How does Hibernate's batch-fetching algorithm work?

Question

I found this description of the batch-fetching algorithm in \"Manning - Java Persistence with Hibernate\":

What is the real batch-fetching algorithm

Answer 1

I couldn't find any information on the web about how hibernate handles batch loading, but judging from your information, one could guess the following:

Why 11 batch loaders?

With a batch size of 20, if you want to minimize the number of loaders required for any combination of proxies, there are basically two options:

• create a loader for 1,2,3,4,5,6,7,...20,21,22,23,... N uninitialized proxies (stupid!) OR
• create a loader for any N between 1..9 and then create more loaders for batch_size/2(recursively)

Example: for batch size 40, you would end up with loaders for 40,20,10,9,8,7,6,5,4,3,2,1 loaders.

1. If you have 33 uninitialized proxies, you can use the following loaders: 20, 10, 3
2. If you have 119 uninitialized proxies, you can use the following loaders, 40(x2), 20, 10, 9
3. ...

Why batch loaders can initialize: 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 proxies ? I think the hibernate team chose this as a balance between the number of loaders required for loading a "common" number N of uninitialized proxies and memory consumption. The could have created a loader for every N between 0 and batch_size, but I suspect that the loaders have a considerable memory footprint so this is a tradeoff. The algorithm can be something like this (educated guess):

1. n = batch_size; while (n > 10)

   1.1. loader(n); n = n / 2

2. for n = 0..10 create loader(n)