[WIP] [Feature]: Add openGauss DB Backend

[initzhang]

Adding openGauss Backend for ContextHub

Part 1. Configurate openGauss Server

• Refer to docs/setup/opengauss-setup-guide-zh.md to add the new server backend.
• When executing CREATE DATABASE, the mode must be set to DBCOMPATIBILITY = 'PG'. This prevents empty strings from being interpreted as NULL.

Part 2. Extension Replacement

• The original ContextHub project relies on the pgvector and pgcrypto extensions, neither of which are supported by openGauss.
• Solution: openGauss 7.0.0 natively supports DataVec, which can replace pgvector. Since openGauss does not support pgcrypto and the alternative uuid-ossp is missing from the official docker image, a custom gen_random_uuid function was implemented manually to bypass the need for the pgcrypto extension.

Part 3. Python Driver Compatibility

• Originally, ContextHub used the asyncpg library to interact with the database; however, asyncpg does not support openGauss's vector data format.
  • Error message: message: unhandled standard data type 'vector' (OID 8305). This can be reproduced using the script opengauss/vector_asyncpg.py.
  • While openGauss maintains its own async_gaussdb library, it also lacks support for the vector format (verified via opengauss/vector_async_gaussdb.py).
• Solution: Implemented a database compatibility layer. The Postgres backend continues to use asyncpg, while the openGauss backend switches to the psycopg3 driver.
  • Since psycopg3 uses %s for positional parameters instead of asyncpg's $n syntax, regular expressions are used for automatic conversion.
  • The compatibility layer handles syntax translation and exposes unified fetch, fetchrow, fetchall, and execute interfaces.
  • Implementation details can be found in src/contexthub/db/repository.py.

Part 4. SQL Dialect Rewriting

• ContextHub originally used PostgreSQL-dialect SQL, which contains features incompatible with openGauss.
• For example, openGauss does not support PostgreSQL's INSERT ON CONFLICT (must be rewritten as ON DUPLICATE KEY UPDATE) and cannot use it in conjunction with RETURNING statements or ROW POLICY.
• Approximately 20+ SQL statements across the project require rewriting. Detailed specifics are available in the opengauss-compatibility-report.md.

Overall Completion Status

• Part 1, 2, and 3: 100% complete. The demo script opengauss/demo_e2e_opengauss.py currently executes the first three steps successfully.
• Part 4: 50% complete. Currently rewriting the SQL for the fourth step of the demo; see the FIXME tags in ContextHub/src/contexthub/services/skill_service.py for details.