I’m trying to write a GQL query that returns N random records of a specific kind. My current implementation works but requires N calls to the datastore. I’d like to make it 1 call to the datastore if possible.
I currently assign a random number to every kind that I put into the datastore. When I query for a random record I generate another random number and query for records > rand ORDER BY asc LIMIT 1.
This works, however, it only returns 1 record so I need to do N queries. Any ideas on how to make this one query? Thanks.
“Under the hood” a single search query call can only return a set of consecutive rows from some index. This is why some GQL queries, including any use of !=, expand to multiple datastore calls.
N independent uniform random selections are not (in general) consecutive in any index.
QED.
You could probably use memcache to store the entities, and reduce the cost of grabbing N of them. Or if you don’t mind the “random” selections being close together in the index, select a randomly-chosen block of (say) 100 in one query, then pick N at random from those. Since you have a field that’s already randomised, it won’t be immediately obvious to an outsider that the N items are related. At least, not until they look at a lot of samples and notice that items A and Z never appear in the same group, because they’re more than 100 apart in the randomised index. And if performance permits, you can re-randomise your entities from time to time.