1. A tick database system for storing and retrieving market data, the system comprising: one or more computers and operatively associated computer-readable media configured to: receive and store, according to a schema, market data comprising data regarding individual transactions for investment instruments; wherein the schema enables at least database column definitions including field identifiers, data type identifiers and corresponding field size indicators; wherein the received data regarding individual transactions is, for each transaction, stored as a discrete tick comprising a plurality of fields including at least (i) a ticksize field where the data in the ticksize field indicates a data size of the discrete tick; and (ii) one or more field pairs comprising field identifier fields and value fields; wherein the data in the field identifier field indicates a particular field identifier and the data in the value field indicates an associated value; wherein the data size of the discrete tick varies based on the received market data for the corresponding discrete tick; and wherein if a value is not available for a particular field identifier, the discrete tick does not include that particular field identifier.