Re: Why query plan is different?

<div class="moz-cite-prefix">On 10.10.2016 19:09, Pavel Stehule wrote:<br /></div><blockquote
cite="mid:CAFj8pRCFZGWtu5NuJ7pcVmXECTiC5wHfYJ7VjhgObcEz+AAzxw@mail.gmail.com"type="cite"><div dir="ltr"><br /><div
class="gmail_extra"><br/><div class="gmail_quote">2016-10-10 17:31 GMT+02:00 Andrzej Zawadzki <span dir="ltr"><<a
href="mailto:zawadaa@wp.pl"moz-do-not-send="true" target="_blank">zawadaa@wp.pl</a>></span>:<br /><blockquote
class="gmail_quote"style="margin:0px 0px 0px             0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex"><divbgcolor="#FFFFFF"> Hi,<br /> Today, I noticed strange situation:<br /><br /> The
samequery run on different servers has very different plan:<br /><br /> Q: SELECT b.* FROM kredytob b  WHERE pesel =
'22222222222' ORDER BY <a href="http://b.id" moz-do-not-send="true" target="_blank">b.id</a> DESC LIMIT 1 <br /><br />
Slowplan:<br /><br /> "Limit  (cost=0.43..28712.33 rows=1 width=4) (actual time=2574.041..2574.044 rows=1 loops=1)"<br
/>"  Output: id"<br /> "  Buffers: shared hit=316132 read=110001"<br /> "  ->  Index Scan Backward using
kredytob_pkeyon public.kredytob b  (cost=0.43..3244444.80 rows=113 width=4) (actual time=2574.034..2574.034 rows=1
loops=1)"<br/> "        Output: id"<br /> "        Filter: (b.pesel = '22222222222'::bpchar)"<br /> "        Rows
Removedby Filter: 433609"<br /></div></blockquote><div><br /></div><div>here is backward index scan with - lot of rows
isthrown <br /><br /> Rows Removed by Filter: 433609" <br /><br /></div><div>probably index definition on these servers
aredifferent<br /></div></div></div></div></blockquote><br /> No! That's binary copy of whole database.<br /> Index are
thesame!<br /> But, when I ask database without "ORDER..."<br /> (SELECT b.id FROM kredytob b  WHERE pesel =
'22222222222';)<br/>  then:<br /><br /> "SLOW"<br /><br /> "Index Scan using kredytob_pesel_typkred_opclass_idx on
public.kredytobb  (cost=0.43..115349.30 rows=113 width=4) (actual time=233.767..392.710 rows=5 loops=1)"<br /> " 
Output:id"<br /> "  Index Cond: (b.pesel = '22222222222'::bpchar)"<br /> "  Buffers: shared hit=19259"<br /> "Planning
time:0.254 ms"<br /> "Execution time: 392.761 ms"<br /><br /> "FAST"<br /><br /> "Index Scan using
kredytob_pesel_typkred_opclass_idxon public.kredytob b  (cost=0.43..115240.10 rows=112 width=4) (actual
time=378.737..836.208rows=5 loops=1)"<br /> "  Output: id"<br /> "  Index Cond: (b.pesel = '22222222222'::bpchar)"<br
/>"  Buffers: shared read=19237"<br /> "Planning time: 0.568 ms"<br /> "Execution time: 836.261 ms"<br /><br /> So,
indexis used in both queries but when is "ORDER" then everything change...<br /> Why?<br /><br /><br /><blockquote
cite="mid:CAFj8pRCFZGWtu5NuJ7pcVmXECTiC5wHfYJ7VjhgObcEz+AAzxw@mail.gmail.com"type="cite"><div dir="ltr"><div
class="gmail_extra"><divclass="gmail_quote"><div><br /></div><div>regards<br /><br /></div><div>Pavel<br
/></div><div><br/></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1pxsolid             rgb(204,204,204);padding-left:1ex"><div bgcolor="#FFFFFF"> "        Buffers:
sharedhit=316132 read=110001"<br /> "Planning time: 0.414 ms"<br /> "Execution time: 2574.139 ms"<br /><br /><br />
Fastplan:<br /> "Limit  (cost=115240.66..115240.66 rows=1 width=4) (actual time=463.275..463.276 rows=1 loops=1)"<br />
" Output: id"<br /> "  Buffers: shared hit=14661 read=4576"<br /> "  ->  Sort  (cost=115240.66..115240.94 rows=112
width=4)(actual time=463.271..463.271 rows=1 loops=1)"<br /> "        Output: id"<br /> "        Sort Key: <a
href="http://b.id"moz-do-not-send="true" target="_blank">b.id</a> DESC"<br /> "        Sort Method: top-N heapsort 
Memory:25kB"<br /> "        Buffers: shared hit=14661 read=4576"<br /> "        ->  Index Scan using
kredytob_pesel_typkred_<wbr/>opclass_idx on public.kredytob b  (cost=0.43..115240.10 rows=112 width=4) (actual
time=311.347..463.183rows=5 loops=1)"<br /> "              Output: id"<br /> "              Index Cond: (b.pesel =
'22222222222'::bpchar)"<br/> "              Buffers: shared hit=14661 read=4576"<br /> "Planning time: 0.383 ms"<br />
"Executiontime: 463.324 ms"<br /><br /> Data is almost equal - "slow" has a few more rows in table. ("Fast" is a copy
from1 am today).<br /> Why runtime is slower?<span class="gmail-HOEnZb"><font color="#888888"><br /><br /> -- <br />
AndrzejZawadzki<br /></font></span></div></blockquote></div><br /></div></div></blockquote><p><br />