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Why are aggregate functions like group_by not supported in hibernate search?

I have a use case where i need to fetch results after applying group by in the query.

Shreyash Hisariya
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There is no technical reason, if this is what you mean. We could probably add it, but there simply wasn't enough demand for this feature to make it to the top of our priority list.

If you want to see a feature added to Hibernate Search, feel free to create a ticket on our JIRA instance, describing in details your use case and the API you would expect.

Note that I am not 100% sure we would implement it for the Lucene backend, since that would probably require a lot of effort. But for people using Elasticsearch behind Hibernate Search, we may at least introduce ways to use Elasticsearch's aggregation support from within Hibernate Search. We are currently experimenting with Hibernate Search 6 and trying this is on my checklist.

In the meantime, if you want us to suggest alternatives, please provide more details about your use case: domain model, mapping, fields you would like to aggregate as part of your "group by"...

yrodiere
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Why it's missing

The primary reason for this to not be support by Hibernate Search is that noone ever asked for it or contributed it.

Another reason is that since the results would be "groups of entities" while the FulltextQuery API returns a List of entities, this would need a new API specifically to run such queries.

How to get it added

We could make that, but if there is not much interest in the feature it would possibly not be worth the maintenance work.

If you need such a feature I suggest you open an issue on the Hibernate Search issue tracker so that other people can also vote or express interest for it. Ideally, someone needing it like yourself might be willing to create a patch or at least start a proof of concept.

Alternatives

Until Hibernate Search provides direct support for it, you can still run such queries yourself. See Using IndexReaders directly to work on the Lucene index directly.

Using the IndexReaders you can always read and Search on Lucene using any advanced feature for which Hibernate Search doesn't provide an API.

Sanne
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