Add merge policies (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request qgis/QGIS#60710
Author: @JuhoErvasti
QGIS version: 3.44

Add merge policies

PR Description:

Fixes #59494.

Similarly to already existing Split and Duplicate "policies" this PR implements Merge Policies, which determine the initial values in the Merge Features dialog. The PR includes the following merge policies:

From the existing Qgis::FieldDomainMergePolicy enum:

• Sum: numeric fields only
• Geometry Weighted: numeric fields only, uses weighted average by geometry
• Default Value: uses the default value set in QGIS

New policies:

• Unset Field: similarly to Split & Duplicate policies clears the field and lets the data provider populate any default value if one exists. If it doesn't the first feature is used (this is the behavior which is currently implemented)
• Largest Geometry: use the value from the feature with the biggest geometry (length/area/number of parts for multipoints)
• Minimum Value: numeric fields only
• Maximum Value: numeric fields only
• Skip Attribute: sets the initial state to "Skip", if not modified manually value will be NULL

Initial merge dialog before PR:

Initial merge dialog after PR:

Configuring the policies:

Funded by the National Land Survey of Finland.

Commits tagged with [need-docs] or [FEATURE]

[JuhoErvasti]

As additional context I've edited the PR description slightly, specifically on these points:

• Skip Attribute: sets the initial state to "Skip", if not modified manually value will be NULL
• -> Set to Null: sets the value of the field for the feature to NULL
• Default Value: uses the default value set in QGIS (expressions use a dummy feature with the merged geometry, but referring to other fields won't work)

Because "Skip Attribute" was changed to "Set to Null" the screenshots are slightly outdated.