The main goal of Apache Tajo project is to build an advanced open source data warehouse system in Hadoop for processing web-scale data sets. Basically, Tajo provides SQL standard as a query language. Tajo is designed for both interactive and batch queries on data sets stored on HDFS and other data sources. Without hurting query response times, Tajo provides fault-tolerance and dynamic load balancing which are necessary for long-running queries. Tajo employs a cost-based and progressive query optimization techniques for reoptimizing running queries in order to avoid the worst query plans.