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- An index column need not be just a column of the underlying table,
- but can be a function or scalar expression computed from one or
- more columns of the table. This feature is useful to obtain fast
- access to tables based on the results of computations.
- </p><p>
- For example, a common way to do case-insensitive comparisons is to
- use the <code class="function">lower</code> function:
- </p><pre class="programlisting">
- SELECT * FROM test1 WHERE lower(col1) = 'value';
- </pre><p>
- This query can use an index if one has been
- defined on the result of the <code class="literal">lower(col1)</code>
- function:
- </p><pre class="programlisting">
- CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));
- </pre><p>
- </p><p>
- If we were to declare this index <code class="literal">UNIQUE</code>, it would prevent
- creation of rows whose <code class="literal">col1</code> values differ only in case,
- as well as rows whose <code class="literal">col1</code> values are actually identical.
- Thus, indexes on expressions can be used to enforce constraints that
- are not definable as simple unique constraints.
- </p><p>
- As another example, if one often does queries like:
- </p><pre class="programlisting">
- SELECT * FROM people WHERE (first_name || ' ' || last_name) = 'John Smith';
- </pre><p>
- then it might be worth creating an index like this:
- </p><pre class="programlisting">
- CREATE INDEX people_names ON people ((first_name || ' ' || last_name));
- </pre><p>
- </p><p>
- The syntax of the <code class="command">CREATE INDEX</code> command normally requires
- writing parentheses around index expressions, as shown in the second
- example. The parentheses can be omitted when the expression is just
- a function call, as in the first example.
- </p><p>
- Index expressions are relatively expensive to maintain, because the
- derived expression(s) must be computed for each row upon insertion
- and whenever it is updated. However, the index expressions are
- <span class="emphasis"><em>not</em></span> recomputed during an indexed search, since they are
- already stored in the index. In both examples above, the system
- sees the query as just <code class="literal">WHERE indexedcolumn = 'constant'</code>
- and so the speed of the search is equivalent to any other simple index
- query. Thus, indexes on expressions are useful when retrieval speed
- is more important than insertion and update speed.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="indexes-unique.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexes.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="indexes-partial.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">11.6. Unique Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 11.8. Partial Indexes</td></tr></table></div></body></html>
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