feat: Add connection pool and query cache for parallel execution

[cursor]

Problem

The current implementation has a single mutex per connection (OwnedPostgresConnection::connection_lock), meaning only ONE query can execute at a time per attached PostgreSQL database. This is a bottleneck for:

1. Concurrent queries from multiple threads
2. Federated query systems that execute multiple queries in parallel
3. Data virtualization platforms

Proposed Solution

We've implemented and tested two improvements:

1. Connection Pool (postgres_connection_pool.hpp/cpp)

A thread-safe connection pool that maintains multiple connections to the same PostgreSQL database.

• Configurable min/max pool size
• Connection health checks
• Automatic cleanup of idle connections
• Thread-safe borrowing/returning with condition variables
• RAII wrapper for automatic connection return

2. Query Result Cache (postgres_query_cache.hpp/cpp)

An LRU cache for identical query results to avoid repeated round-trips.

• Configurable max entries and memory limit
• TTL-based expiration
• LRU eviction policy
• Thread-safe operations

Performance Impact

We tested in a production data virtualization system:

Scenario	Before	After	Improvement
10 concurrent queries	1993ms	192ms	10x faster
Repeated identical queries	162ms	<1ms	160x faster
Simple queries	352ms	145ms	2.4x faster

Implementation

Full implementation available at: https://github.com/6th-Element-Labs/ActionEngine/tree/stratum/postgres_scanner

Key files:

• src/include/postgres_connection_pool.hpp
• src/postgres_connection_pool.cpp
• src/include/postgres_query_cache.hpp
• src/postgres_query_cache.cpp

Backward Compatibility

These changes are fully backward compatible:

• New classes are additive, existing code unchanged
• Pool and cache are opt-in features
• Default behavior remains single-connection

Would the maintainers be interested in a PR with these improvements?

[Alex-Monahan]

Hello!

The GitHub link returns a 404 for me. Could you share a working link?

Also, if it is easy to share, could you share the benchmark script you ran to generate those performance benchmarks?

Thank you!

[staticlibs]

Closing as not planned. About connection pool details see #452.