Using the QGIS expression string builder, there is a preview window at the bottom that shows the pre-calculated output for the current feature selected. However, with more complex expressions, calculation can become complex and takes a while. The expression string builder can become slow or even hang. This is especially true for aggregate or (nested) array_foreach functions.
Editing an expression in such circumstances is difficult as each character entered or deleted starts calculation from the start and each time it takes a while until it becomes responsive again.
Is there a way to disable/deactivate the preview in QGIS expression string builder?
Best Answer
A simple workaround is to make the expression invalid, e.g. by removing the closing bracket. Add the bracket back when you are done with your changes to test the expression.