Posts in category 'tips'

Move PostGIS extension to a different schema

As of PostGIS 2.3, the postgis extension was changed to no longer allow relocation. All function calls within the extension are now schema qualified.

While this change fixed some issues with database restore, it created the issue of if you installed PostGIS in a schema other than the one you wanted to it is not intuitive how to move it to a different schema. Luckily there is a way to do this.

For this exercise, I will install PostGIS in the default schema and then demonstrate how to move it into another schema location.

You can run these steps using psql or pgAdmin or any other PostgreSQL tool you want.

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Causes for 'postgis.backend' is already set

The error ‘postgis.backend’ is already set comes up every so often in PostGIS mailing list. The issue arises often during or after an upgrade. I’ll go over causes for this I am aware of and how to fix.

The question goes something like this

After upgrading to Postgis 2.3 from 2.1, my server log is filled with these messages :

“WARNING ‘postgis.backend’ is already set and cannot be changed until you reconnect”

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Selecting only pixels of particular range of values with ST_Reclass

This raster question comes up quite a bit on PostGIS mailing lists and stack overflow and the best answer often involves the often forgotten ST_Reclass function that has existed since PostGIS 2.0.
People often resort to the much slower though more flexible ST_MapAlgebra or dumping out their rasters as Pixel valued polygons they then filter with WHERE val > 90, where ST_Reclass does the same thing but orders of magnitude faster.

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Why can't I do CREATE EXTENSION postgis_sfcgal

PostGIS 2.2.0 came out this month, and the SFCGAL extension that offers advanced 3D and volumetric support, in addition to some extended 2D functions like ST_ApproximateMedialAxis became a standard PostgreSQL extension and seems to be a fairly popular extension.

I’ve seen several reports on GIS Stack Exchange of people trying to install PostGIS SFCGAL and getting things such as

ERROR: could not open extension control file /Applications/Postgres.app/Contents/Versions/9.5/share/postgresql/extension/postgis_sfcgal.control

as in this question: SFCGAL in PostGIS problem.

There are two main causes for this:

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Enabling PostGIS raster drivers and out-of-db raster support

I’ve been asked this question in some shape or form at least 3 times, mostly from people puzzled why they get this error. The last iteration went something like this:

I can’t use ST_AsPNG when doing something like

SELECT ST_AsPNG(rast) 
    FROM  sometable;

Gives error: Warning: pg_query(): Query failed: ERROR: rt_raster_to_gdal: Could not load the output GDAL driver.

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