Lucene search engine allows to define one data structure, NUTCH and other search engines have a pre defined structure ( ex. title, url, body). In other side, in a RDBMS we build different tables for different data structure.
How can I store various XML formats, documents together and search it? For Nutch andLucene, we need to remove fieldtypes from filteration criteria. In a database, each table, each column needs to be looked for finding the data. It will be very slow.
These constraints push us develop a schema which can allow search while preserving the structure and allowing write operations.