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How do indexes work on non-unique values ?

From: rhian <williams.rhian_at_freenet.de>
Date: 23 Jan 2003 12:42:44 -0800
Message-ID: <a0310e5c.0301231242.1a8c1512_at_posting.google.com> 





i have been told that creating an index on a field which can only
contain 2 possible values (i.e male/female) is a bad idea be because
order(n/2) is still order(n) and you get no benefit from doing a
binary search on the index file.



I think i understand how an index works and is beneficial when created
on a unique value, such as primary key. e.g



primary key values    page number

1                      1
101                    2
201                    3
301                    4





so if i were searching for 250 i would find the relevant page in
minimal number of goes, then the page would be load into memory and
scanned sequentially until the relevant record was found



I don't understand how this works for indexes on foreign keys, which i
understand to be beneficial or how it would work for my initial
example
Received on Thu Jan 23 2003 - 21:42:44 CET












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