Let's say I have several entities stored in the AppEngine DataStore that all have an indexed numerical property and I need to find the largest currently existing value.
I could simply do a projection query sorted on this property with descending sort-order and a limit of 1. The returned result would then contain my answer.
Is this the best approach or is there a more efficient way to get this value?
(Consistency is not too much of a worry in this case. If I miss a couple values that are being committed simultaneously, I don't care too much. So, no need for transactions here.)
Since GAE datastore does not have an equivalent to MAX on an SQL database, it is indeed best to order by the column and retrieve the first entry. For example:
max_val = MyEntity.query().order(-MyEntity.my_numeric_field).get().my_numeric_field
See also this forum discussion.
Your second option would be to store the maximum value in some other entity and then update it (probably in a transaction) whenever you add one of your first entities.