ADR-0027: Migrate from Psycopg2 to Psycopg3¶

• Status: Accepted
• Date: 2025-01-15
• Deciders: Mihai Criveti

Context¶

The MCP Gateway uses PostgreSQL as one of its supported production database backends. Historically, the project used psycopg2 (via psycopg2-binary) as the PostgreSQL adapter. Psycopg2 has been the standard PostgreSQL adapter for Python for over 15 years.

Psycopg3 (the psycopg package) is a complete rewrite of the PostgreSQL adapter, offering significant improvements in architecture, performance, and features. SQLAlchemy 2.x provides native support for psycopg3 via the postgresql+psycopg:// dialect.

Key benefits of psycopg3:

1. Parameter binding: Psycopg3 uses server-side parameter binding by default (parameters sent separately from query), improving security
2. Prepared statements: Native support for prepared statements, improving performance for repeated queries
3. Binary protocol: Support for binary data transfer, reducing parsing overhead
4. Async support: First-class async support built into the core library
5. Connection pooling: Native connection pool implementation (psycopg_pool)
6. Active maintenance: Psycopg3 is actively developed; psycopg2 is in maintenance mode

Decision¶

We will use psycopg[binary] (psycopg3) as the only supported PostgreSQL adapter. Psycopg2 is no longer supported.

Changes Made¶

1. Dependencies (pyproject.toml):

2. Use psycopg[c,binary]>=3.2.0 for the postgres extra (C extension + pre-compiled libpq)

3. Driver Detection (mcpgateway/db.py):

4. Only psycopg3 driver is supported: driver in ("psycopg", "default", "")

5. Keep-alive parameters work via libpq

6. Connection URLs:

7. postgresql+psycopg:// - Required for psycopg3
8. postgresql:// - Does NOT work (defaults to psycopg2 in SQLAlchemy)

Migration Path for Users¶

1. URL format: Must use postgresql+psycopg:// (not postgresql://)
2. Install: Use pip install 'psycopg[c,binary]' or install the gateway with [postgres] extra
3. Breaking change: All PostgreSQL URLs must be updated to use postgresql+psycopg://

Performance Optimizations¶

The migration to psycopg3 enabled several performance optimizations:

1. COPY Protocol Utility (for Large Bulk Imports)¶

A utility module provides psycopg3's COPY protocol for very large bulk inserts (1000+ rows).

Location: mcpgateway/utils/psycopg3_optimizations.py

Important: COPY is only faster for large batches. Testing showed: - 1000+ rows: COPY is 5-10x faster than INSERT - 10-100 rows: COPY is actually 2x slower due to protocol overhead - For small batches, use SQLAlchemy's bulk_insert_mappings() instead

Use cases: - Bulk data imports (tools, resources, prompts) - Initial data seeding - NOT recommended for metrics buffering (small frequent batches)

2. Automatic Prepared Statements¶

Psycopg3 automatically prepares frequently-executed queries server-side after a configurable threshold.

Configuration: DB_PREPARE_THRESHOLD (default: 5)

After N executions of the same query template, psycopg3 creates a server-side prepared statement, reducing: - Query parsing overhead - Network round-trips for query plans - PostgreSQL CPU usage for repeated queries

Set to 0 to disable, 1 to prepare immediately, or higher values to reduce memory usage.

3. Optimized Utility Module¶

New utility module provides psycopg3-specific optimizations with automatic fallbacks:

Location: mcpgateway/utils/psycopg3_optimizations.py

Features: - bulk_insert_with_copy() - COPY protocol for bulk inserts - bulk_insert_metrics() - Optimized metrics insertion - execute_pipelined() - Pipeline mode for batch queries - is_psycopg3_backend() - Runtime backend detection

Consequences¶

Positive¶

• Better security: Server-side parameter binding prevents SQL injection at the protocol level
• Improved performance: Prepared statements and binary protocol reduce overhead
• COPY protocol: 5-10x faster bulk inserts for large import operations (1000+ rows)
• Future-ready: Native async support enables future async migration if needed
• Active maintenance: Psycopg3 receives regular updates and security fixes
• Better connection pooling: Option to use psycopg_pool for advanced scenarios
• Simpler codebase: Only one driver to support and test

Negative¶

• Breaking change: Users with psycopg2 must migrate
• Server-side binding limitations: Some DDL statements don't support parameters (mitigated by using psycopg.sql module)

Neutral¶

• Keep-alive parameters: Work the same way via libpq
• SQLAlchemy abstraction: Most application code unchanged

Compatibility Notes¶

SQL Patterns That Work Unchanged¶

• Standard SELECT/INSERT/UPDATE/DELETE with named parameters (:name style)
• Simple queries without parameters
• SQLAlchemy ORM operations
• Alembic migrations (all use compatible patterns)

Patterns Requiring Attention¶

These patterns are not used in the MCP Gateway codebase but should be noted:

1. Tuple IN clause: IN %s with tuple doesn't work; use = ANY(%s) with list
2. IS NULL with parameter: IS %s doesn't work; use IS NOT DISTINCT FROM %s
3. Multiple statements: Can't execute multiple statements with parameters in one call

Alternatives Considered¶

References¶

• Psycopg3 Documentation
• Differences from psycopg2
• SQLAlchemy psycopg Dialect
• Server-side Binding

Status¶

This decision is implemented. The MCP Gateway uses psycopg3 as the only supported PostgreSQL adapter.