#1 Dictionary is mainly for storage saving Consider a column like ³country² , by using dictionary, we can encode the country into only 1 byte (let¹s say there are no more than 256 countries in the world) while if not using dictionary ³People¹s republic of China² will cost a lot more space to store.
#2. Dictionary is more like a k-v map, k is a order-preserved id with same size(currently, maybe in the future some other encoding like Huffman will be introduced), and v is the actual value. By using dictionary, all value in the same column can have the same size, thus the size of a record is also the same. This will make us easier to decode and encode. (Maybe some other encoding like Huffman) Currently we use TRIE to build dictionary. Best Regard Zhou QianHao On 3/18/15, 5:51 PM, "koteswara Vemu" <[email protected]> wrote: >Hi, > > What is the use of creating dimension dictionary? It will be a >great >help if you can explain how the data is stored in dictionary > > >Regards, >Koti
