[postgis-devel] Postgis documentation clarification request.

[Birkett, Scott]

Dear postgis developers,

"PostgreSQL 9.5.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.3 20140911 (Red Hat 4.8.3-9), 64-bit POSTGIS="2.2.5 r15298" GEOS="3.5.1-CAPI-1.9.1 r4246" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 2.0.3, released 2016/07/01" LIBXML="2.9.1" LIBJSON="0. (...)"

Please consider adding documentation to explain why the following SQL statement returns true.

SELECT ( ST_AsTiff(raster, ARRAY[1,2]) = ST_AsTiff(raster, ARRAY[3]) )
FROM
(
SELECT '0100000200000000000000F03F000000000000F0BF00000000000000000000000000000000000000000000000000000000000000000000000002000200080000000000000000000000000000000000000000080000000001000000010000000100000001000000'::raster
)
as rast;

The raster created by …
SELECT '0100000200000000000000F03F000000000000F0BF00000000000000000000000000000000000000000000000000000000000000000000000002000200080000000000000000000000000000000000000000080000000001000000010000000100000001000000'::raster
is a two band raster with zeros in the first band and ones in the second band.

( ST_AsTiff(raster, ARRAY[1,2]) = ST_AsTiff(raster, ARRAY[3]) ) appears to evaluate to true because any raster band selection on a band that doesn’t exists (ARRAY[3] in this example) returns ALL the raster bands.

The functionality is especially confusing as some raster functions (ST_SummaryStats, ST_DumpValues, etc.) take the band argument more literally and return nothing when requesting bands that don’t exist.

I am not asking for ST_Band or ST_AsTiff to change functionality, but if the documentation https://postgis.net/docs/RT_ST_AsTIFF.html made it apparent that parameter integer[] nbands fetches you ALL the bands when band request are made that don’t exists might help people of the future.



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