"You should almost always index foreign keys. The only exception is when the matching unique or primary key is never updated or deleted." When a foreign key is unindexed, DML on the parent primary key results in a share row exclusive table lock (or share-subexclusive table lock, SSX) on the child table, preventing DML from other transactions against the child table. If the DML affects several rows in the parent table, the lock on the child table is obtained and released immediately for each row in turn. Despite the speed of the lock-release process, this can cause significant amounts of contention on the child table during periods of heavy update/delete activity on the parent table. When a foreign key is indexed, DML on the parent primary key results in a row share table lock (or subshare table lock, SS) on the child table. This type of lock prevents other transactions from issuing whole table locks on the child table, but does not block DML on either the parent or the child table. Only the rows relating to the parent primary key are locked in the child table.
REM List foreign keys with no matching index on child table - causes locks set linesize 150; col owner for a20; col COLUMN_NAME for a20; SELECT c.owner, c.constraint_name, c.table_name, cc.column_name, c.status FROM dba_constraints c, dba_cons_columns cc WHERE c.constraint_type = 'R' AND c.owner NOT IN ('SYS', 'SYSTEM', 'SYSMAN', 'EXFSYS', 'WMSYS', 'OLAPSYS', 'OUTLN', 'DBSNMP', 'ORDSYS', 'ORDPLUGINS', 'MDSYS', 'CTXSYS', 'AURORA$ORB$UNAUTHENTICATED', 'XDB', 'FLOWS_030000', 'FLOWS_FILES') AND c.owner = cc.owner AND c.constraint_name = cc.constraint_name AND NOT EXISTS (SELECT 'x' FROM dba_ind_columns ic WHERE cc.owner = ic.table_owner AND cc.table_name = ic.table_name AND cc.column_name = ic.column_name AND cc.position = ic.column_position AND NOT EXISTS (SELECT owner, index_name FROM dba_indexes i WHERE i.table_owner = c.owner AND i.index_Name = ic.index_name AND i.owner = ic.index_owner AND (i.status = 'UNUSABLE' OR i.partitioned = 'YES' AND EXISTS (SELECT 'x' FROM dba_ind_partitions ip WHERE status = 'UNUSABLE' AND ip. index_owner = i. owner AND ip. index_Name = i. index_name UNION ALL SELECT 'x' FROM dba_ind_subpartitions isp WHERE status = 'UNUSABLE' AND isp. index_owner = i. owner AND isp. index_Name = i. index_name)))) ORDER BY 1, 2 /