ST_MapAlgebra not respecting NULL nodatavalues

[robe]

As someone noted on irc/matrix

If you have a NULL nodatavalue constraint on your raster table, and do something like

UPDATE yourtable SET rast = ST_MapAlgebra(rast,1, '32BF'::text, '[rast.val]*1'::text, NULL::float8);

You get a constraint vialoation

To test, I tested on demo.postgis.net in the nyc db, which has our development version of postgis running

CREATE TABLE test_robe AS 
SELECT *
FROM o_2_nyc_dem;

SELECT AddRasterConstraints('test_robe'::name, 'rast'::name);
UPDATE test_robe SET rast = ST_MapAlgebra(rast,1, '16BUI'::text, '[rast.val]*1'::text, NULL::float8) WHERE rid = 1 ;

and received

ERROR:  Failing row contains (1, 0100000100000000000000344000000000000034C09A999999FECA2B41333333..., nyc_dem.tif).new row for relation "test_robe" violates check constraint "enforce_nodata_values_rast" 

The work around would be to set the bandnodata value as follows:

UPDATE test_robe SET rast = ST_SetBandNoDataValue(ST_MapAlgebra(rast,1, '16BUI'::text, '[rast.val]*1'::text, NULL::float8), NULL) WHERE rid = 1 ;

[robe]

Looking at this

SELECT ST_BandNoDataValue(ST_MapAlgebra(rast,1, '16BUI'::text, '[rast.val]*1'::text, NULL::float8)) FROM test_robe WHERE rid = 1 ;

What is returned is 0

but this also returns 0

SELECT ST_BandNoDataValue(ST_MapAlgebra(rast,1, '16BUI'::text, '[rast.val]*1'::text, 1)) FROM test_robe WHERE rid = 1 ;

I'm not sure if this is a bug, or just a documentation error. It might have become an issue when we merged all variants of ST_MapAlgebra under one umbrella.

​https://postgis.net/docs/RT_ST_MapAlgebra_expr.html

Because the documentation speaks of using an expression to set no-data-value, and yet none of the signatures listed would allow that.

[robe]

I also tried the original signature:

st_mapalgebraexpr(
	rast raster,
	band integer,
	pixeltype text,
	expression text,
	nodataval double precision DEFAULT NULL::double precision)

and this also seems to ignore the input nodataval even calling by named args

SELECT ST_BandNoDataValue(st_mapalgebraexpr(rast => rast, band => 1, pixeltype => NULL, expression => '[rast.val]*1'::text, nodataval => 1)) 
FROM test_robe WHERE rid = 1 ;