Add a "cancel" button for Query Builder get "Sample" Values

[qgib]

Author Name: Nikos Tsarmpopoulos (@ntsarb)
Original Redmine Issue: 14627

Redmine category:vectors

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Hi,

When hitting the "Sample" button, Query Builder freezes. I presume this is because there's a large database associated with the layer. However, this should not happen. Either the function should quickly return a small resultset, or it should run in a separate thread and the user interface should provide a button to interrupt it and return a small sample.

To reproduce, open a layer with a sufficiently large set, e.g. polygons and data for UK postcodes, open the Query builder and click on "Sample" button.

This has been found on QGIS 2.14.1 64bit, running on Windows 7 Pro 64bit.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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Please link/attach a dataset that can be used to replicate the issue.

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• category_id was configured as Vectors
• status_id was changed from Open to Feedback

[qgib]

Author Name: Saber Razmjooei (@saberraz)

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QGIS does some work in the background to generate the sample.

To have it in a separate thread with option to cancel the operation is more of a feature request than a bug.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• subject was changed from Query Builder (Not Responding) when selecting to get "Sample" Values to Add a "cancel" button for Query Builder get "Sample" Values
• status_id was changed from Feedback to Open
• tracker_id was changed from 1 to 2

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• tracker_id was changed from 2 to 1
• crashes_corrupts_data was changed from 1 to 0

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• tracker_id was changed from 1 to 2

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0

[MrChebur]

I would suggest extending the proposed functionality by adding the ability to cancel the execution of any request in the Query Builder. For example, by pressing the Esc key.

[elpaso]

I agree this would be a nice feature.
We have recently improved this for the new connection API and in DB manager, with #38870 and the upstream OSGeo/gdal#2953 but there is no QGIS data provider API support for that functionality and it's not trivial to add it.