Relation aggregate function error in QGIS using Geometry by expression

Question

I have a point layer, consisting of 12 points with fid 1 to 12. I want to render points with fid > median (fid) - thus in this case, point 7 to 12. That works perfectly if using geometry generator with this expression - see screenshot, where only 7 to 12 are rendered, whereas 1 to 6 only have labels/callouts:

if ( 
    "point_id" > median (
        "point_id"
    ), 
    $geometry , 
    ''
)

However, when I use the same expression with Menu Processing / Toolbox /Geometry by expression on the same point layer, I get an error: Cannot use relation aggregate function in this context. Why is that - or asked the other way round: how to correct the expression? It's the same expression as in geometry generator in rather the same context.

I am asking specifically about this relation aggregate function error, as I already encountered it several times in more complex cases. In this simple case, of course there would be easier ways to achieve the goal. However, I wanted to present a simple case to make the error easier to reproduce.

Answer 1

When using aggregate functions with processing it seems there is a scope problem.

For your problem, you should use the aggregate function and refer to the layername of your source point layer.

Something like that :

if (

    "point_id" > aggregate('<layername>','median', "point_id"), 
    $geometry , 
    geom_from_wkt('POINT EMPTY')
)

You need also to create a valid geometry point with geom_from_wkt('POINT EMPTY')