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MCPg - Production-grade PostgreSQL MCP Server

MCPg - Production-grade PostgreSQL MCP Server

by devopam
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Find blocking chains

find_blocking_chains
Read-only

Identify PostgreSQL lock blocking chains by returning blocked and blocking backend PID pairs, including cycles, to diagnose sessions waiting on locks.

Instructions

Return (blocked, blocking) backend pairs via pg_blocking_pids. Each row pairs a backend waiting on a Lock with one PID holding the lock that's preventing progress. Cycles are possible (A blocks B, B blocks A); render with care. Read-only.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "find_blocking_chains"
  }
}

Input Schema

NameRequiredDescriptionDefault
limitNo
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
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

B3.2/5.0
Behavior4/5

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

Annotations already cover safety (readOnlyHint, openWorldHint), so 'Read-only' is redundant, but the description adds real value: it defines what each row means (a waiter paired with a holder), warns that cycles can occur, and cautions about rendering them. It says nothing about the effect of the default limit of 50 or output volume, which would have completed the picture.

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?

Four short sentences with the return shape front-loaded and no filler; the cycle caveat earns its place. The trailing 'Read-only' duplicates the annotation and could be dropped.

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

Completeness3/5

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

The tool is simple (two optional params) and has an output schema, so return values need not be explained. However, the role of the limit parameter and the choice against sibling lock tools remain unexplained, leaving the definition only minimally complete.

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 50%: 'database' is documented in the schema, but 'limit' has no description anywhere. The description mentions neither parameter, so it adds no meaning beyond the schema and fails to compensate for the undocumented limit.

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

Purpose4/5

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

States a specific verb and resource ('Return (blocked, blocking) backend pairs') and even names the underlying mechanism (pg_blocking_pids). An agent understands exactly what comes back, but the description never distinguishes this from close siblings like walk_blocking_chains or list_locks.

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

Usage Guidelines2/5

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

There is no when-to-use or when-not-to-use guidance. With a near-duplicate sibling (walk_blocking_chains) and adjacent tools (list_locks, analyze_lock_hotspots, read_pg_stat_lock), an agent is left to guess which one to call; only the lock-diagnosis context is implied.

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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