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Oracle Tips by Burleson |
Indexing Foreign Keys
We talked about foreign keys and how they
enforced a parent child relationship. One problem with foreign keys
is that when you UPDATE/INSERT to the child column, you must scan
the parent column to validate the key. If the parent is not a
primary key, this can result in constantly reading the entire table
to validate the child value. Placing an index on the parent column
will allow the foreign key to validate the child with the index,
avoiding the costly full table scan.
When you start designing your database, you
need to develop a naming scheme. This will allow you to determine
the function of objects in the database. Tables are pretty easy,
but indexes are normally left out. You should name each object so
that you know what it does. The emp_job_fdx index could be the
index on the emp_job_fk constraint. A primary key index could be
name_pk, a unique index name_udx.
Sometimes, you can find that the database
over indexes, causing performance problems on INSERTs and UPDATEs.
You must determine which indexes you can remove to improve
performance. If you unknowingly delete an index on a foreign key,
the performance impact may be drastic. By using a logical naming
convention, you will know by the object name why you placed it into
the database in the first place.
The above book excerpt is from:
Easy Oracle
SQL
Get Started
Fast writing SQL Reports with SQL*Plus
ISBN 0-9727513-7-8
John Garmany
http://www.rampant-books.com/book_2005_1_easy_sql.htm |
