[mssql] Small fixes to expression compilation

- Use [] to correctly quote field names instead of ""
- Don't invalidate provider side limit when not required

Forward port of 8dec70c, because I'm only human and forgot

Author: nyalldawson

src/providers/mssql/qgsmssqlexpressioncompiler.cpp

@@ -91,6 +91,15 @@ QString QgsMssqlExpressionCompiler::quotedValue( const QVariant &value, bool &ok
   }
 }
 
+QString QgsMssqlExpressionCompiler::quotedIdentifier( const QString &identifier )
+{
+  QString quoted = identifier;
+  quoted.replace( '[', "[[" );
+  quoted.replace( ']', "]]" );
+  quoted = quoted.prepend( '[' ).append( ']' );
+  return quoted;
+}
+
 QString QgsMssqlExpressionCompiler::castToReal( const QString &value ) const
 {
   return QStringLiteral( "CAST((%1) AS REAL)" ).arg( value );

src/providers/mssql/qgsmssqlexpressioncompiler.h

@@ -29,6 +29,7 @@ class QgsMssqlExpressionCompiler : public QgsSqlExpressionCompiler
   protected:
     Result compileNode( const QgsExpressionNode *node, QString &result ) override;
     QString quotedValue( const QVariant &value, bool &ok ) override;
+    QString quotedIdentifier( const QString& identifier ) override;
     QString castToReal( const QString &value ) const override;
     QString castToInt( const QString &value ) const override;
     QString sqlFunctionFromFunctionName( const QString &fnName ) const override;

src/providers/mssql/qgsmssqlfeatureiterator.cpp

@@ -265,7 +265,7 @@ void QgsMssqlFeatureIterator::BuildStatement( const QgsFeatureRequest &request )
     mOrderByCompiled = false;
   }
 
-  if ( !mOrderByCompiled )
+  if ( !mOrderByCompiled && !request.orderBy().isEmpty() )
     limitAtProvider = false;
 
   if ( request.limit() >= 0 && limitAtProvider )