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Comment by csytan | Hacker News Reader

Appengine's GQL is a very limited subset of SQL. The only queries which can be performed are those that can scale well, so it's doubtful that even simple things like JOINs will be supported in the future.

http://code.google.com/appengine/docs/python/datastore/gqlre...

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Appengine datastore works by creating indexes for every query. In relational databases you make joins execute fast by creating indexes. I don't see why joins couldn't be added to appengine.

I believe there's a few reasons. one is that the google file system blocks that bigtable runs on and "reads" for your query could be on any number of machines. been a while since I read the google architecture papers but I would recommend it to all. it is fascinating how they implemented gfs, bigtable, chubby lock, etc. the work around is essentially to write your own code that does what a database does - something to the effect of getting a list of keys for the records you are interested in and using that to then scan just the area in your data structure you want.

It hasn't got anything to do with that. Creating an index on a join means that possibly hundreds of indexes need to be updated every time a you make a write - but that is where appengine would actually shine since those writes can be parallelized to hundreds of machines. It could easily make postgres/mysql/oracle redundant.

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