GH-5343 Make LMDBSail Size() 36000x Faster 🚀🚀🚀🚀

[odysa]

GitHub issue resolved: # #5343 Briefly describe the changes proposed in this PR:

This PR introduces an optimization for the size(...) method in the LMDBstore implementation. Introduce a cardinalityExact to calcualte the exact size, leverageingmdb_stats when possible.

Key Changes

• ChangeSet

  • Fast-path Optimization

    • When there are no approved or deprecated changes in the current transaction or not in a transcation, the method directly delegates to the derivedFrom store for fast cardinality estimation.

  • Fallback to Iterator

    • When changes exist in the transaction, the method falls back to streaming through matching statements using getStatements(...).stream().count(). This bypasses LMDB’s lazy evaluation to ensure consistency, even with uncommitted changes.

• Low-level Size Calculation (cardinalityExact)

  • If the pattern is completely unspecified (i.e. all wildcards), the method uses LMDB's mdb_stat to return the total size efficiently.
  • For specific patterns, it iterates over both explicit and implicit triples and counts the results.

Perf

I created a LMDBSail with 10M triples. Original size(): 21802ms

Optimized size(): 685.6 μs to get the full size by leveraging mdb_stats. 274.2 ms to get the size of a context of 5000000 triples.

Total Size: 10000000, Time taken: 685.6 μs
Size in context: 5000000, Time taken: 274.2 ms

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PR Author Checklist (see the contributor guidelines for more details):

• [x] my pull request is self-contained
• [x] I've added tests for the changes I made
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