Improve performance on SQL query with Nested Loop - PostgreSQL
<p>I am using <strong>PostgreSQL</strong> and I have a weird problem with my SQL query. <strong>Depending on wich date paramter I'm using</strong>. My request doesn't do the same operation.</p>

<p>This is my working query :</p>

<pre><code>SELECT DISTINCT app.id_application 
FROM stat sj
LEFT OUTER JOIN groupe gp ON gp.id_groupe = sj.id_groupe 
LEFT OUTER JOIN application app ON app.id_application = gp.id_application 
WHERE date_stat &gt;= '2016/3/01' 
AND date_stat &lt;= '2016/3/31' 
AND ( date_stat = date_gen-1 or (date_gen = '2016/04/01' AND date_stat = '2016/3/31')) 
AND app.id_application IS NOT NULL 
</code></pre>

<p>This query takes around 2 secondes (which is OKAY for me because I have a lots of rows). When I run EXPLAIN ANALYSE for this query I have this:</p>

<pre class="lang-none prettyprint-override"><code>HashAggregate  (cost=375486.95..375493.62 rows=667 width=4) (actual time=2320.541..2320.656 rows=442 loops=1)
    -&gt;  Hash Join  (cost=254.02..375478.99 rows=3186 width=4) (actual time=6.144..2271.984 rows=263274 loops=1)
    Hash Cond: (gp.id_application = app.id_application)
    -&gt;  Hash Join  (cost=234.01..375415.17 rows=3186 width=4) (actual time=5.926..2200.671 rows=263274 loops=1)
          Hash Cond: (sj.id_groupe = gp.id_groupe)
          -&gt;  Seq Scan on stat sj  (cost=0.00..375109.47 rows=3186 width=8) (actual time=3.196..2068.357 rows=263274 loops=1)
                Filter: ((date_stat &gt;= '2016-03-01'::date) AND (date_stat &lt;= '2016-03-31'::date) AND ((date_stat = (date_gen - 1)) OR ((date_gen = '2016-04-01'::date) AND (date_stat = '2016-03-31'::date))))
                Rows Removed by Filter: 7199514
          -&gt;  Hash  (cost=133.45..133.45 rows=8045 width=12) (actual time=2.677..2.677 rows=8019 loops=1)
                Buckets: 1024  Batches: 1  Memory Usage: 345kB
                -&gt;  Seq Scan on groupe gp  (cost=0.00..133.45 rows=8045 width=12) (actual time=0.007..1.284 rows=8019 loops=1)
    -&gt;  Hash  (cost=11.67..11.67 rows=667 width=4) (actual time=0.206..0.206 rows=692 loops=1)
          Buckets: 1024  Batches: 1  Memory Usage: 25kB
          -&gt;  Seq Scan on application app  (cost=0.00..11.67 rows=667 width=4) (actual time=0.007..0.101 rows=692 loops=1)
                Filter: (id_application IS NOT NULL)
    Total runtime: 2320.855 ms
</code></pre>

<p>Now, When I'm trying the same query for the current month (<strong>we are the 6th of April, so I'm trying to get all the application_id of April</strong>) with the same query</p>

<pre><code>SELECT DISTINCT app.id_application 
FROM stat sj
LEFT OUTER JOIN groupe gp ON gp.id_groupe = sj.id_groupe 
LEFT OUTER JOIN application app ON app.id_application = gp.id_application 
WHERE date_stat &gt;= '2016/04/01' 
AND date_stat &lt;= '2016/04/30' 
AND ( date_stat = date_gen-1 or ( date_gen = '2016/05/01' AND date_job = '2016/04/30')) 
AND app.id_application IS NOT NULL 
</code></pre>

<p>This query takes now 120 seconds. So I also ran EXPLAIN ANALYZE on this query and now it doesn't have the same operations:</p>

<pre class="lang-none prettyprint-override"><code>HashAggregate  (cost=375363.50..375363.51 rows=1 width=4) (actual time=186716.468..186716.532 rows=490 loops=1)
-&gt;  Nested Loop  (cost=0.00..375363.49 rows=1 width=4) (actual time=1.945..186619.404 rows=118990 loops=1)
    Join Filter: (gp.id_application = app.id_application)
    Rows Removed by Join Filter: 82222090
    -&gt;  Nested Loop  (cost=0.00..375343.49 rows=1 width=4) (actual time=1.821..171458.237 rows=118990 loops=1)
          Join Filter: (sj.id_groupe = gp.id_groupe)
          Rows Removed by Join Filter: 954061820
          -&gt;  Seq Scan on stat sj  (cost=0.00..375109.47 rows=1 width=8) (actual time=0.235..1964.423 rows=118990 loops=1)
                Filter: ((date_stat &gt;= '2016-04-01'::date) AND (date_stat &lt;= '2016-04-30'::date) AND ((date_stat = (date_gen - 1)) OR ((date_gen = '2016-05-01'::date) AND (date_stat = '2016-04-30'::date))))
                Rows Removed by Filter: 7343798
          -&gt;  Seq Scan on groupe gp  (cost=0.00..133.45 rows=8045 width=12) (actual time=0.002..0.736 rows=8019 loops=118990)
    -&gt;  Seq Scan on application app  (cost=0.00..11.67 rows=667 width=4) (actual time=0.003..0.073 rows=692 loops=118990)
          Filter: (id_application IS NOT NULL)
  Total runtime: 186716.635 ms
</code></pre>

<p>So I decided to search where the problem came from by reducing the number of conditions from my query until the performances is acceptable again.</p>

<p>So with only this parameter</p>

<pre><code>WHERE date_stat &gt;= '2016/04/01'
</code></pre>

<p>It takes only 1.9secondes (like the first working query)
and it's also working with 2 parameters :</p>

<pre><code>WHERE date_stat &gt;= '2016/04/01' 
AND app.id_application IS NOT NULL 
</code></pre>

<p>BUT when I try to add one of those line I have the Nested loop in the Explain</p>

<pre><code>AND date_stat &lt;= '2016/04/30' 
AND ( date_stat = date_gen-1 or ( date_gen = '2016/05/01' AND date_stat = '2016/04/30')) 
</code></pre>

<p>Does someone have any idea where it could come from?</p>