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javapostgresqljpaentitymanager

Select all records from offset to limit using a postgres index

I want to get all data from offset to limit from a table with about 40 columns and 1.000.000 rows. I tried to index the id column via postgres and get the result of my select query via java and an entitymanager.

My query needs about 1 minute to get my results, which is a bit too long. I tried to use a different index and also limited my query down to 100 but still it needs this time. How can i fix it up? Do I need a better index or is anything wrong with my code?

CriteriaQuery<T> q = entityManager.getCriteriaBuilder().createQuery(Entity.class);
TypedQuery<T> query = entityManager.createQuery(q);
List<T> entities = query.setFirstResult(offset).setMaxResults(limit).getResultList();
Solution

  • Right now you probably do not utilize the index at all. There is some ambiguity how a hibernate limit/offset will translate to database operations (see this comment in the case of postgres). It may imply overhead as described in detail in a reply to this post.

    If you have a direct relationship of offset and limit to the values of the id column you could use that in a query of the form

    SELECT e
FROM Entity
WHERE id >= offset and id < offset + limit

    Given the number of records asked for is significantly smaller than the total number of records int the table the database will use the index.

    The next thing is, that 40 columns is quite a bit. If you actually need significantly less for your purpose, you could define a restricted entity with just the attributes required and query for that one. This should take out some more overhead.

    If you're still not within performance requirements you could chose to take a jdbc connection/query instead of using hibernate.

    Btw. you could log the actual sql issued by jpa/hibernate and use it to get an execution plan from postgress, this will show you what the query actually looks like and if an index will be utilized or not. Further you could monitor the database's query execution times to get an idea which fraction of the processing time is consumed by it and which is consumed by your java client plus data transfer overhead.