Added cache for computation of partition keys

[RanVaknin]

The DDB Enhanced Client originally used direct field access to retrieve single partition key names from table metadata on every operation. PR #6574 extended this to support composite keys (up to 4 partition + 4 sort keys) by introducing stream processing to extract key names from lists of metadata objects.

Profiling revealed this stream processing is a performance bottleneck. For smaller payload this bottleneck accounts for roughly 20%~ of overall CPU time, with larger and more nested the payloads, the performance profile shifts away from this (for example an item with 80 nested objects that is 4 levels deep this accounts for 4%~ of overall CPU time).

In the current implementation Key.keyMap() is repeatedly calling indexPartitionKeys() to build request key maps for every operation. This PR adds a cache to StaticTableMetadata that computes the key name lists once on first access and return the cached results on subsequent access.

Design Considerations:

Do we need a bounded cache?

While technically nothing prevents the user from creating an infinite amount of indexes, the DDB service side limits on the number of indexes per table:

• up to 20 GSIs
• up to 5 LSIs
• 1 primary index

Since StaticTableMetadata is immutable, the cache contents are fixed for the lifetime of the instance.

Flamegraphs:

Before:

After:

---

Operation	Size	Master (us/op)	Optimization (us/op)	Improvement
Delete	TINY	0.396	0.263	-33.59%
Delete	SMALL	0.387	0.26	-32.82%
Delete	HUGE	0.392	0.263	-32.91%
Delete	HUGE_FLAT	0.397	0.262	-34.01%
Get	TINY	0.462	0.312	-32.47%
Get	SMALL	0.975	0.695	-28.72%
Get	HUGE	10.198	9.289	-8.91%
Get	HUGE_FLAT	3.024	2.698	-10.78%
Put	TINY	0.688	0.552	-19.77%
Put	SMALL	1.284	1.108	-13.71%
Put	HUGE	13.482	13.047	-3.23%
Put	HUGE_FLAT	7.595	7.301	-3.87%
Query	TINY	2.557	2.339	-8.53%
Query	SMALL	4.142	3.923	-5.29%
Query	HUGE	31.065	30.026	-3.34%
Query	HUGE_FLAT	8.975	9.202	2.53%
Scan	TINY	1.080	1.082	0.19%
Scan	SMALL	2.479	2.585	4.28%
Scan	HUGE	30.366	29.667	-2.30%
Scan	HUGE_FLAT	8.105	8.273	2.07%
Update	TINY	1.277	1.247	-2.35%
Update	SMALL	9.241	9.122	-1.29%
Update	HUGE	39.486	38.822	-1.68%
Update	HUGE_FLAT	232.677	232.115	-0.24%

[sonarqubecloud]

Quality Gate failed

Failed conditions
54.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud