Allow setting order by clause for aggregate calculation

python/core/auto_generated/qgsaggregatecalculator.sip.in

@@ -66,6 +66,8 @@ to a data provider for remote calculation.
       QString filter;
 
       QString delimiter;
+
+      QgsFeatureRequest::OrderBy orderBy;
     };
 
     QgsAggregateCalculator( const QgsVectorLayer *layer );

src/core/qgsaggregatecalculator.cpp

@@ -39,6 +39,7 @@ void QgsAggregateCalculator::setParameters( const AggregateParameters &parameter
 {
   mFilterExpression = parameters.filter;
   mDelimiter = parameters.delimiter;
+  mOrderBy = parameters.orderBy;
 }
 
 QVariant QgsAggregateCalculator::calculate( QgsAggregateCalculator::Aggregate aggregate,
@@ -82,6 +83,8 @@ QVariant QgsAggregateCalculator::calculate( QgsAggregateCalculator::Aggregate ag
                                          QgsFeatureRequest::NoFlags :
                                          QgsFeatureRequest::NoGeometry )
                               .setSubsetOfAttributes( lst, mLayer->fields() );
+  if ( !mOrderBy.empty() )
+    request.setOrderBy( mOrderBy );
   if ( !mFilterExpression.isEmpty() )
     request.setFilterExpression( mFilterExpression );
   if ( context )

src/core/qgsaggregatecalculator.h

@@ -22,6 +22,7 @@
 #include "qgsstatisticalsummary.h"
 #include "qgsdatetimestatisticalsummary.h"
 #include "qgsstringstatisticalsummary.h"
+#include "qgsfeaturerequest.h"
 #include <QVariant>
 
 
@@ -103,6 +104,12 @@ class CORE_EXPORT QgsAggregateCalculator
        * \see QgsAggregateCalculator::delimiter()
        */
       QString delimiter;
+
+      /**
+       * Optional order by clauses.
+       * \since QGIS 3.8
+       */
+      QgsFeatureRequest::OrderBy orderBy;
     };
 
     /**
@@ -183,6 +190,9 @@ class CORE_EXPORT QgsAggregateCalculator
     //! Filter expression, or empty for no filter
     QString mFilterExpression;
 
+    //! Order by clause
+    QgsFeatureRequest::OrderBy mOrderBy;
+
     //! Delimiter to use for concatenate aggregate
     QString mDelimiter;
 

tests/src/python/test_qgsaggregatecalculator.py

@@ -21,6 +21,7 @@
                        QgsExpressionContext,
                        QgsExpressionContextScope,
                        QgsGeometry,
+                       QgsFeatureRequest,
                        NULL
                        )
 from qgis.PyQt.QtCore import QDateTime, QDate, QTime
@@ -151,6 +152,20 @@ def testNumeric(self):
             val, ok = agg.calculate(t, 'flddbl')
             self.assertFalse(ok)
 
+        # with order by
+        agg = QgsAggregateCalculator(layer)
+        val, ok = agg.calculate(QgsAggregateCalculator.ArrayAggregate, 'fldint')
+        self.assertEqual(val, [4, 2, 3, 2, 5, NULL, 8])
+        params = QgsAggregateCalculator.AggregateParameters()
+        params.orderBy = QgsFeatureRequest.OrderBy([QgsFeatureRequest.OrderByClause('fldint')])
+        agg.setParameters(params)
+        val, ok = agg.calculate(QgsAggregateCalculator.ArrayAggregate, 'fldint')
+        self.assertEqual(val, [2, 2, 3, 4, 5, 8, NULL])
+        params.orderBy = QgsFeatureRequest.OrderBy([QgsFeatureRequest.OrderByClause('flddbl')])
+        agg.setParameters(params)
+        val, ok = agg.calculate(QgsAggregateCalculator.ArrayAggregate, 'fldint')
+        self.assertEqual(val, [2, 2, 4, 8, 3, 5, NULL])
+
     def testString(self):
         """ Test calculation of aggregates on string fields"""
 
@@ -208,6 +223,18 @@ def testString(self):
             val, ok = agg.calculate(t, 'fldstring')
             self.assertFalse(ok)
 
+        # with order by
+        agg = QgsAggregateCalculator(layer)
+        val, ok = agg.calculate(QgsAggregateCalculator.ArrayAggregate, 'fldstring')
+        self.assertEqual(val, ['cc', 'aaaa', 'bbbbbbbb', 'aaaa', 'eeee', '', 'eeee', '', 'dddd'])
+        params = QgsAggregateCalculator.AggregateParameters()
+        params.orderBy = QgsFeatureRequest.OrderBy([QgsFeatureRequest.OrderByClause('fldstring')])
+        agg.setParameters(params)
+        val, ok = agg.calculate(QgsAggregateCalculator.ArrayAggregate, 'fldstring')
+        self.assertEqual(val, ['', '', 'aaaa', 'aaaa', 'bbbbbbbb', 'cc', 'dddd', 'eeee', 'eeee'])
+        val, ok = agg.calculate(QgsAggregateCalculator.StringConcatenate, 'fldstring')
+        self.assertEqual(val, 'aaaaaaaabbbbbbbbccddddeeeeeeee')
+
     def testDateTime(self):
         """ Test calculation of aggregates on date/datetime fields"""