From 1d2bb41752c51e6c9b84124a1ce1599d3d42b41c Mon Sep 17 00:00:00 2001
From: Nyall Dawson <nyall.dawson@gmail.com>
Date: Mon, 12 Oct 2020 08:22:13 +1000
Subject: [PATCH] Lazy construct expression context for memory provider feature
 sources

It's not free to calculate, and is only used when
iterating over a memory layer with a subset string set

Results in a big speed up to scripts which fire off many individual
feature requests to a memory provider layer
---
 .../memory/qgsmemoryfeatureiterator.cpp       | 30 +++++++++++++------
 .../memory/qgsmemoryfeatureiterator.h         |  4 ++-
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/src/core/providers/memory/qgsmemoryfeatureiterator.cpp b/src/core/providers/memory/qgsmemoryfeatureiterator.cpp
index 20b041e866ee..4a0a8a5ec9cf 100644
--- a/src/core/providers/memory/qgsmemoryfeatureiterator.cpp
+++ b/src/core/providers/memory/qgsmemoryfeatureiterator.cpp
@@ -47,7 +47,7 @@ QgsMemoryFeatureIterator::QgsMemoryFeatureIterator( QgsMemoryFeatureSource *sour
   if ( !mSource->mSubsetString.isEmpty() )
   {
     mSubsetExpression = qgis::make_unique< QgsExpression >( mSource->mSubsetString );
-    mSubsetExpression->prepare( &mSource->mExpressionContext );
+    mSubsetExpression->prepare( mSource->expressionContext() );
   }
 
   if ( !mFilterRect.isNull() && mRequest.flags() & QgsFeatureRequest::ExactIntersect )
@@ -63,7 +63,7 @@ QgsMemoryFeatureIterator::QgsMemoryFeatureIterator( QgsMemoryFeatureSource *sour
   {
     mUsingFeatureIdList = true;
     mFeatureIdList = mSource->mSpatialIndex->intersects( mFilterRect );
-    QgsDebugMsg( "Features returned by spatial index: " + QString::number( mFeatureIdList.count() ) );
+    QgsDebugMsgLevel( "Features returned by spatial index: " + QString::number( mFeatureIdList.count() ), 2 );
   }
   else if ( mRequest.filterType() == QgsFeatureRequest::FilterFid )
   {
@@ -138,8 +138,8 @@ bool QgsMemoryFeatureIterator::nextFeatureUsingList( QgsFeature &feature )
 
     if ( hasFeature && mSubsetExpression )
     {
-      mSource->mExpressionContext.setFeature( candidate );
-      if ( !mSubsetExpression->evaluate( &mSource->mExpressionContext ).toBool() )
+      mSource->expressionContext()->setFeature( candidate );
+      if ( !mSubsetExpression->evaluate( mSource->expressionContext() ).toBool() )
         hasFeature = false;
     }
 
@@ -198,8 +198,8 @@ bool QgsMemoryFeatureIterator::nextFeatureTraverseAll( QgsFeature &feature )
 
     if ( mSubsetExpression )
     {
-      mSource->mExpressionContext.setFeature( *mSelectIterator );
-      if ( !mSubsetExpression->evaluate( &mSource->mExpressionContext ).toBool() )
+      mSource->expressionContext()->setFeature( *mSelectIterator );
+      if ( !mSubsetExpression->evaluate( mSource->expressionContext() ).toBool() )
         hasFeature = false;
     }
 
@@ -257,9 +257,6 @@ QgsMemoryFeatureSource::QgsMemoryFeatureSource( const QgsMemoryProvider *p )
   , mSubsetString( p->mSubsetString )
   , mCrs( p->mCrs )
 {
-  mExpressionContext << QgsExpressionContextUtils::globalScope()
-                     << QgsExpressionContextUtils::projectScope( QgsProject::instance() );
-  mExpressionContext.setFields( mFields );
 }
 
 QgsFeatureIterator QgsMemoryFeatureSource::getFeatures( const QgsFeatureRequest &request )
@@ -267,4 +264,19 @@ QgsFeatureIterator QgsMemoryFeatureSource::getFeatures( const QgsFeatureRequest
   return QgsFeatureIterator( new QgsMemoryFeatureIterator( this, false, request ) );
 }
 
+QgsExpressionContext *QgsMemoryFeatureSource::expressionContext()
+{
+  // lazy construct expression context -- it's not free to calculate, and is only used when
+  // iterating over a memory layer with a subset string set
+  if ( !mExpressionContext )
+  {
+    mExpressionContext = qgis::make_unique< QgsExpressionContext >(
+                           QList<QgsExpressionContextScope *>()
+                           << QgsExpressionContextUtils::globalScope()
+                           << QgsExpressionContextUtils::projectScope( QgsProject::instance() ) );
+    mExpressionContext->setFields( mFields );
+  }
+  return mExpressionContext.get();
+}
+
 ///@endcond PRIVATE
diff --git a/src/core/providers/memory/qgsmemoryfeatureiterator.h b/src/core/providers/memory/qgsmemoryfeatureiterator.h
index 1c771dab6edd..404fccec17b4 100644
--- a/src/core/providers/memory/qgsmemoryfeatureiterator.h
+++ b/src/core/providers/memory/qgsmemoryfeatureiterator.h
@@ -38,12 +38,14 @@ class QgsMemoryFeatureSource final: public QgsAbstractFeatureSource
 
     QgsFeatureIterator getFeatures( const QgsFeatureRequest &request ) override;
 
+    QgsExpressionContext *expressionContext();
+
   private:
     QgsFields mFields;
     QgsFeatureMap mFeatures;
     std::unique_ptr< QgsSpatialIndex > mSpatialIndex;
     QString mSubsetString;
-    QgsExpressionContext mExpressionContext;
+    std::unique_ptr< QgsExpressionContext > mExpressionContext;
     QgsCoordinateReferenceSystem mCrs;
 
     friend class QgsMemoryFeatureIterator;