The enormous amount of data generated from different sources in different fields and sectors and in different formats, which poses a challenge to existing data mining techniques, is referred to as big data. Today only big giants in different sectors like retail, medical, sports, banking etc. can think of handling big data because of huge cost and expenses attached to it. Big data relevant to customer purchase behavior is voluminous, volatile, and the veracity attached with big data has forced big retail industries like Walmart ($473 billion), Tesco ($130 billion), and Target ($73 billion), scrambling to bring up new data mining techniques and shifting to cheaper open source software to precisely store and examine data in real time. To be precise in knowing the interest ofcustomers, companies need to regularly mine the data related to each individual customerand gain insight of his or her shopping pattern like which goods he or she buys, customersbudget, customers likes and dislikes etc.The research paper concentrates on highlighting the changes brought in the retail sector by big data. The paper discusses the working of MapReduce technology with an appropriate example.