[postgis-users] ST_Mapalgebraexpr requires same alignment

[Pierre Racine]

> The resulted tiles are empty. Does ST_Mapalgebraexpr  require same alignment?
> According to the docs I guess not but then dont know why im getting that
> message.

What in the doc let you think that it does not require same alignment? Is does...

> In another example I didnt use st_union with the same result.
> 
> insert into demdif (rast) select st_mapalgebraexpr (d1.rast, 1, st_union (d2.rast),
> 1, 'abs([rast1.val] - [rast2-val])','16BSI', 'FIRST', null,
> null,null) from dem200 d1, dem d2 where d1.rast && d2.rast and st_relate
> (d1.rast::geometry, d2.rast::geometry, 'T********') group by d1.rid;
> 
> Another issue is st_union (internally is st_mapalgebraexpr) is extremely slow. I
> can not manage raster layers bigger than a couple of thousand pixel in x or y
> even with 128 tiled blocks.

Right. This is documented:

http://postgis.org/documentation/manual-svn/RT_ST_Union.html

Pierre