, 2014

Indexing Foreign Key: the proof of the FK index use

Filed under: Deadlock — hourim @ 1:51 pm

If you have been looking for a material proof showing Oracle using the foreign key index created on the child table while deleting from a parent table then here it is

drop table t2;
drop table t1;

create table t1
(col1 number primary key);

create table t2
(col1    number primary key
,status  varchar2(12) not null
,col2    number
,col2v   number generated always as (case when status = 'ACTIVE' then col2 end) VIRTUAL
,constraint t2_fk foreign key (col2v) references t1(col1)
,constraint t2_ck check (status in ('ACTIVE','INACTIVE') and (status = 'INACTIVE' or col2 is not null))

create index t2_ind_fk on t2(col2v);

insert into t1
 select rownum
from dual
connect by level <=100;


insert into t2 (col1, status, col2) values (1, 'ACTIVE',50);

alter session set skip_unusable_indexes = false;

alter index t2_ind_fk unusable; -- implicit commit

I have created a pair of parent-child table (t1 and t2), an index on the foreign key on the t2 child table, set this index into an unusable state and changed the default skip_unusable_indexes parameter to false so that unusable indexes will not be skipped.

Now, I am going in the next PL/SQL anonymous block, to simulate a delete from a parent table using an autonomous transaction in order to mimic a different session (in fact a different transaction within the same session)

 pragma autonomous_transaction;
 delete from t1 –- deleting from the parent table
 where col1 = 99;

ERROR at line 1:
ORA-01502: index 'XXX.T2_IND_FK' or partition of such index is in unusable state
ORA-06512: at line 4

See how deleting from the parent table (t1) triggered an error on the index of the foreign key constraint created on the child table (t2). This is a simple way to show the mechanism used by Oracle in order to avoid a child table lock (before eventually a deadlock situation) simply by using the index on the foreign key.

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