[mssql] Small fixes to expression compilation

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

src/providers/mssql/qgsmssqlexpressioncompiler.cpp

@@ -77,3 +77,12 @@ QString QgsMssqlExpressionCompiler::quotedValue( const QVariant& value, bool& ok
       return QgsSqlExpressionCompiler::quotedValue( value, ok );
   }
 }
+
+QString QgsMssqlExpressionCompiler::quotedIdentifier( const QString &identifier )
+{
+  QString quoted = identifier;
+  quoted.replace( '[', "[[" );
+  quoted.replace( ']', "]]" );
+  quoted = quoted.prepend( '[' ).append( ']' );
+  return quoted;
+}

src/providers/mssql/qgsmssqlexpressioncompiler.h

@@ -29,6 +29,7 @@ class QgsMssqlExpressionCompiler : public QgsSqlExpressionCompiler
   protected:
     virtual Result compileNode( const QgsExpression::Node* node, QString& result ) override;
     virtual QString quotedValue( const QVariant& value, bool& ok ) override;
+    virtual QString quotedIdentifier( const QString& identifier ) override;
 
 };
 

src/providers/mssql/qgsmssqlfeatureiterator.cpp

@@ -109,7 +109,7 @@ void QgsMssqlFeatureIterator::BuildStatement( const QgsFeatureRequest& request )
     mStatement += QString( ",[%1]" ).arg( mSource->mGeometryColName );
   }
 
-  mStatement += QString( "FROM [%1].[%2]" ).arg( mSource->mSchemaName, mSource->mTableName );
+  mStatement += QString( " FROM [%1].[%2]" ).arg( mSource->mSchemaName, mSource->mTableName );
 
   bool filterAdded = false;
   // set spatial filter
@@ -247,7 +247,7 @@ void QgsMssqlFeatureIterator::BuildStatement( const QgsFeatureRequest& request )
     mOrderByCompiled = false;
   }
 
-  if ( !mOrderByCompiled )
+  if ( !mOrderByCompiled && !request.orderBy().isEmpty() )
     limitAtProvider = false;
 
   if ( request.limit() >= 0 && limitAtProvider )