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Postgres FD Implementation
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$Header: /cvsroot/pgsql/doc/src/sgml/Attic/keys.sgml,v 1.7 2000/12/22 21:51:58 petere Exp $
Indices and Keys
-->
<chapter id="keys">
<docinfo>
<authorgroup>
<author>
<firstname>Herouth</firstname>
<surname>Maoz</surname>
</author>
</authorgroup>
<date>1998-03-02</date>
</docinfo>
<title>Indices and Keys</title>
<note>
<title>Author</title>
<para>
Written by Herouth Maoz
(<email>herouth@oumail.openu.ac.il</email>)
</para>
</note>
<note>
<title>Editor's Note</title>
<para>
This originally appeared on the mailing list
in response to the question:
"What is the difference between PRIMARY KEY and UNIQUE constraints?".
</para>
</note>
<programlisting>
Subject: Re: [QUESTIONS] PRIMARY KEY | UNIQUE
What's the difference between:
PRIMARY KEY(fields,...) and
UNIQUE (fields,...)
- Is this an alias?
- If PRIMARY KEY is already unique, then why
is there another kind of key named UNIQUE?
</programlisting>
<para>
A primary key is the field(s) used to identify a specific row. For example,
Social Security numbers identifying a person.
</para>
<para>
A simply UNIQUE combination of fields has nothing to do with identifying
the row. It's simply an integrity constraint. For example, I have
collections of links. Each collection is identified by a unique number,
which is the primary key. This key is used in relations.
</para>
<para>
However, my application requires that each collection will also have a
unique name. Why? So that a human being who wants to modify a collection
will be able to identify it. It's much harder to know, if you have two
collections named "Life Science", the the one tagged 24433 is the one you
need, and the one tagged 29882 is not.
</para>
<para>
So, the user selects the collection by its name. We therefore make sure,
withing the database, that names are unique. However, no other table in the
database relates to the collections table by the collection Name. That
would be very inefficient.
</para>
<para>
Moreover, despite being unique, the collection name does not actually
define the collection! For example, if somebody decided to change the name
of the collection from "Life Science" to "Biology", it will still be the
same collection, only with a different name. As long as the name is unique,
that's OK.
</para>
<para>
So:
<itemizedlist>
<listitem>
<para>
Primary key:
<itemizedlist>
<listitem>
<para>
Is used for identifying the row and relating to it.
</para>
</listitem>
<listitem>
<para>
Is impossible (or hard) to update.
</para>
</listitem>
<listitem>
<para>
Should not allow NULLs.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
<listitem>
<para>
Unique field(s):
<itemizedlist>
<listitem>
<para>
Are used as an alternative access to the row.
</para>
</listitem>
<listitem>
<para>
Are updateable, so long as they are kept unique.
</para>
</listitem>
<listitem>
<para>
NULLs are acceptable.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</itemizedlist>
</para>
<para>
As for why no non-unique keys are defined explicitly in standard
<acronym>SQL</acronym> syntax?
Well, you
must understand that indices are implementation-dependent. <acronym>SQL</acronym> does not
define the implementation, merely the relations between data in the
database. <productname>Postgres</productname> does allow non-unique indices, but indices
used to enforce <acronym>SQL</acronym> keys are always unique.
</para>
<para>
Thus, you may query a table by any combination of its columns, despite the
fact that you don't have an index on these columns. The indexes are merely
an implementational aid that each <acronym>RDBMS</acronym> offers you, in order to cause
commonly used queries to be done more efficiently. Some <acronym>RDBMS</acronym> may give you
additional measures, such as keeping a key stored in main memory. They will
have a special command, for example
<programlisting>
CREATE MEMSTORE ON &lt;table&gt; COLUMNS &lt;cols&gt;
</programlisting>
(this is not an existing command, just an example).
</para>
<para>
In fact, when you create a primary key or a unique combination of fields,
nowhere in the <acronym>SQL</acronym> specification does it say that an index is created, nor that
the retrieval of data by the key is going to be more efficient than a
sequential scan!
</para>
<para>
So, if you want to use a combination of fields that is not unique as a
secondary key, you really don't have to specify anything - just start
retrieving by that combination! However, if you want to make the retrieval
efficient, you'll have to resort to the means your <acronym>RDBMS</acronym> provider gives you
- be it an index, my imaginary MEMSTORE command, or an intelligent
<acronym>RDBMS</acronym>
that creates indices without your knowledge based on the fact that you have
sent it many queries based on a specific combination of keys... (It learns
from experience).
</para>
</chapter>
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