Add optimized version of QgsPostgresProvider::empty()

qgis · Jun 4, 2018 · b568d6b · b568d6b
1 parent 381a766
commit b568d6b
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diff --git a/python/core/auto_generated/qgsvectorlayer.sip.in b/python/core/auto_generated/qgsvectorlayer.sip.in
@@ -937,6 +937,18 @@ Number of features rendered with specified legend key. Features must be first
 calculated by countSymbolFeatures()
 
 :return: number of features rendered by symbol or -1 if failed or counts are not available
+%End
+
+    virtual bool empty() const;
+
+%Docstring
+Determines if this vector layer is empty.
+A layer is considered empty if either the data provider or
+the edit buffer have features in them.
+This means, in case the data provider contains features which have all
+been deleted in the edit buffer, the result will be false.
+
+.. versionadded:: 3.2
 %End
 
  void setDataSource( const QString &dataSource, const QString &baseName, const QString &provider, bool loadDefaultStyleFlag = false ) /Deprecated/;

diff --git a/src/core/qgsvectorlayer.cpp b/src/core/qgsvectorlayer.cpp
@@ -704,6 +704,8 @@ long QgsVectorLayer::featureCount( const QString &legendKey ) const
   return mSymbolFeatureCountMap.value( legendKey );
 }
 
+
+
 QgsVectorLayerFeatureCounter *QgsVectorLayer::countSymbolFeatures()
 {
   if ( mSymbolFeatureCounted || mFeatureCounter )
@@ -2763,6 +2765,11 @@ long QgsVectorLayer::featureCount() const
          ( mEditBuffer ? mEditBuffer->mAddedFeatures.size() - mEditBuffer->mDeletedFeatureIds.size() : 0 );
 }
 
+bool QgsVectorLayer::empty() const
+{
+  return mDataProvider->empty() && ( !mEditBuffer || mEditBuffer->addedFeatures().empty() );
+}
+
 bool QgsVectorLayer::commitChanges()
 {
   mCommitErrors.clear();

diff --git a/src/core/qgsvectorlayer.h b/src/core/qgsvectorlayer.h
@@ -929,6 +929,17 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
      */
     long featureCount( const QString &legendKey ) const;
 
+    /**
+     * Determines if this vector layer is empty.
+     * A layer is considered empty if either the data provider or
+     * the edit buffer have features in them.
+     * This means, in case the data provider contains features which have all
+     * been deleted in the edit buffer, the result will be false.
+     *
+     * \since QGIS 3.2
+     */
+    bool empty() const override;
+
     /**
      * Update the data source of the layer. The layer's renderer and legend will be preserved only
      * if the geometry type of the new data source matches the current geometry type of the layer.

diff --git a/src/providers/postgres/qgspostgresprovider.cpp b/src/providers/postgres/qgspostgresprovider.cpp
@@ -3187,6 +3187,19 @@ long QgsPostgresProvider::featureCount() const
   return num;
 }
 
+bool QgsPostgresProvider::empty() const
+{
+  QString sql = QStringLiteral( "SELECT EXISTS (SELECT * FROM %1%2 LIMIT 1)" ).arg( mQuery, filterWhereClause() );
+  QgsPostgresResult res( connectionRO()->PQexec( sql ) );
+  if ( res.PQresultStatus() != PGRES_TUPLES_OK )
+  {
+    pushError( res.PQresultErrorMessage() );
+    return false;
+  }
+
+  return res.PQgetvalue( 0, 0 ) != 't';
+}
+
 QgsRectangle QgsPostgresProvider::extent() const
 {
   if ( mGeometryColumn.isNull() )

diff --git a/src/providers/postgres/qgspostgresprovider.h b/src/providers/postgres/qgspostgresprovider.h
@@ -109,6 +109,16 @@ class QgsPostgresProvider : public QgsVectorDataProvider
 
     long featureCount() const override;
 
+    /**
+     * Determines if there is at least one feature avaiable on this table.
+     *
+     * \note In contrast to the featureCount() method, this method is not
+     *       affected by estimated metadata.
+     *
+     * \since QGIS 3.2
+     */
+    bool empty() const override;
+
     /**
      * Returns a string representation of the endian-ness for the layer
      */