feat(sqlite): Improve throughput of SqliteEventCacheStore::load_all_chunks_metadata by 1140%

[Hywan]

This patch changes the query used by SqliteEventCacheStore::load_all_chunks_metadata. It was the cause of severe slowness (see #5407). The new query improves the throughput by +1140% and the time by -91.916%.

This query will visit all chunks of a linked chunk with ID hashed_linked_chunk_id. For each chunk, it collects its ID (ChunkIdentifier), previous chunk, next chunk, and number of events (num_events). If it's a gap, num_events is equal to 0, otherwise it counts the number of events in event_chunks where event_chunks.chunk_id = linked_chunks.id.

Why not using a (LEFT) JOIN + COUNT? Because for gaps, the entire event_chunks will be traversed every time. It's extremely inefficient. To speed that up, we could use an INDEX but it will consume more storage space. Finally, traversing an INDEX boils down to traversing a B-tree, which is O(log n), whilst this CASE approach is O(1). This solution is a nice trade-off and offers great performance.

Note

Please, note that the more gaps, the slower the query was. Now, the number of gaps isn't impactful.

This change doesn't involve a new schema, no migration is necessary.

Metrics for 10'000 events (with 1 gap every 80 events)

Before

	Lower bound	Estimate	Upper bound
Throughput	20.650 Kelem/s	20.686 Kelem/s	20.722 Kelem/s
R²	0.1032688	0.1374475	0.1035309
Mean	482.58 ms	483.43 ms	484.27 ms
Std. Dev.	929.07 µs	1.4376 ms	1.7083 ms
Median	481.90 ms	483.82 ms	484.60 ms
MAD	173.82 µs	2.1061 ms	2.4203 ms

After

	Lower bound	Estimate	Upper bound
Slope	39.322 ms	39.444 ms	39.607 ms
Throughput	252.48 Kelem/s	253.52 Kelem/s	254.31 Kelem/s
R²	0.9993976	0.9995784	0.9992583
Mean	39.381 ms	39.478 ms	39.596 ms
Std. Dev.	75.457 µs	184.96 µs	260.35 µs
Median	39.354 ms	39.459 ms	39.546 ms
MAD	23.761 µs	124.48 µs	267.83 µs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.81%. Comparing base (7d9d5bf) to head (9e2da60).
Report is 5 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5411   +/-   ##
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  Coverage   88.80%   88.81%           
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  Files         334      334           
  Lines       91256    91266   +10     
  Branches    91256    91266   +10     
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+ Hits        81044    81059   +15     
+ Misses       6399     6393    -6     
- Partials     3813     3814    +1     

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