In JPA 2, using a CriteriaQuery, how to count results
Sweet CriteriaBuilder
, you counting star! Let's meet EntityManager
for a quick rude count:
Replace MyEntity
with your entity class. The cb.count(entityRoot)
is the paparazzi focusing on the counting operation.
Counting delicacies: Distinct, Joins and a Pinch of Reuse
Counting distinct entities
Fancy counting distinct records? No problemo!
Navigating the joins
An optimized join count, saving you from "Oops, I loaded all the data"!
Reusable criteria pattern
Repetition? Not on my watch! A reusable method:
Mastering complex counts
Sometimes things get real: more complex queries, special JPA providers, and counting teeth... uh, I mean performance considerations.
Optimizing the counts
Beware, each additional join may cause a performance drain. Steer clear of fetch joins which inflate the entity counts.
Reusable geniuses
Imagine a utility class with EntityManager
and domainClass
as constructor params generating counts on demand. Einstein-level reusability, eh?
Provider-specific dance
JPA implementations like Hibernate or EclipseLink can have specific quirks. Check their specs and adjust your counts. The devil, as always, is in the details.
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