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MCPg - Production-grade PostgreSQL MCP Server

MCPg - Production-grade PostgreSQL MCP Server

by devopam
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Generate schema diagram

generate_schema_diagram
Read-only

Render a Mermaid ER diagram for a PostgreSQL schema, excluding views and foreign tables; set include_partitions=true to show partitions as entities. Output is ready for Markdown.

Instructions

Render a Mermaid ER diagram for a schema. Views and foreign tables are excluded; partitions are excluded by default — pass include_partitions=true to draw each partition as its own entity. Returns the Mermaid erDiagram as a string ready to paste into a Markdown block.

Example: generate_schema_diagram(schema='public', include_partitions=false)

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "schema": "<schema>"
    },
    "name": "generate_schema_diagram"
  }
}

Input Schema

NameRequiredDescriptionDefault
schemaYes
databaseNoOptional: a database id from list_databases, to target a configured secondary (read-only) database; omit for the primary. Database names that are not configured ids are rejected.
include_partitionsNo

Output Schema

NameRequiredDescriptionDefault
resultYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv0.8.4
    • changedInput schema / properties / database / description
      Previous value: -"Optional: target a configured secondary (read-only) database by name; omit for the primary. Call list_databases to see the configured ids."New value: +"Optional: a database id from list_databases, to target a configured secondary (read-only) database; omit for the primary. Database names that are not configured ids are rejected."
  2. First observedv0.6.8

TDQS

A4.3/5.0
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds real value by disclosing the exclusion rules and the exact return shape (a Mermaid erDiagram string ready to paste into Markdown). It does not mention size limits, performance on large schemas, or error behavior for unknown schemas.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two tight sentences plus a concrete example call; the scope exclusions and the return format are front-loaded with no filler. Every sentence carries information an agent needs.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a read-only rendering tool with an output schema, the description covers what is produced, what is omitted, and how to change the default. Nothing needed to invoke it correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is only 33%, so the description must carry weight, and it does for include_partitions by explaining both the default and the effect of enabling it. The required 'schema' parameter is left implicit, and 'database' semantics live only in the schema, but the example call reinforces the two main parameters.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Render a Mermaid ER diagram for a schema') and immediately narrows scope by declaring what is excluded (views, foreign tables, partitions by default). An agent can distinguish this from generate_schema_docs or generate_fk_cascade_graph without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives a clear conditional rule for the main branch point ('partitions are excluded by default — pass include_partitions=true to draw each partition as its own entity'), which is the key usage decision. It does not explicitly name alternatives such as generate_schema_docs or describe_table, so routing among the many sibling generators is left to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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