Data storage for unstructured data such as JSON data stored as collections of documents transforms the JSON data into a columnar form of storing unstructured data by grouping similar fields together for facilitating retrieval of the individual fields from a range of documents. Groups of fields are stored in individual files for each field. Compound data such as arrays and subdocuments are also broken down into files for each atomic field. In other words, a compound document structure that defines a hierarchy or “tree” of fields is flattened such that each “leaf” of the tree is stored in a separate file.