private boolean checkExpressions(SelectOperator op) {
    SelectDesc desc = op.getConf();
    if (desc.isSelectStar() || desc.isSelStarNoCompute()) {
      return true;
    }
    for (ExprNodeDesc expr : desc.getColList()) {
      if (!checkExpression(expr)) {
        return false;
      }
    }
    return true;
  }
Tries to convert simple fetch query to single fetch task, which fetches rows directly
from location of table/partition.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/SimpleFetchOptimizer.java