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<h1 class="topictitle1">Think about join order</h1>


<div>
<div class="p">For some queries, join order can make the difference between a table scan
(expensive) and an index scan (cheap). Here's an example:  
<pre><strong>select ht.hotel_id, ha.stay_date, ht.depart_time
from hotels ht, Hotelavailability ha
where ht.hotel_id = ha.hotel_id and 
ht.room_number = ha.room_number
and ht.bed_type = 'KING'
and ht.smoking_room = 'NO'
order by ha.stay_date</strong></pre>
</div>

<p>If <span>Derby</span> chooses <em>Hotels</em> as the outer table,
it can use the index on <em>Hotels</em> to retrieve qualifying
rows. Then it
need only look up data in <em>HotelAvailability</em> three
times, once for each qualifying row. And to retrieve the appropriate rows
from <em>HotelAvailability</em>, it can use an index for <em>HotelAvailability</em>'s <em>hotel_id</em> column
instead of scanning the entire table.</p>

<p>If <span>Derby</span> chooses the other order, with <em>HotelAvailability</em> as the outer table, it will have to probe the <em>Hotels</em> table for <em>every row</em>, not just three rows, because
there are no qualifications on the <em>HotelAvailability</em> table.</p>

<p>For more information about join order, see <a href="ctunoptimz12168.html#ctunoptimz12168">Joins and performance</a>.</p>

<p><span>Derby</span> usually chooses a good join order. However, as with index use,
you should make sure. Analyze the way <span>Derby</span> is executing your application's
queries. See <a href="ttundepth33391.html#ttundepth33391">Analyzing statement execution</a> for information on
how to do this.</p>

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<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="ctundepth23033.html" title="">Always create indexes</a></div>
</div>
<div class="relconcepts"><strong>Related concepts</strong><br />
<div><a href="ctundepth1002853.html" title="Indexes are useful when a query contains a WHERE clause.">Create useful indexes</a></div>
<div><a href="ctundepth1003116.html" title="">Make sure indexes are being used, and rebuild them</a></div>
<div><a href="ctundepth1003209.html" title="">Think about index order</a></div>
<div><a href="ctundepth1003672.html" title="">Decide whether a descending index would be useful</a></div>
</div>
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