Google Datastore [1] works the same way. I imagine it's because indexes can be configured per row. So for example if you have a boolean value and only ever quer

Google Datastore [1] works the same way. I imagine it's because indexes can be configured per row. So for example if you have a boolean value and only ever query for it to be true then you might set the index on only if it's true. The reason you might want to use indexes scarcely is because they cost money. For Google Datastore they used to cost extra write operations and storage. Now they just cost storage. -- [1] https://cloud.google.com/datastore/ https://cloud.google.com/datastore/