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<p class="MsoNormal">Hi All,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In 10.2.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I have a query:<o:p></o:p></p>
<p class="MsoNormal">SELECT count(*)<o:p></o:p></p>
<p class="MsoNormal">FROM comms c<o:p></o:p></p>
<p class="MsoNormal"> LEFT JOIN researcher r ON c.launch_id = r.researcher_id AND c.launch_type in (0,2)<o:p></o:p></p>
<p class="MsoNormal"> LEFT JOIN user_details ud ON ud.user_id = r.user_id<o:p></o:p></p>
<p class="MsoNormal">WHERE c.active=1 AND c.comm_type=2<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The part of the qep which handles the left join between comms and researcher looks like this:<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> /
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> left join
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> K Join(launch_id)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> Sort on(tidp)
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> Pages 1
<span style="background:yellow;mso-highlight:yellow">Tups 11</span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> D203 C539<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> / \<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> Proj-rest researcher_id_idx<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> Heap I(r)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> Pages 4 <span style="background:yellow;mso-highlight:yellow">
Tups 745</span> B-Tree(researcher_id)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> D170 C525 Pages 36 Tups 6117<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">/<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">comms<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">(c)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">B-Tree(NU)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Pages 680 Tups 52520<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">How can a left join have fewer result rows (11) than the left table (745)?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The fun part is that this only occurs if:<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]> the left join between comms and researcher has the ON cond1 AND cond2<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>The query has multiple left joins.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If you remove either of those then the tuple estimate returns to 745. So it looks like the it has treated the second join condition as a row restriction just like it had been in the where clause of the query.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Martin Bowes<o:p></o:p></p>
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