Fixed interval graduated renderer always falls back to interval size 1 and immediately runs classify

[anitagraser]

What is the bug or the crash?

When switching to "fixed interval" mode, the graduated renderer immediately triggers the "classify" operation with interval size 1. This can take very long if the value range of the selected value / attribute is huge.

When switching to another mode, the (potentially huge) number of classes is retained, resulting in another very long wait.

If the user changes the interval size, it will be reset to 1 if the user switches to a different mode and then back to fixed interval.

Overall, it feels like the "classify" button is useless now since any change will automatically trigger a reclassification.

Steps to reproduce the issue

1. Load a vector layer with a numerical attribute
2. Enable graduated renderer
3. Switch mode to "fixed interval" --> will immediately classify with interval size 1
4. Switch to "equal interval" --> will retain whatever number of classes was created by the fixed interval mode
5. Switch back to "fixed interval"
6. Change interval to a larger value
7. Change to "equal interval"
8. Change back to "fixed interval" --> you'll see that interval size is reset to 1

Versions

QGIS version
3.38.1-Grenoble
QGIS code branch
Release 3.38
Qt version
5.15.8
Python version
3.10.14
GDAL/OGR version
3.9.1
PROJ version
9.4.1
EPSG Registry database version
v11.006 (2024-03-13)
GEOS version
3.12.2-CAPI-1.18.2
SQLite version
3.46.0
PDAL version
2.7.2
PostgreSQL client version
unknown
SpatiaLite version
5.1.0
QWT version
6.3.0
QScintilla2 version
2.14.1
OS version
Windows 10 Version 2009

Active Python plugins
assets
0.5
DataPlotly
4.1.0
Hqgis
1.2.3
MemoryLayerSaver
5.0.2
plugin_reloader
0.9.4
processing_trajectory
2.2
qforst-project-generator
0.2
qgis-here-api-plugin
1.1
qgis2web
3.21.0
qgis_resource_sharing
1.0.0
quick-geojson
1.0
QuickOSM
2.2.3
QuickWKT
3.1
quick_map_services
0.19.34
db_manager
0.1.20
grassprovider
2.12.99
processing
2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[tsjordan-eng]

It looks like the symptom was kind of patched here: #57722, but It would be better to give a more sensitble default like choosing an interval to give "current number of intervals" or "5" for example.

[jeryadrian]

Still experiencing this today in QGIS 3.42 using MBA M1 8GB, it keeps crashing when selecting fixed interval. However, I do not experience this when using my PC with higher RAM.