Eclipselink batch fetch VS join fetch

Question

When should I use "eclipselink.join-fetch", when should I use "eclipselink.batch" (batch type = IN)?

Is there any limitations for join fetch, such as the number of tables being fetched?

Answer 1

Answer is alway specific to your query, the specific use case, and the database, so there is no hard rule on when to use one over the other, or if to use either at all. You cannot determine what to use unless you are serious about performance and willing to test both under production load conditions - just like any query performance tweaking.

Join-fetch is just what it says, causing all the data to be brought back in the one query. If your goal is to reduce the number of SQL statements, it is perfect. But it comes at costs, as inner/outer joins, cartesian joins etc can increase the amount of data being sent across and the work the database has to do.

Batch fetching is one extra query (1+1), and can be done a number of ways. IN collects all the foreign key values and puts them into one statement (more if you have >1000 on oracle). Join is similar to fetch join, as it uses the criteria from the original query to select over the relationship, but won't return as much data, as it only fetches the required rows. EXISTS is very similar using a subquery to filter.