Details
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Improvement
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Status: Open
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Major
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Resolution: Unresolved
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Description
The idea of this enhancement is that, if we know that a collection field is empty there is no need to fetch it. Therefore an optimization can be done if we store a flag in collection owner table indicating whether it is null//empty/not empty and awoid fetching null and empty collections.
It can provide a truly dramatic performance improvement when in a large set of instance only some of them have non-empty collection field. Consider a very common case - composite (tree like) data structures. Unlike true composite pattern typical tree structure does not have a special leaf class that is any node of a tree can potentially have sub-nodes. When traversing such a tree as many as 70% of fetches of child nodes will yield empty collection because obviously leaf level is the larges in a tree structure 
I wrote a prototype custom 1-N mapping for Kodo 3.4 which allow to store "empty" flag (whether the collection is empty) on commit and will store empty collection into StateManager on collection field load if the flag is set to true (empty) instead of going to database to fetch it.
The results were dramatic - when traversing 800-node tree number of "fetch-sub-nodes" SQL statements was cut from 800 to 130.
Non-Tree cases when objects have sparsely populated collection field can be even more dramatic.
If concurrency of the collection field is controlled on owned class level (default) I think there is no dander of this flag being out of synch with actual collection content without entering concurrent modification state.
I have not had chance to think through transaction commit implications if any.
There is a very nice facility in ContainerFieldMappling for indicating null container fields. I wonder why it so much hard wired to empty/null and does not allow non-empty/empty/null differentiation and optimization.
The documentation states:
" 6.2.3.7. jdbc-container-meta
Container metadata is used to record non-essential information about collection and map fields. If this extension is set to true, collections and maps will be able to distinguish between the empty state and the null state. If this extension is set to false or is unset, then it will not be possible for Kodo to differentiate between these two states. In this situation, all collections and maps in persistent objects loaded from the database will be non-null"
Actual implementation of jdbc-container-meta is very restrictive (at least in 3.4 branch) and does not seem to be easily adaptable for not-fetching empty collections
It would be great if it were enhanced and no-fetch optimization for null/empty collections could be performed
Best Regards
Alex Roytman
Peace Technology, Inc