presto-main/src/main/java/io/prestosql/operator/LookupJoinOperator.java/JoinProcessor/buildPage
class JoinProcessor:        private void buildPage()
        {
            verify(outputPage == null);
            verify(probe != null);

            if (pageBuilder.isEmpty()) {
                return;
            }

            outputPage = pageBuilder.build(probe);
            pageBuilder.reset();
        }