Layer Properties > Source > Provider feature filter: accept Query which includes global or project variables

[qgib]

Author Name: Garth Fletcher (@fgfletcher)
Original Redmine Issue: 19250

Redmine category:expressions

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At present the Query Builder only supports layer fields in its queries.
It would be useful to also be able to reference global or project variables, for example:
"disp_lev" >= @Wshed_disp_lev
where "disp_lev" is a layer variable but @Wshed_disp_lev is a global or project variable.
In the above example, the layers consisted of watersheds of which only the top (largest) watersheds should be drawn.
Wanted to draw a series of maps starting with L1 (smallest/highest watersheds), L2 (next level in which some L1 watersheds were absorbed into L2 watersheds), L3...
@Wshed_disp_lev was used to set which map was being draw, and "disp_lev" to determine whether a watershed in lower levels needed to be drawn...

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• subject was changed from QGIS 3.0.3 Layer Properties > Source > Provider feature filter: accept Query which includes global or project variables to Layer Properties > Source > Provider feature filter: accept Query which includes global or project variables

[alexbruy]

Duplicates #24662.