[FEATURE] Add QgsVectorDataProvider::truncate for clearing layers

Adds a new method to QgsVectorDataProvider to truncate the layer.
The base implementation requires DeleteFeatures capability and
is not optimised. Providers can return the FastTruncate capability
and override the base implementation with a provider specific
optimised version. This is done in this commit for the Postgres
and Spatialite providers.

Author: nyalldawson

python/core/qgsvectordataprovider.sip

@@ -49,6 +49,8 @@ class QgsVectorDataProvider : QgsDataProvider
       ChangeFeatures,
       /** Supports renaming attributes (fields). Added in QGIS 2.16 */
       RenameAttributes,
+      //! Supports fast truncation of the layer (removing all features). Added in QGIS 3.0
+      FastTruncate,
     };
     typedef QFlags<QgsVectorDataProvider::Capability> Capabilities;
 
@@ -191,11 +193,21 @@ class QgsVectorDataProvider : QgsDataProvider
     virtual bool addFeatures( QList<QgsFeature> &flist /In,Out/ );
 
     /**
-     * Deletes one or more features
+     * Deletes one or more features from the provider. This requires the DeleteFeatures capability.
      * @param id list containing feature ids to delete
      * @return true in case of success and false in case of failure
+     * @see truncate()
      */
-    virtual bool deleteFeatures( const QSet<qint64> &id );
+    virtual bool deleteFeatures( const QgsFeatureIds &id );
+
+    /**
+     * Removes all features from the layer. This requires either the FastTruncate or DeleteFeatures capability.
+     * Providers with the FastTruncate capability will use an optimised method to truncate the layer.
+     * @returns true in case of success and false in case of failure.
+     * @note added in QGIS 3.0
+     * @see deleteFeatures()
+     */
+    virtual bool truncate();
 
     /**
      * Adds new attributes

src/core/qgsvectordataprovider.cpp

@@ -69,6 +69,20 @@ bool QgsVectorDataProvider::deleteFeatures( const QgsFeatureIds &ids )
   return false;
 }
 
+bool QgsVectorDataProvider::truncate()
+{
+  if ( !( capabilities() & DeleteFeatures ) )
+    return false;
+
+  QgsFeatureIds toDelete;
+  QgsFeatureIterator it = getFeatures( QgsFeatureRequest().setFlags( QgsFeatureRequest::NoGeometry ).setSubsetOfAttributes( QgsAttributeList() ) );
+  QgsFeature f;
+  while ( it.nextFeature( f ) )
+    toDelete << f.id();
+
+  return deleteFeatures( toDelete );
+}
+
 bool QgsVectorDataProvider::addAttributes( const QList<QgsField> &attributes )
 {
   Q_UNUSED( attributes );

src/core/qgsvectordataprovider.h

@@ -100,6 +100,8 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
       ChangeFeatures =                              1 << 18,
       //! Supports renaming attributes (fields). Added in QGIS 2.16
       RenameAttributes =                            1 << 19,
+      //! Supports fast truncation of the layer (removing all features). Added in QGIS 3.0
+      FastTruncate =                                    1 << 20,
     };
 
     Q_DECLARE_FLAGS( Capabilities, Capability )
@@ -245,12 +247,22 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
     virtual bool addFeatures( QgsFeatureList &flist );
 
     /**
-     * Deletes one or more features
+     * Deletes one or more features from the provider. This requires the DeleteFeatures capability.
      * @param id list containing feature ids to delete
      * @return true in case of success and false in case of failure
+     * @see truncate()
      */
     virtual bool deleteFeatures( const QgsFeatureIds &id );
 
+    /**
+     * Removes all features from the layer. This requires either the FastTruncate or DeleteFeatures capability.
+     * Providers with the FastTruncate capability will use an optimised method to truncate the layer.
+     * @returns true in case of success and false in case of failure.
+     * @note added in QGIS 3.0
+     * @see deleteFeatures()
+     */
+    virtual bool truncate();
+
     /**
      * Adds new attributes
      * @param attributes list of new attributes

src/providers/postgres/qgspostgresprovider.cpp

@@ -1135,7 +1135,7 @@ bool QgsPostgresProvider::hasSufficientPermsAndCapabilities()
       if ( testAccess.PQgetvalue( 0, 0 ) == QLatin1String( "t" ) )
       {
         // DELETE
-        mEnabledCapabilities |= QgsVectorDataProvider::DeleteFeatures;
+        mEnabledCapabilities |= QgsVectorDataProvider::DeleteFeatures | QgsVectorDataProvider::Truncate;
       }
 
       if ( testAccess.PQgetvalue( 0, 1 ) == QLatin1String( "t" ) )
@@ -2260,6 +2260,63 @@ bool QgsPostgresProvider::deleteFeatures( const QgsFeatureIds & id )
   return returnvalue;
 }
 
+bool QgsPostgresProvider::truncate()
+{
+  bool returnvalue = true;
+
+  if ( mIsQuery )
+  {
+    QgsDebugMsg( "Cannot truncate (is a query)" );
+    return false;
+  }
+
+  QgsPostgresConn* conn = connectionRW();
+  if ( !conn )
+  {
+    return false;
+  }
+  conn->lock();
+
+  try
+  {
+    conn->begin();
+
+    QString sql = QStringLiteral( "TRUNCATE %1" ).arg( mQuery );
+    QgsDebugMsg( "truncate sql: " + sql );
+
+    //send truncate statement and do error handling
+    QgsPostgresResult result( conn->PQexec( sql ) );
+    if ( result.PQresultStatus() != PGRES_COMMAND_OK && result.PQresultStatus() != PGRES_TUPLES_OK )
+      throw PGException( result );
+
+    returnvalue &= conn->commit();
+
+    if ( returnvalue )
+    {
+      if ( mSpatialColType == sctTopoGeometry )
+      {
+        // NOTE: in presence of multiple TopoGeometry objects
+        //       for the same table or when deleting a Geometry
+        //       layer _also_ having a TopoGeometry component,
+        //       orphans would still be left.
+        // TODO: decouple layer from table and signal table when
+        //       records are added or removed
+        dropOrphanedTopoGeoms();
+      }
+      mShared->clear();
+    }
+  }
+  catch ( PGException &e )
+  {
+    pushError( tr( "PostGIS error while truncating: %1" ).arg( e.errorMessage() ) );
+    conn->rollback();
+    returnvalue = false;
+  }
+
+  conn->unlock();
+  return returnvalue;
+}
+
 bool QgsPostgresProvider::addAttributes( const QList<QgsField> &attributes )
 {
   bool returnvalue = true;
@@ -4706,3 +4763,12 @@ QVariantList QgsPostgresSharedData::lookupKey( QgsFeatureId featureId )
     return it.value();
   return QVariantList();
 }
+
+void QgsPostgresSharedData::clear()
+{
+  QMutexLocker locker( &mMutex );
+  mFidToKey.clear();
+  mKeyToFid.clear();
+  mFeaturesCounted = -1;
+  mFidCounter = 0;
+}

src/providers/postgres/qgspostgresprovider.h

@@ -126,9 +126,6 @@ class QgsPostgresProvider : public QgsVectorDataProvider
     void setExtent( QgsRectangle& newExtent );
 
     virtual QgsRectangle extent() const override;
-
-    /** Update the extent
-     */
     virtual void updateExtents() override;
 
     /**
@@ -145,13 +142,7 @@ class QgsPostgresProvider : public QgsVectorDataProvider
     void determinePrimaryKeyFromUriKeyColumn();
 
     QgsFields fields() const override;
-
-    /**
-     * Return a short comment for the data that this provider is
-     * providing access to (e.g. the comment for postgres table).
-     */
     QString dataComment() const override;
-
     QVariant minimumValue( int index ) const override;
     QVariant maximumValue( int index ) const override;
     virtual void uniqueValues( int index, QList<QVariant> &uniqueValues, int limit = -1 ) const override;
@@ -165,42 +156,14 @@ class QgsPostgresProvider : public QgsVectorDataProvider
     QString defaultValueClause( int fieldId ) const override;
     QVariant defaultValue( int fieldId ) const override;
     bool skipConstraintCheck( int fieldIndex, QgsFieldConstraints::Constraint constraint, const QVariant& value = QVariant() ) const override;
-
-    /** Adds a list of features
-      @return true in case of success and false in case of failure*/
     bool addFeatures( QgsFeatureList & flist ) override;
-
-    /** Deletes a list of features
-      @param id list of feature ids
-      @return true in case of success and false in case of failure*/
     bool deleteFeatures( const QgsFeatureIds & id ) override;
-
+    bool truncate() override;
     bool addAttributes( const QList<QgsField> &attributes ) override;
     bool deleteAttributes( const QgsAttributeIds &name ) override;
     virtual bool renameAttributes( const QgsFieldNameMap& renamedAttributes ) override;
-
-    /** Changes attribute values of existing features
-      @param attr_map a map containing the new attributes. The integer is the feature id,
-      the first QString is the attribute name and the second one is the new attribute value
-      @return true in case of success and false in case of failure*/
     bool changeAttributeValues( const QgsChangedAttributesMap &attr_map ) override;
-
-    /**
-       Changes geometries of existing features
-       @param geometry_map   A QMap containing the feature IDs to change the geometries of.
-                             the second map parameter being the new geometries themselves
-       @return               true in case of success and false in case of failure
-     */
     bool changeGeometryValues( const QgsGeometryMap &geometry_map ) override;
-
-    /**
-     * Changes attribute values and geometries of existing features.
-     * @param attr_map a map containing changed attributes
-     * @param geometry_map   A QgsGeometryMap whose index contains the feature IDs
-     *                       that will have their geometries changed.
-     *                       The second map parameter being the new geometries themselves
-     * @return true in case of success and false in case of failure
-     */
     bool changeFeatures( const QgsChangedAttributesMap &attr_map, const QgsGeometryMap &geometry_map ) override;
 
     //! Get the postgres connection
@@ -559,6 +522,7 @@ class QgsPostgresSharedData
     QVariantList removeFid( QgsFeatureId fid );
     void insertFid( QgsFeatureId fid, const QVariantList& k );
     QVariantList lookupKey( QgsFeatureId featureId );
+    void clear();
 
   protected:
     QMutex mMutex; //!< Access to all data members is guarded by the mutex

src/providers/spatialite/qgsspatialiteprovider.cpp

@@ -532,7 +532,7 @@ QgsSpatiaLiteProvider::QgsSpatiaLiteProvider( QString const &uri )
   if (( mTableBased || mViewBased ) &&  !mReadOnly )
   {
     // enabling editing only for Tables [excluding Views and VirtualShapes]
-    mEnabledCapabilities |= QgsVectorDataProvider::DeleteFeatures;
+    mEnabledCapabilities |= QgsVectorDataProvider::DeleteFeatures | QgsVectorDataProvider::Truncate;
     if ( !mGeometryColumn.isEmpty() )
       mEnabledCapabilities |= QgsVectorDataProvider::ChangeGeometries;
     mEnabledCapabilities |= QgsVectorDataProvider::ChangeAttributeValues;
@@ -4119,6 +4119,36 @@ bool QgsSpatiaLiteProvider::deleteFeatures( const QgsFeatureIds &id )
   return true;
 }
 
+bool QgsSpatiaLiteProvider::truncate()
+{
+  char *errMsg = nullptr;
+  QString sql;
+
+  int ret = sqlite3_exec( mSqliteHandle, "BEGIN", nullptr, nullptr, &errMsg );
+  if ( ret != SQLITE_OK )
+  {
+    handleError( sql, errMsg );
+    return false;
+  }
+
+  sql = QStringLiteral( "DELETE FROM %1" ).arg( quotedIdentifier( mTableName ) );
+  ret = sqlite3_exec( mSqliteHandle, sql.toUtf8().constData(), nullptr, nullptr, &errMsg );
+  if ( ret != SQLITE_OK )
+  {
+    handleError( sql, errMsg, true );
+    return false;
+  }
+
+  ret = sqlite3_exec( mSqliteHandle, "COMMIT", nullptr, nullptr, &errMsg );
+  if ( ret != SQLITE_OK )
+  {
+    handleError( sql, errMsg, true );
+    return false;
+  }
+
+  return true;
+}
+
 bool QgsSpatiaLiteProvider::addAttributes( const QList<QgsField> &attributes )
 {
   char *errMsg = nullptr;

src/providers/spatialite/qgsspatialiteprovider.h

@@ -141,10 +141,8 @@ class QgsSpatiaLiteProvider: public QgsVectorDataProvider
       @return true in case of success and false in case of failure*/
     bool addFeatures( QgsFeatureList & flist ) override;
 
-    /** Deletes a list of features
-      @param id list of feature ids
-      @return true in case of success and false in case of failure*/
     bool deleteFeatures( const QgsFeatureIds & id ) override;
+    bool truncate() override;
 
     /** Adds new attributes
       @param name map with attribute name as key and type as value

tests/src/python/providertestbase.py

@@ -737,6 +737,28 @@ def testDeleteFeatures(self):
             self.assertFalse(l.dataProvider().deleteFeatures(to_delete),
                              'Provider reported no DeleteFeatures capability, but returned true to deleteFeatures')
 
+    def testTruncate(self):
+        if not getattr(self, 'getEditableLayer', None):
+            return
+
+        l = self.getEditableLayer()
+        self.assertTrue(l.isValid())
+
+        features = [f['pk'] for f in l.dataProvider().getFeatures()]
+
+        if l.dataProvider().capabilities() & QgsVectorDataProvider.FastTruncate or l.dataProvider().capabilities() & QgsVectorDataProvider.DeleteFeatures:
+            # expect success
+            result = l.dataProvider().truncate()
+            self.assertTrue(result, 'Provider reported FastTruncate or DeleteFeatures capability, but returned False to truncate()')
+
+            # check result
+            features = [f['pk'] for f in l.dataProvider().getFeatures()]
+            self.assertEqual(len(features), 0)
+        else:
+            # expect fail
+            self.assertFalse(l.dataProvider().truncate(),
+                             'Provider reported no FastTruncate or DeleteFeatures capability, but returned true to truncate()')
+
     def testChangeAttributes(self):
         if not getattr(self, 'getEditableLayer', None):
             return