Mongo count operation called twice in QuerydslMongoPredicateExecutor.findAll(Predicate, Pageable) [DATAMONGO-1860]

[spring-projects-issues]

Erik T opened DATAMONGO-1860 and commented

https://github.com/spring-projects/spring-data-mongodb/blob/master/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/QuerydslMongoPredicateExecutor.java#L172

return PageableExecutionUtils.getPage(applyPagination(query, pageable).fetchResults().getResults(), pageable,
() -> createQueryFor(predicate).fetchCount());

Count is a very expensive operation in MongoDB https://jira.mongodb.org/browse/SERVER-1752

In snippet above, fetchResults() internally calls count to populate total count of QueryResults.
Again, fetchCount also calls count.

final QueryResults<T> queryResults = applyPagination(query, pageable).fetchResults();
return PageableExecutionUtils.getPage(queryResults.getResults(), pageable,
() -> queryResults.getTotal());

would end up calling count once.

Moreover, giving the option of returning Slice<T> as well as Page<T>, and do away with count altogether would also be a nice addition.

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Affects: 2.0.3 (Kay SR3)

Reference URL: https://github.com/spring-projects/spring-data-mongodb/blob/master/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/QuerydslMongoPredicateExecutor.java#L172

Referenced from: pull request #529

Backported to: 2.0.4 (Kay SR4), 1.10.11 (Ingalls SR11)

[spring-projects-issues]

Mark Paluch commented

Hey Erik T, thanks for raising this issue. Reusing the count from QueryResults makes sense. Going forward with this topic we could use fetch() instead and retrieve the count only if necessary