Why does Google App Engine restrict GQL queries?

Question

I was reading about App Engine on wikipedia and came across some GQL restrictions:

• JOIN is not supported

• can SELECT from at most one table at a time

• can put at most 1 column in the WHERE clause

What are the advantages of these restrictions?

Are these restrictions common in other places where scalability is a priority?

Answer 1

I believe the answer is in fact to do with the underlying technology of the datastore rather than any kind of restriction on what is available. Google aren't using a relational database under the hood, but instead BigTable, they have just added a nice API which uses SQL like queries to limit the learning curve for those who are used to using a relational database. For those who are more used to using ORM's will take to it like a duck to water.

Answer 2

The datastore that GQL talks to is:

• not a relational database like MySQL or PostgreSQL
• is a Column-oriented DBMS called BigTable

One reason to have a database like this is to have a very high performance database that you can scale across hundreds of servers.

GQL is not SQL it is SQL-like.

Here are some references:

• http://en.wikipedia.org/wiki/Column-oriented_DBMS
• http://en.wikipedia.org/wiki/BigTable
• http://code.google.com/appengine/docs/datastore/overview.html
• http://code.google.com/appengine/docs/datastore/gqlreference.html

Answer 3

the existing answers do a good job with the high-level question.

one additional note: the third restriction you mention isn't actually true. GQL queries can include as many columns in the WHERE clause as you like. there are a few caveats, but number of columns is not explicitly limited. more:

http://code.google.com/appengine/docs/python/datastore/queries.html