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MCPg - Production-grade PostgreSQL MCP Server

MCPg - Production-grade PostgreSQL MCP Server

by devopam
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

List policies

list_policies
Read-only

Retrieve Row-Level Security policies on a table plus whether RLS is enabled, returning each policy's command, permissive flag, roles, USING and WITH CHECK expressions.

Instructions

List the Row-Level-Security policies on a table, and whether row security is enabled. Returns an object with rls_enabled (bool) and policies — a list of {name, command (SELECT/INSERT/UPDATE/DELETE/ALL), permissive (bool), roles, using_expression, check_expression}.

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>",
      "table": "<table>"
    },
    "name": "list_policies"
  }
}

Input Schema

NameRequiredDescriptionDefault
tableYes
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.

Output Schema

NameRequiredDescriptionDefault
policiesYes
rls_enabledYes

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

A3.6/5.0
Behavior3/5

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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered. The description adds the concrete result shape (rls_enabled flag plus per-policy command, permissive, roles, using/check expressions), but says nothing about pagination, permissions required, or behavior when the table has no policies.

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

Conciseness4/5

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

Two tight sentences, purpose front-loaded before the return detail. The second sentence is long but information-dense; only mild redundancy with the declared output schema.

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

Completeness4/5

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

For a simple read-only metadata lister with an output schema and safety annotations, the description covers what it does and what comes back. The remaining gap is the undocumented schema/table parameters and absence of any when-to-use routing against the RLS testing siblings.

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

Parameters2/5

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

Schema coverage is only 33%: the `database` parameter is documented inline, but `schema` and `table` carry no descriptions. The phrase 'on a table' hints at the table parameter, yet the description adds no clarification of qualification, case sensitivity, or the primary-vs-secondary database target.

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?

Specific verb plus resource ('List the Row-Level-Security policies on a table') with an added scope detail (also reports whether row security is enabled). It is plainly distinguishable from siblings like test_rls_for_role or list_grants without opening a schema.

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

Usage Guidelines3/5

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

Usage is only implied: an agent can infer this is the inspection step for RLS, but the description never says when to pick it over test_rls_for_role or list_grants, and offers no prerequisites or exclusions.

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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