if (null != spec) {
spec.setQuery(query);
spec.setConf(planConf);
// Given a flow specification from the AST, run it through
// necessary post-processing and optimization phases.
spec.bfs(new PropagateSchemas());
if (retContext.isExplain()) {
// We just should explain this flow, but not actually add it.
msgBuilder.append("Execution plan:\n");
msgBuilder.append(spec.toString());
msgBuilder.append("\n");