JPA: Group by and select function in Postgres - postgresql

Question

I have a simple reporting query group by id and day that looks like the following:
select id,
avg(case when name = 'temp' then value end) as average_temp,
DATE_TRUNC('day', timestamp) as day
from data
group by id, day
order by id;
The query basically needs to show the average daily temperature for each asset.
The user is able to specify a bunch of different aggregation functions beyond just 'average', the above is only a simple example. For example, avg temp, max temp, max speed, etc.
I'm trying to translate that into JPA as follows:
CriteriaQuery<Object[]> query = criteriaBuilder.createQuery(Object[].class);
Root<AssetMetricDataPoint> root = query.from(Data.class);
List<Selection<?>> selectionList = getSelections(aggregationQuery, criteriaBuilder, root);
Expression<Instant> groupDate = criteriaBuilder.function("date_trunc", Instant.class, criteriaBuilder.literal("day"), root.get("timestamp"));
selectionList.add(groupDate.alias("day"));
query.multiselect(selectionList);
query.where(getWherePredicates(aggregationQuery, criteriaBuilder, root));
query.orderBy(getOrderBy(aggregationQuery, criteriaBuilder, root));
query.groupBy(root.get("id"), groupDate);
return this.setupPagination(entityManager.createQuery(query), aggregationQuery);
I'm using criteriaBuilder.function to group by the date. However, when I execute the query using JPA I get the following exception:
org.postgresql.util.PSQLException: ERROR: column "data0_.timestamp" must appear in the GROUP BY clause or be used in an aggregate function
This appears to occur because the query is parametized and Postgres doesn't realize that the 'day' parameter that appears in both the select and group by clauses are the same.
Is there any way around this. Can I somehow bake in the 'day' value so it's not sent a parameter? Or some other method?

Answer 1

In the end there's a relatively solution to the problem. Rather than grouping by the expression, I thought I'd try to group by the alias instead: criteriaBuilder.literal("day"). This didn't work, however, with Postgres complaining about a non-integer literal.
I then realised I could group by a positional integer instead, which in my case ended up looking like:
query.groupBy(root.get("id"), criteriaBuilder.literal(selectionList.size()));
This works as expected.