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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?

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2 Answers 2

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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.

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  • But if you make the function invalid, you can't tell if you make syntax errors.
    – pigreco
    Dec 19, 2021 at 16:28
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When I have to study or create complex expressions I put a filter on the layer:

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