TAJO-836

Hi Hyunsik,
I also deeply thought about the directory structure. 
In MHO, your suggestion might cause unnecessary operations while reading table data. That is, the scanner should check that the file(or directory) is index or table data.

Also, in PostgreSQL, catalog maintains indexes as well as tables as relations. In other words, indexes are treated a kind of relations. This policy prevents any indexes in a database from having the same name.

So, how about the directory structure of ```${DATABASE_NAME}/${INDEX_NAME}```? I think that this structure can help the index management and also dose not disturb other operations.