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Hi</div>
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Do mean something like this ?</div>
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Here is the result from postgres 11 -</div>
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<span style="font-variant-ligatures: no-common-ligatures"></span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures">EXPLAIN ANALYZE</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">select (ST_dump(st_union(geo))).geom as geo</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">from sde_markslag.markslag_myrikilden_temp </span></p>
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<span style="font-variant-ligatures: no-common-ligatures">where gid < 10000</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">;</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> QUERY PLAN </span></p>
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<span style="font-variant-ligatures: no-common-ligatures">---------------------------------------------------------------------------------------------------------------------------------------------------</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Result (cost=62634.26..62899.53 rows=1000 width=32) (actual time=25249.956..25331.325 rows=9205 loops=1)</span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures"> -> ProjectSet (cost=62634.26..62639.53 rows=1000 width=32) (actual time=25249.953..25330.483 rows=9205 loops=1)</span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures"> -> Aggregate (cost=62634.26..62634.28 rows=1 width=32) (actual time=25244.718..25244.719 rows=1 loops=1)</span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures"> -> Seq Scan on markslag_myrikilden_temp (cost=0.00..62609.51 rows=9900 width=1593) (actual time=0.008..173.033 rows=9999 loops=1)</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Filter: (gid < 10000)</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Rows Removed by Filter: 557242</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Planning Time: 0.064 ms</span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures"> Execution Time: 25335.136 ms</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">(8 rows)</span></p>
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Here is the same postgres 9.5 -</div>
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<span style="font-variant-ligatures: no-common-ligatures"></span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures">EXPLAIN ANALYZE</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">[more] - > select (ST_dump(st_union(geo))).geom as geo</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">[more] - > from sde_markslag.markslag_myrikilden_temp </span></p>
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<span style="font-variant-ligatures: no-common-ligatures">[more] - > where gid < 10000</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">[more] - > ;</span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures"> QUERY PLAN </span></p>
<p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo">
<span style="font-variant-ligatures: no-common-ligatures">----------------------------------------------------------------------------------------------------------------------------------------</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Aggregate (cost=170462.27..170467.28 rows=1000 width=1604) (actual time=4311.911..4409.250 rows=9205 loops=1)</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> -> Seq Scan on markslag_myrikilden_temp (cost=0.00..170437.51 rows=9903 width=1604) (actual time=0.054..629.045 rows=9999 loops=1)</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Filter: (gid < 10000)</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Rows Removed by Filter: 557242</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Planning time: 0.045 ms</span></p>
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<span style="font-variant-ligatures: no-common-ligatures"> Execution time: 4409.919 ms</span></p>
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<span style="font-variant-ligatures: no-common-ligatures">(6 rows)</span></p>
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Thanks </div>
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Lars</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> postgis-users <postgis-users-bounces@lists.osgeo.org> on behalf of Martin Davis <mtnclimb@gmail.com><br>
<b>Sent:</b> Thursday, April 11, 2019 8:07 PM<br>
<b>To:</b> PostGIS Users Discussion<br>
<b>Subject:</b> Re: [postgis-users] diffrent execution plan on Postgres 9.5 and Postgres 11 for ST_union and performance problem Postgres 11</font>
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<div dir="ltr" class="x_gmail_attr">On Thu, Apr 11, 2019 at 3:27 AM Lars Aksel Opsahl <<a href="mailto:Lars.Opsahl@nibio.no">Lars.Opsahl@nibio.no</a>> wrote:</div>
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Can it be problem related ST_Union and aggregate ?</div>
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<div>Can you test the performance of aggregate ST_Union on it's own by dropping the GROUP BY and instead using a WHERE clause to union a small-ish subset of the geometries? (But large enough to give a measurable query time.)</div>
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