[FEATURE] Aggregate calculation in expression engine #3073
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This can be more efficient for large quantities of values, since it avoids the need to create a list of all values in advance
aggregates from vector layer fields and expressions
(not implemented for any providers yet)
Can be used to store the results of expensive sub-expression calculations (eg layer aggregates), so that future expression evaluation using the same context does not have to recalculate the cached values.
Also - Begin unit tests for QgsRelationManager - Make project optional when constructing QgsRelationManager
This commit adds a number of different forms of aggregates to
the expression engine.
1. Aggregates within the current layer, eg sum("passengers")
Supports sub expressions (ie sum("passengers"/2) ), group by
( sum("passengers", group_by:="line_segment") ), and optional
filters ( sum("passengers", filter:= "station_class" > 3 ) )
2. Relational aggregates, which calculate an aggregate over
all matching child features from a relation, eg
relation_aggregate( 'my_relation', 'mean', "some_child_field" )
3. A summary aggregate function, for calculating aggregates
on other layers. Eg aggregate('rail_station_layer','sum',"passengers")
The summary aggregate function supports an optional filter,
making it possible to calculate things like:
aggregate('rail_stations','sum',"passengers",
intersects(@atlas_geometry, $geometry ) )
for calculating the total number of passengers for the stations
inside the current atlas feature
In all cases the calculations are cached inside the expression
context, so they only need to be calculated once for each
set of expression evaluations.
Sponsored by Kanton of Zug, Switzerland
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Manually merged |
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Implements qgis/QGIS-Enhancement-Proposals#59