Custom Indexes and Complex Queries

All queries not covered by the automatic indexes must have corresponding indexes defined in the app’s index configuration file. We’ll refer to these as “custom indexes,” in contrast with “automatic indexes.” App Engine needs these hints because building every possible index for every combination of property and sort order would take a gargantuan amount of space and time, and an app isn’t likely to need more than a fraction of those possibilities.

In particular, the following queries require custom indexes:

A query that uses just equality filters on properties does not need a custom index in most cases thanks to a specialized query algorithm for this case, which we’ll look at in a moment. Also, filters on keys do not require custom indexes; they can operate on whatever indexes are used to fulfill the rest of the query.