I have a table A and a table B. A has a foreign key to B on B's primary key, B_ID.
For some reason (I know there are legitimate reasons) it is not using an index when I join these two tables on the key.
Do I need to separately create an index on A.B_ID or should the existence of a foreign key provide that?
Creating a foreign key does not automatically create an index on A.B_ID. So it would generally make sense from a query performance perspective to create a separate index on A.B_ID.
If you ever delete rows in B, you definitely want A.B_ID to be indexed. Otherwise, Oracle will have to do a full table scan on A every time you delete a row from B to make sure that there are no orphaned records (depending on the Oracle version, there may be additional locking implications as well, but those are diminished in more recent Oracle versions).
Just for more info: Oracle doesn't create an index automatically (as it does for unique constraints) because (a) it is not required to enforce the constraint, and (b) in some cases you don't need one.
Most of the time, however, you will want to create an index (in fact, in Oracle Apex there's a report of "unindexed foreign keys").
Whenever the application needs to be able to delete a row in the parent table, or update the PK value (which is rarer), the DML will suffer if no index exists, because it will have to lock the entire child table.
A case where I usually choose not to add an index is where the FK is to a "static data" table that defines the domain of a column (e.g. a table of status codes), where updates and deletes on the parent table are never done directly by the application. However, if adding an index on the column gives benefits to important queries in the application, then the index will still be a good idea.
SQL Server has never put indexes onto foreign key columns automatically - check out Kim Tripp's excellent blog post on the background and history of this urban myth.
It's usually a good idea to index your foreign key columns, however - so yes, I would recommend making sure each FK column is backed up by an index; not necessarily on that one column alone - maybe it can make sense to create an index on two or three columns with the FK column as the first one in there. Depends on your scenario and your data.
For performance reasons an index should be created. Is used in delete operations on primary table (to check that the record you are deleting is not used) and in joins that usually a foreign key is involved. Only few tables (I do not create them in logs) could be that do not need the index but probably, in this cases probably you don't need the foreign key constraint as well.
BUT
There are some databases that already automatically create indexes on foreign Keys. Jet Engine (Microsoft Access Files) Firebird MySQL
FOR SURE
SQL Server Oracle
DOES NOT
As with anything relating to performance, it depends on many factors and there is no silve bullet e.g. in a very high activilty environment the maintainance of an index may be unacceptable.
Most salient here would seem to be selectivity: if the values in the index would be highly duplicated then it may give better performance to drop the index (if possible) and allow a table scan.
UNIQUE, PRIMARY KEY, and FOREIGN KEY constraints generate indexes that enforce or "back" the constraint (and are sometimes called backing indexes). PRIMARY KEY constraints generate unique indexes. FOREIGN KEY constraints generate non-unique indexes. UNIQUE constraints generate unique indexes if all the columns are non-nullable, and they generate non-unique indexes if one or more columns are nullable. Therefore, if a column or set of columns has a UNIQUE, PRIMARY KEY, or FOREIGN KEY constraint on it, you do not need to create an index on those columns for performance.
