Speed up spatialite provider

No need to iterate over all the features in the datasource only to find
out that there is no feature which is not in an empty list of feature
ids.

src/providers/spatialite/qgsspatialitefeatureiterator.cpp

@@ -53,7 +53,7 @@ QgsSpatiaLiteFeatureIterator::QgsSpatiaLiteFeatureIterator( QgsSpatiaLiteFeature
   catch ( QgsCsException & )
   {
     // can't reproject mFilterRect
-    mClosed = true;
+    close();
     return;
   }
 
@@ -90,10 +90,13 @@ QgsSpatiaLiteFeatureIterator::QgsSpatiaLiteFeatureIterator( QgsSpatiaLiteFeature
   }
   else if ( request.filterType() == QgsFeatureRequest::FilterFids )
   {
-    whereClause = whereClauseFids();
-    if ( ! whereClause.isEmpty() )
+    if ( request.filterFids().isEmpty() )
     {
-      whereClauses.append( whereClause );
+      close();
+    }
+    else
+    {
+      whereClauses.append( whereClauseFids() );
     }
   }
   //IMPORTANT - this MUST be the last clause added!
@@ -144,77 +147,80 @@ QgsSpatiaLiteFeatureIterator::QgsSpatiaLiteFeatureIterator( QgsSpatiaLiteFeature
     }
   }
 
-  whereClause = whereClauses.join( QStringLiteral( " AND " ) );
+  if ( !mClosed )
+  {
+    whereClause = whereClauses.join( QStringLiteral( " AND " ) );
 
-  // Setup the order by
-  QStringList orderByParts;
+    // Setup the order by
+    QStringList orderByParts;
 
-  mOrderByCompiled = true;
+    mOrderByCompiled = true;
 
-  if ( QgsSettings().value( QStringLiteral( "qgis/compileExpressions" ), true ).toBool() )
-  {
-    Q_FOREACH ( const QgsFeatureRequest::OrderByClause &clause, request.orderBy() )
+    if ( QgsSettings().value( QStringLiteral( "qgis/compileExpressions" ), true ).toBool() )
     {
-      QgsSQLiteExpressionCompiler compiler = QgsSQLiteExpressionCompiler( source->mFields );
-      QgsExpression expression = clause.expression();
-      if ( compiler.compile( &expression ) == QgsSqlExpressionCompiler::Complete )
+      Q_FOREACH ( const QgsFeatureRequest::OrderByClause &clause, request.orderBy() )
       {
-        QString part;
-        part = compiler.result();
+        QgsSQLiteExpressionCompiler compiler = QgsSQLiteExpressionCompiler( source->mFields );
+        QgsExpression expression = clause.expression();
+        if ( compiler.compile( &expression ) == QgsSqlExpressionCompiler::Complete )
+        {
+          QString part;
+          part = compiler.result();
 
-        if ( clause.nullsFirst() )
-          orderByParts << QStringLiteral( "%1 IS NOT NULL" ).arg( part );
-        else
-          orderByParts << QStringLiteral( "%1 IS NULL" ).arg( part );
+          if ( clause.nullsFirst() )
+            orderByParts << QStringLiteral( "%1 IS NOT NULL" ).arg( part );
+          else
+            orderByParts << QStringLiteral( "%1 IS NULL" ).arg( part );
 
-        part += clause.ascending() ? " COLLATE NOCASE ASC" : " COLLATE NOCASE DESC";
-        orderByParts << part;
-      }
-      else
-      {
-        // Bail out on first non-complete compilation.
-        // Most important clauses at the beginning of the list
-        // will still be sent and used to pre-sort so the local
-        // CPU can use its cycles for fine-tuning.
-        mOrderByCompiled = false;
-        break;
+          part += clause.ascending() ? " COLLATE NOCASE ASC" : " COLLATE NOCASE DESC";
+          orderByParts << part;
+        }
+        else
+        {
+          // Bail out on first non-complete compilation.
+          // Most important clauses at the beginning of the list
+          // will still be sent and used to pre-sort so the local
+          // CPU can use its cycles for fine-tuning.
+          mOrderByCompiled = false;
+          break;
+        }
       }
     }
-  }
-  else
-  {
-    mOrderByCompiled = false;
-  }
+    else
+    {
+      mOrderByCompiled = false;
+    }
 
-  if ( !mOrderByCompiled )
-    limitAtProvider = false;
+    if ( !mOrderByCompiled )
+      limitAtProvider = false;
 
-  // also need attributes required by order by
-  if ( !mOrderByCompiled && mRequest.flags() & QgsFeatureRequest::SubsetOfAttributes && !mRequest.orderBy().isEmpty() )
-  {
-    QSet<int> attributeIndexes;
-    Q_FOREACH ( const QString &attr, mRequest.orderBy().usedAttributes() )
+    // also need attributes required by order by
+    if ( !mOrderByCompiled && mRequest.flags() & QgsFeatureRequest::SubsetOfAttributes && !mRequest.orderBy().isEmpty() )
     {
-      attributeIndexes << mSource->mFields.lookupField( attr );
+      QSet<int> attributeIndexes;
+      Q_FOREACH ( const QString &attr, mRequest.orderBy().usedAttributes() )
+      {
+        attributeIndexes << mSource->mFields.lookupField( attr );
+      }
+      attributeIndexes += mRequest.subsetOfAttributes().toSet();
+      mRequest.setSubsetOfAttributes( attributeIndexes.toList() );
     }
-    attributeIndexes += mRequest.subsetOfAttributes().toSet();
-    mRequest.setSubsetOfAttributes( attributeIndexes.toList() );
-  }
 
-  // preparing the SQL statement
-  bool success = prepareStatement( whereClause, limitAtProvider ? mRequest.limit() : -1, orderByParts.join( QStringLiteral( "," ) ) );
-  if ( !success && useFallbackWhereClause )
-  {
-    //try with the fallback where clause, e.g., for cases when using compiled expression failed to prepare
-    mExpressionCompiled = false;
-    success = prepareStatement( fallbackWhereClause, -1, orderByParts.join( QStringLiteral( "," ) ) );
-  }
+    // preparing the SQL statement
+    bool success = prepareStatement( whereClause, limitAtProvider ? mRequest.limit() : -1, orderByParts.join( QStringLiteral( "," ) ) );
+    if ( !success && useFallbackWhereClause )
+    {
+      //try with the fallback where clause, e.g., for cases when using compiled expression failed to prepare
+      mExpressionCompiled = false;
+      success = prepareStatement( fallbackWhereClause, -1, orderByParts.join( QStringLiteral( "," ) ) );
+    }
 
-  if ( !success )
-  {
-    // some error occurred
-    sqliteStatement = nullptr;
-    close();
+    if ( !success )
+    {
+      // some error occurred
+      sqliteStatement = nullptr;
+      close();
+    }
   }
 }