Re: Selecting records not present in related tables - Mailing list pgsql-sql

<div class="Section1"><p class="MsoNormal"><span class="SpellE"><font color="navy" face="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial;color:navy">Jeziel</span></font></span><fontcolor="navy" face="Arial"
size="2"><spanstyle="font-size:10.0pt;font-family:Arial; 
color:navy">,</span></font><p class="MsoNormal"><font color="navy" face="Arial" size="2"><span style="font-size:
10.0pt;font-family:Arial;color:navy"><span style="mso-spacerun:yes">  </span><span class="GramE">there</span> are a
coupletechniques you can try, two I like are set difference and anti-</span></font><font color="navy" face="Arial"
size="2"><spanstyle="font-size:10.0pt;font-family:Arial;color:navy">j</span></font><font color="navy" face="Arial"
size="2"><spanstyle="font-size:10.0pt;font-family:Arial;color:navy">oins.</span></font><p class="MsoNormal"><font
color="navy"face="Arial" size="2"><span style="font-size: 
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial;color:navy">here’s</span></font></span><font
color="navy"face="Arial" size="2"><span style="font-size:10.0pt;font-family:Arial; 
color:navy"> the set diff:</span></font><p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier
New";color:navy">select</span></font></span><fontcolor="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:
"Courier New";color:navy"> id </span></font><p class="MsoNormal"><font color="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew";color:navy"><span style="mso-spacerun:yes">  </span><span
class="GramE">from</span>messages </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier
New";color:navy">except</span></font></span><fontcolor="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:
"Courier New";color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy" face="Courier New"
size="2"><spanstyle="font-size:10.0pt;font-family:"Courier New";color:navy">select</span></font></span><font
color="navy"face="Courier New" size="2"><span style="font-size:10.0pt;font-family: 
"Courier New";color:navy"> id </span></font><p class="MsoNormal"><font color="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew";color:navy"><span style="mso-spacerun:yes">  </span><span
class="GramE">from</span><span class="SpellE">usermessages</span></span></font><p class="MsoNormal"><font color="navy"
face="Arial"size="2"><span style="font-size: 
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial;color:navy">that</span></font></span><font
color="navy"face="Arial" size="2"><span style="font-size:10.0pt;font-family:Arial; 
color:navy"> will returns all id from messages not in <span class="SpellE">usermessages</span></span></font><p
class="MsoNormal"><fontcolor="navy" face="Arial" size="2"><span style="font-size: 
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial;color:navy">if</span></font></span><font
color="navy"face="Arial" size="2"><span style="font-size:10.0pt;font-family:Arial; 
color:navy"> ID is index</span></font><font color="navy" face="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial;color:navy">ed</span></font><font color="navy" face="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial;
color:navy">on both tables, you may <span class="SpellE">wanna</span> try an anti </span></font><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial;color:navy">j</span></font><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial; 
color:navy">oin:</span></font><p class="MsoNormal"><font color="navy" face="Arial" size="2"><span style="font-size:
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier
New";color:navy">select</span></font></span><fontcolor="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:
"Courier New";color:navy"> <span class="SpellE">m.id</span></span></font><p class="MsoNormal"><font color="navy"
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New";color:navy"><span
style="mso-spacerun:yes"> </span><span class="GramE">from</span> messages m </span></font><p class="MsoNormal"><font
color="navy"face="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier New";color:navy"><span
style="mso-spacerun:yes"> </span><span style="mso-spacerun:yes">     </span><span class="GramE">left</span>
</span></font><fontcolor="navy" face="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New";color:navy">j</span></font><fontcolor="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"; 
color:navy">oin </span></font><p class="MsoNormal"><font color="navy" face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew";color:navy"><span style="mso-spacerun:yes">       </span><span
class="SpellE"><spanclass="GramE">usermessages</span></span> um</span></font><p class="MsoNormal"><font color="navy"
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New";color:navy"><span
style="mso-spacerun:yes"> </span><spanstyle="mso-spacerun:yes"> </span><span style="mso-spacerun:yes">  </span><span
class="GramE">on</span>( <span class="SpellE">m.id</span> = <span class="SpellE">um.id</span> )</span></font><p
class="MsoNormal"><fontcolor="navy" face="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New";color:navy"><spanstyle="mso-spacerun:yes"> </span><span class="GramE">where</span> <span
class="SpellE">um.id</span>is null</span></font><font color="navy" face="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial;color:navy"></span></font><pclass="MsoNormal"><font color="navy" face="Arial"
size="2"><spanstyle="font-size: 
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial;color:navy">both</span></font></span><font
color="navy"face="Arial" size="2"><span style="font-size:10.0pt;font-family:Arial; 
color:navy"> techniques can be <span class="SpellE">visciously</span> efficient.</span></font><p
class="MsoNormal"><fontcolor="navy" face="Arial" size="2"><span style="font-size: 
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal"><span class="GramE"><font color="navy"
face="Arial"size="2"><span style="font-size:10.0pt;font-family:Arial;color:navy">good</span></font></span><font
color="navy"face="Arial" size="2"><span style="font-size:10.0pt;font-family:Arial; 
color:navy"> luck,</span></font><p class="MsoNormal"><font color="navy" face="Arial" size="2"><span style="font-size:
10.0pt;font-family:Arial;color:navy"><span style="mso-spacerun:yes">  </span>Anthony <span
style="mso-spacerun:yes"> </span></span></font><pclass="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial;color:navy"><span style="mso-spacerun:yes">           </span></span></font><p
class="MsoNormal"><fontcolor="navy" face="Arial" size="2"><span style="font-size: 
10.0pt;font-family:Arial;color:navy"> </span></font><p class="MsoNormal" style="margin-left:.5in"><font face="Tahoma"
size="2"><spanstyle="font-size:10.0pt;font-family:Tahoma">-----Original Message-----<br /><b><span
style="font-weight:bold">From:</span></b>pgsql-sql-owner@postgresql.org [mailto:pgsql-sql-owner@postgresql.org]
<b><spanstyle="font-weight:bold">On Behalf Of </span></b>Hector Rosas<br /><b><span
style="font-weight:bold">Sent:</span></b>Thursday, October 06, 2005 3:44 PM<br /><b><span
style="font-weight:bold">To:</span></b>pgsql-sql@postgresql.org<br /><b><span
style="font-weight:bold">Subject:</span></b>[SQL] Selecting records not present in related tables</span></font><p
class="MsoNormal"style="margin-left:.5in"><font face="Times New Roman" size="3"><span
style="font-size:12.0pt"> </span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Times New Roman"
size="3"><spanstyle="font-size:12.0pt">Hello, I'm trying to select records in a table not present in a related table,
inexample, I've a table with message information (subject, message, date, etc) and another (usermessages) with where
user(s)has that message, its state, etc. Records in this table will be deleted in a certain time (just some extra
info).<br /> I want to select messages records that aren't present in the other table (usermessages), I got the next
twoqueries, maybe someone can suggest a better one.<br /><br /></span></font><font face="Courier New"><span
style="font-family:"CourierNew"">SELECT <a href="http://m.id">m.id</a> FROM messages AS m<br /> WHERE (SELECT
count(um.*)FROM usermessages AS um WHERE um.idmessage=m.id )=0;</span></font><br /><br /><font face="Courier New"><span
style="font-family:"CourierNew"">SELECT <a href="http://m.id">m.id</a> FROM messages AS m where id NOT IN (select
um.idmessageFROM usermessages um);</span></font><br /><br /> Both queries work, but doing a <font face="Courier
New"><spanstyle="font-family: 
"Courier New"">EXPLAIN ANALYZE</span></font> I got the next results.<br /><br /><font face="Courier New"><span
style="font-family:"CourierNew"">bd=# explain analyze SELECT <a href="http://m.id">m.id</a> FROM messages AS m <br />
bd-#WHERE (SELECT count(um.*) FROM usermessages AS um WHERE um.idmessage=m.id)=0;<br />
                                                                   QUERY
PLAN                                                                                                                                       
<br/>
---------------------------------------------------------------------------------------------------------------------------------------------------
<br/>  Seq Scan on messages m  (cost=0.00..3915.75 rows=3 width=4) (actual time=40.531..40.531 rows=0 loops=1)<br />   
Filter:((subplan) = 0)<br />    SubPlan<br />      ->  Aggregate  (cost=9.11..9.11 rows=1 width=4) (actual
time=0.098..0.104rows=1 loops=355)<br />            ->  Index Scan using message_selection on usermessages um 
(cost=0.00..9.10rows=3 width=4) (actual time=0.067..0.078 rows=1 loops=355)<br />                  Index Cond:
(idmessage= $0)<br />  Total runtime: 40.605 ms<br /> (7 rows)<br />  <br /> bd=# explain analyze select <a
href="http://m.id">m.id</a>FROM messages AS m where id NOT IN (select um.idmessage FROM usermessages um);<br />
                                                     QUERY PLAN<br />
----------------------------------------------------------------------------------------------------------------------
<br/>  Seq Scan on messages m  (cost=9.68..43.00 rows=213 width=4) (actual time=20.329..20.329 rows=0 loops=1)<br />   
Filter:(NOT (hashed subplan))<br />    SubPlan<br />      ->  Seq Scan on usermessages um  (cost=0.00..8.54 rows=454
width=4)(actual time=0.008..13.094 rows=454 loops=1)<br />  Total runtime: 20.386 ms<br /> (5 rows)<br
/></span></font><br/> In first query, cost can be between 0 and almost 4 sec, and also I see that loops value, that I
don'tknow what performance issues could arise.<br /> In second query, I see a seq scan, which I don't like, I think
thatwith too many records this query could take ages, or maybe not, but loops value is 1. <br /><br /> I hope someone
cangive some advice with those queries , or maybe a better query. I've not decided which query I'm going to use,
thanks!<br/><br /> Jeziel.</div>