Don't wastefully recalculate memory provider extent after every

feature addition

Previously, the memory provider would automatically recalculate
the extent of the layer after new features are added by
looping through the entire set of existing features and calculating
the bounding boxes. This is very wasteful, as many code paths
add features one-by-one, so with every new feature added to
the provider every existing feature is iterated over. This caused
memory layers to slow to a crawl after many features are added.

This commit improves the logic so that IF an existing layer
extent is known, then it's updated on the fly as each individual
feauture is added. Instead of looping through all features, we
just expand the existing known extent with the added features
bounds. If the extent isn't known, we just invalidate it
when adding/deleting/modifying features, and defer the actual
extent calculation until it's next requested.

Makes memory layers many thousands of magnitudes of orders faster
when adding lots of features (e.g. when memory providers
are used as temporary outputs in processing)

Author: nyalldawson

src/core/providers/memory/qgsmemoryprovider.cpp

@@ -272,6 +272,16 @@ QgsFeatureIterator QgsMemoryProvider::getFeatures( const QgsFeatureRequest &requ
 
 QgsRectangle QgsMemoryProvider::extent() const
 {
+  if ( mExtent.isEmpty() && !mFeatures.isEmpty() )
+  {
+    mExtent.setMinimal();
+    Q_FOREACH ( const QgsFeature &feat, mFeatures )
+    {
+      if ( feat.hasGeometry() )
+        mExtent.unionRect( feat.geometry().boundingBox() );
+    }
+  }
+
   return mExtent;
 }
 
@@ -315,6 +325,9 @@ QgsCoordinateReferenceSystem QgsMemoryProvider::crs() const
 
 bool QgsMemoryProvider::addFeatures( QgsFeatureList &flist )
 {
+  // whether or not to update the layer extent on the fly as we add features
+  bool updateExtent = mFeatures.isEmpty() || !mExtent.isEmpty();
+
   // TODO: sanity checks of fields and geometries
   for ( QgsFeatureList::iterator it = flist.begin(); it != flist.end(); ++it )
   {
@@ -323,15 +336,19 @@ bool QgsMemoryProvider::addFeatures( QgsFeatureList &flist )
 
     mFeatures.insert( mNextFeatureId, *it );
 
-    // update spatial index
-    if ( mSpatialIndex )
-      mSpatialIndex->insertFeature( *it );
+    if ( it->hasGeometry() )
+    {
+      if ( updateExtent )
+        mExtent.combineExtentWith( it->geometry().boundingBox() );
+
+      // update spatial index
+      if ( mSpatialIndex )
+        mSpatialIndex->insertFeature( *it );
+    }
 
     mNextFeatureId++;
   }
 
-  updateExtent();
-
   return true;
 }
 
@@ -352,7 +369,7 @@ bool QgsMemoryProvider::deleteFeatures( const QgsFeatureIds &id )
     mFeatures.erase( fit );
   }
 
-  updateExtent();
+  updateExtents();
 
   return true;
 }
@@ -471,7 +488,7 @@ bool QgsMemoryProvider::changeGeometryValues( const QgsGeometryMap &geometry_map
       mSpatialIndex->insertFeature( *fit );
   }
 
-  updateExtent();
+  updateExtents();
 
   return true;
 }
@@ -522,21 +539,9 @@ QgsVectorDataProvider::Capabilities QgsMemoryProvider::capabilities() const
 }
 
 
-void QgsMemoryProvider::updateExtent()
+void QgsMemoryProvider::updateExtents()
 {
-  if ( mFeatures.isEmpty() )
-  {
-    mExtent = QgsRectangle();
-  }
-  else
-  {
-    mExtent.setMinimal();
-    Q_FOREACH ( const QgsFeature &feat, mFeatures )
-    {
-      if ( feat.hasGeometry() )
-        mExtent.unionRect( feat.geometry().boundingBox() );
-    }
-  }
+  mExtent.setMinimal();
 }
 
 QString QgsMemoryProvider::name() const

src/core/providers/memory/qgsmemoryprovider.h

@@ -68,22 +68,18 @@ class QgsMemoryProvider : public QgsVectorDataProvider
     QString name() const override;
     QString description() const override;
     virtual QgsRectangle extent() const override;
+    void updateExtents() override;
     bool isValid() const override;
     virtual QgsCoordinateReferenceSystem crs() const override;
 
-  protected:
-
-    // called when added / removed features or geometries has been changed
-    void updateExtent();
-
   private:
     // Coordinate reference system
     QgsCoordinateReferenceSystem mCrs;
 
     // fields
     QgsFields mFields;
     QgsWkbTypes::Type mWkbType;
-    QgsRectangle mExtent;
+    mutable QgsRectangle mExtent;
 
     // features
     QgsFeatureMap mFeatures;