# How the MCP Server Automatically Adapts Queries for PostgreSQL Versions 12–17

> Discover how the MCP server auto-adapts SQL queries for PostgreSQL 12-17. Learn about runtime detection, feature flags, and version-specific templates for seamless compatibility.

- Repository: [JungJungIn/mcp-postgresql-ops](https://github.com/call518/mcp-postgresql-ops)
- Tags: internals
- Published: 2026-02-26

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**The MCP server detects the PostgreSQL version at runtime using `get_postgresql_version()` and builds version-aware SQL statements through feature flags, dynamic column selection, and version-specific query templates to ensure seamless compatibility across PostgreSQL 12 through 17.**

The `call518/mcp-postgresql-ops` repository implements a robust compatibility layer that allows a single MCP (Model Context Protocol) server to operate against any PostgreSQL release from version 12 to 17. By automatically detecting server capabilities and adjusting SQL generation accordingly, the tool eliminates manual version management while maintaining consistent output formats across all supported versions.

## Runtime Version Detection and Caching

The adaptation process begins with precise version identification. The `get_postgresql_version()` function in [`src/mcp_postgresql_ops/version_compat.py`](https://github.com/call518/mcp-postgresql-ops/blob/main/src/mcp_postgresql_ops/version_compat.py) (lines 87–128) executes `SELECT version()` against the connected database and parses the result into a structured `PostgreSQLVersion` object.

This object stores `major`, `minor`, and `patch` version numbers and exposes boolean feature helpers such as:

- `has_pg_stat_io` – Available in PostgreSQL 16+
- `has_checkpointer_split` – Available in PostgreSQL 14+
- `has_enhanced_wal_receiver` – Available in PostgreSQL 13+
- `has_replication_slot_wal_status` – Available in PostgreSQL 13+

To minimize overhead, the parsed version is cached in the module-level variable `_cached_version`. Subsequent calls return the cached object unless `force_refresh=True` is passed, triggering a fresh detection cycle.

## Dynamic Column Selection for Schema Compatibility

When querying system catalogs that vary between releases, the server uses `get_compatible_column_list()` (lines 153–185) to construct SELECT statements that maintain identical result schemas regardless of the underlying PostgreSQL version.

The function accepts a list of all possible columns and a mapping of version-specific requirements. It preserves columns that exist in all versions, includes conditional columns only when the version requirement is satisfied, and injects `NULL::text AS column` placeholders for missing columns.

```python
from .version_compat import get_compatible_column_list, PostgreSQLVersion

all_cols = ["pid", "datname", "usename", "leader_pid", "query_id"]
version_specific = {
    "leader_pid": PostgreSQLVersion(14),   # added in PG 14

    "query_id":   PostgreSQLVersion(13),   # added in PG 13

}
cols_sql = await get_compatible_column_list(
    "pg_stat_activity",
    all_cols,
    version_specific,
)

# Returns: "pid, datname, usename, leader_pid, query_id" on PG ≥14

# Returns: "pid, datname, usename, NULL::text AS leader_pid, query_id" on PG 13

```

This guarantees that application code receives a predictable column structure without version-specific branching logic.

## Version-Specific Query Templates

The `VersionAwareQueries` class (lines 214–246) encapsulates static async methods for every high-level database operation, including background writer statistics, I/O metrics, replication status, and WAL receiver monitoring. Each method follows a consistent pattern:

1. Calls `await get_postgresql_version()` to obtain the version object
2. Checks the relevant feature flag (`has_checkpointer_split`, `has_pg_stat_io`, etc.)
3. Returns a **different** SQL string based on the detected capabilities
4. Falls back to legacy views or NULL placeholders when features are unavailable

For example, the replication slots query (lines 308–357) conditionally includes `wal_status` and `safe_wal_size` columns only when `version.has_replication_slot_wal_status` returns true (PostgreSQL 13+). On older versions, it selects the same logical columns using `NULL::text AS wal_status` placeholders to maintain schema consistency.

```python
from .version_compat import VersionAwareQueries

# Automatically uses pg_stat_io on PG 16+, falls back to pg_statio_all_tables on older versions

io_sql = await VersionAwareQueries.get_io_statistics()
rows = await execute_query(io_sql)

```

## Centralized Query Selection and Execution

For operations requiring simple query variants, `get_version_appropriate_query()` (lines 88–111) selects among multiple pre-written SQL templates using a mapping of `PostgreSQLVersion` to query strings. The `execute_version_aware_query()` helper (lines 80–99) combines selection and execution into a single asynchronous call, further reducing boilerplate in the tool implementation.

## Integration in the MCP Tool Layer

High-level MCP tools in [`src/mcp_postgresql_ops/mcp_main.py`](https://github.com/call518/mcp-postgresql-ops/blob/main/src/mcp_postgresql_ops/mcp_main.py) (lines 65–71) invoke these version-aware builders without concerning themselves with underlying PostgreSQL versions:

```python
slots_query = await VersionAwareQueries.get_replication_slots_query()
repl_slots = await execute_query(slots_query)

receiver_query = await VersionAwareQueries.get_wal_receiver_query()
wal_receiver = await execute_query(receiver_query)

```

The rest of the codebase remains completely isolated from version-specific logic. The compatibility layer guarantees consistent output formats, allowing operators to upgrade PostgreSQL from version 12 through 17 without modifying client code or tool configurations.

## Summary

- **Runtime detection**: `get_postgresql_version()` parses `SELECT version()` output into a cached `PostgreSQLVersion` object with feature flags for capabilities like `has_pg_stat_io` and `has_checkpointer_split`.
- **Schema normalization**: `get_compatible_column_list()` dynamically builds SELECT statements with `NULL` placeholders for missing columns, ensuring uniform result sets across versions 12–17.
- **Template selection**: The `VersionAwareQueries` class provides static methods that return version-specific SQL strings, automatically falling back to legacy views when newer features are unavailable.
- **Clean abstraction**: High-level tools in [`mcp_main.py`](https://github.com/call518/mcp-postgresql-ops/blob/main/mcp_main.py) consume these utilities transparently, requiring no version-specific logic in the application layer.

## Frequently Asked Questions

### How does the MCP server detect which PostgreSQL version is running?

The server executes `SELECT version()` via `get_postgresql_version()` in [`src/mcp_postgresql_ops/version_compat.py`](https://github.com/call518/mcp-postgresql-ops/blob/main/src/mcp_postgresql_ops/version_compat.py), parses the result into a `PostgreSQLVersion` object containing major, minor, and patch components, and caches this in the module-level `_cached_version` variable to avoid repeated queries on subsequent calls.

### What happens when a query references columns that don't exist in older PostgreSQL versions?

The `get_compatible_column_list()` function (lines 153–185) automatically replaces unavailable columns with `NULL::text AS column` placeholders. This ensures the result set schema remains identical whether running against PostgreSQL 12 or PostgreSQL 17, preventing application errors due to missing columns.

### Which specific PostgreSQL features does the version compatibility layer handle?

According to the source code, the system specifically manages **pg_stat_io** statistics (16+), split **checkpointer** statistics (14+), enhanced **WAL receiver** information (13+), and replication slot **wal_status** and **safe_wal_size** columns (13+), falling back to legacy system views or NULL values when these features are absent.

### Can the version detection cache be refreshed without restarting the server?

Yes. While the parsed version is stored in `_cached_version` for performance, passing `force_refresh=True` to `get_postgresql_version()` bypasses the cache and re-executes `SELECT version()` against the database, allowing the server to adapt to PostgreSQL upgrades or connection pool changes dynamically.