What does the distinct on clause mean in cloud datastore and how does it effect the reads?

Question

This is what the cloud datastore doc says but I'm having a hard time understanding what exactly this means:

A projection query that does not use the distinct on clause is a small operation and counts as only a single entity read for the query itself.

Grouping

Projection queries can use the distinct on clause to ensure that only the first result for each distinct combination of values for the specified properties will be returned. This will return only the first result for entities which have the same values for the properties that are being projected.

Let's say i have a table for questions and i only want to get the question text sorted by the created date would this be counted as a single read and rest as small operations?

Answer 1

If your goal is to just project the date and text fields, you can create a composite index on those two fields. When you query, this is a small operation with all the results as a single read. You are not trying to de-duplicate (so no distinct/on) in this case and so it is a small operation with a single read.