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<p class="MsoNormal"><span lang="EN-US">This seems like one of the most asked questions in the PostgreSQL world, but I guess I haven’t understood all the answers yet:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Why won’t my SELECT query use the INDEX I have created for it?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I have a table with about 18mio rows.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">My SELECT statement looks like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">SELECT "id",encode(ST_AsEWKB("geom"),'base64') as "geometry"<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">FROM my_table<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">WHERE geom &&
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">ST_GeomFromText('POLYGON ((892267.1937422 6148208.34251139, 892500.497129949 6148208.34251139, 892500.497129949 6148009.40012468, 892267.1937422 6148009.40012468, 892267.1937422 6148208.34251139))',
ST_SRID("geom"))<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The EXPLAIN ANALYZE of the above statement returned this:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">"Seq Scan on my_table (cost=0.00..4329124.83 rows=1731 width=1700) (actual time=194785.745..1553525.244 rows=138 loops=1)"<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">" Filter: (geom && st_geomfromtext('POLYGON ((892267.1937422 6148208.34251139, 892500.497129949 6148208.34251139, 892500.497129949 6148009.40012468, 892267.1937422 6148009.40012468,
892267.1937422 6148208.34251139))'::text, st_srid(geom)))"<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">" Rows Removed by Filter: 17311187"<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">"Total runtime: 1553525.352 ms"<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The POLYGON described above is located at the outer edge of the 17mio geometries and the extent is pretty small.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I have executed a VACUUM ANALYZE to clean up the statistics, which didn’t seem to improve the results.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">My INDEX has been created like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">CREATE INDEX my_table_geom_idx<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> ON my_table<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> USING gist<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> (geom);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Upon reading up on this issue I have changed the following in my postgresql.conf:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">random_page_cost = 2.0<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">shared_buffers = 512MB<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">work_mem = 8MB<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">maintenance_work_mem = 256MB<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">effective_cache_size = 8GB<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The HARDWARE is a Windows 2012R2 with 12GB ram and SAS HDDs. PostgreSQL 9.3 x64 and PostGIS 2.1.1.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Can any ask me the right questions so I can solve my INDEX problem?<o:p></o:p></span></p>
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