title: Order by ASC 100x faster than Order by DESC ? Why?
body: <p>I have one complexe query generated by Hibernate for JBPM. I can't really modify it and i'm searching to optimize it as much as possible.</p>
  
  <p>I found out that ORDER BY DESC is way slower than ORDER BY ASC, do you have any idea ?</p>
  
  <p>PostgreSQL Version : 9.4
  Schema : <a href="https://pastebin.com/qNZhrbef" rel="nofollow noreferrer">https://pastebin.com/qNZhrbef</a>
  Query : </p>
  
  <pre><code>select 
  taskinstan0_.ID_ as ID1_27_, 
  taskinstan0_.VERSION_ as VERSION3_27_, 
  taskinstan0_.NAME_ as NAME4_27_, 
  taskinstan0_.DESCRIPTION_ as DESCRIPT5_27_, 
  taskinstan0_.ACTORID_ as ACTORID6_27_, 
  taskinstan0_.CREATE_ as CREATE7_27_, 
  taskinstan0_.START_ as START8_27_, 
  taskinstan0_.END_ as END9_27_,
  taskinstan0_.DUEDATE_ as DUEDATE10_27_, 
  taskinstan0_.PRIORITY_ as PRIORITY11_27_, 
  taskinstan0_.ISCANCELLED_ as ISCANCE12_27_, 
  taskinstan0_.ISSUSPENDED_ as ISSUSPE13_27_, 
  taskinstan0_.ISOPEN_ as ISOPEN14_27_, 
  taskinstan0_.ISSIGNALLING_ as ISSIGNA15_27_, 
  taskinstan0_.ISBLOCKING_ as ISBLOCKING16_27_, 
  taskinstan0_.LOCKED as LOCKED27_, 
  taskinstan0_.QUEUE as QUEUE27_, 
  taskinstan0_.TASK_ as TASK19_27_, 
  taskinstan0_.TOKEN_ as TOKEN20_27_, 
  taskinstan0_.PROCINST_ as PROCINST21_27_, 
  taskinstan0_.SWIMLANINSTANCE_ as SWIMLAN22_27_, 
  taskinstan0_.TASKMGMTINSTANCE_ as TASKMGM23_27_ 
  from JBPM_TASKINSTANCE taskinstan0_, JBPM_VARIABLEINSTANCE stringinst1_, JBPM_PROCESSINSTANCE processins2_, JBPM_VARIABLEINSTANCE variablein3_ 
  
  where stringinst1_.CLASS_='S' 
      and taskinstan0_.PROCINST_=processins2_.ID_ 
      and taskinstan0_.ID_=variablein3_.TASKINSTANCE_ 
      and variablein3_.NAME_ = 'NIR' 
      and taskinstan0_.QUEUE = 'ERT_TPS'
      and (processins2_.ORGAPATH_ like '/ERT%')
      and taskinstan0_.ISOPEN_= 't'
      and variablein3_.ID_=stringinst1_.ID_
  order by stringinst1_.STRINGVALUE_ ASC limit '10';
  </code></pre>
  
  <p>Explain result for ASC :</p>
  
  <pre><code> Limit  (cost=1.71..11652.93 rows=10 width=646) (actual time=6.588..82.407 rows=10 loops=1)
     -&gt;  Nested Loop  (cost=1.71..6215929.27 rows=5335 width=646) (actual time=6.587..82.402 rows=10 loops=1)
           -&gt;  Nested Loop  (cost=1.29..6213170.78 rows=5335 width=646) (actual time=6.578..82.363 rows=10 loops=1)
                 -&gt;  Nested Loop  (cost=1.00..6159814.66 rows=153812 width=13) (actual time=0.537..82.130 rows=149 loops=1)
                       -&gt;  Index Scan Backward using totoidx10 on jbpm_variableinstance stringinst1_  (cost=0.56..558481.07 rows=11199905 width=13) (actual time=0.018..11.914 rows=40182 loops=1)
                             Filter: (class_ = 'S'::bpchar)
                       -&gt;  Index Scan using jbpm_variableinstance_pkey on jbpm_variableinstance variablein3_  (cost=0.43..0.49 rows=1 width=16) (actual time=0.002..0.002 rows=0 loops=40182)
                             Index Cond: (id_ = stringinst1_.id_)
                             Filter: ((name_)::text = 'NIR'::text)
                             Rows Removed by Filter: 1
                 -&gt;  Index Scan using jbpm_taskinstance_pkey on jbpm_taskinstance taskinstan0_  (cost=0.29..0.34 rows=1 width=641) (actual time=0.001..0.001 rows=0 loops=149)
                       Index Cond: (id_ = variablein3_.taskinstance_)
                       Filter: (isopen_ AND ((queue)::text = 'ERT_TPS'::text))
                       Rows Removed by Filter: 0
           -&gt;  Index Only Scan using idx_procin_2 on jbpm_processinstance processins2_  (cost=0.42..0.51 rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=10)
                 Index Cond: (id_ = taskinstan0_.procinst_)
                 Filter: ((orgapath_)::text ~~ '/ERT%'::text)
                 Heap Fetches: 0
   Planning time: 2.598 ms
   Execution time: 82.513 ms
  </code></pre>
  
  <p>Explain result for DESC :</p>
  
  <pre><code> Limit  (cost=1.71..11652.93 rows=10 width=646) (actual time=8144.871..8144.986 rows=10 loops=1)
     -&gt;  Nested Loop  (cost=1.71..6215929.27 rows=5335 width=646) (actual time=8144.870..8144.984 rows=10 loops=1)
           -&gt;  Nested Loop  (cost=1.29..6213170.78 rows=5335 width=646) (actual time=8144.858..8144.951 rows=10 loops=1)
                 -&gt;  Nested Loop  (cost=1.00..6159814.66 rows=153812 width=13) (actual time=8144.838..8144.910 rows=20 loops=1)
                       -&gt;  Index Scan using totoidx10 on jbpm_variableinstance stringinst1_  (cost=0.56..558481.07 rows=11199905 width=13) (actual time=0.066..2351.727 rows=2619671 loops=1)
                             Filter: (class_ = 'S'::bpchar)
                             Rows Removed by Filter: 906237
                       -&gt;  Index Scan using jbpm_variableinstance_pkey on jbpm_variableinstance variablein3_  (cost=0.43..0.49 rows=1 width=16) (actual time=0.002..0.002 rows=0 loops=2619671)
                             Index Cond: (id_ = stringinst1_.id_)
                             Filter: ((name_)::text = 'NIR'::text)
                             Rows Removed by Filter: 1
                 -&gt;  Index Scan using jbpm_taskinstance_pkey on jbpm_taskinstance taskinstan0_  (cost=0.29..0.34 rows=1 width=641) (actual time=0.002..0.002 rows=0 loops=20)
                       Index Cond: (id_ = variablein3_.taskinstance_)
                       Filter: (isopen_ AND ((queue)::text = 'ERT_TPS'::text))
           -&gt;  Index Only Scan using idx_procin_2 on jbpm_processinstance processins2_  (cost=0.42..0.51 rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=10)
                 Index Cond: (id_ = taskinstan0_.procinst_)
                 Filter: ((orgapath_)::text ~~ '/ERT%'::text)
                 Heap Fetches: 0
   Planning time: 2.080 ms
   Execution time: 8145.053 ms
  </code></pre>
  
  <p>Tables infos :
  jbpm_variableinstance 12100592 rows
  jbpm_taskinstance 69913 rows
  jbpm_processinstance 97546 rows</p>
  
  <p>If you have any idea, thanks</p>