v.21.4Improvements

Add PostgreSQL Connection Pool to ClickHouse

Add connection pool for PostgreSQL table/database engine and dictionary source. Should fix #21444. #21839 (Kseniia Sumarokova).
Added connection pooling support for PostgreSQL table and dictionary engines in ClickHouse.

Why it matters

This feature addresses the problem of inefficient and potentially overloaded PostgreSQL connections by introducing connection pooling, which improves performance and stability when querying PostgreSQL data sources from ClickHouse.

How to use it

To enable connection pooling, configure the PostgreSQL table engine or dictionary source to use the new connection pool settings as documented. This typically involves specifying connection pool parameters in the table or dictionary configuration to manage and reuse connections efficiently.