@yawlabs/postgres-mcp

Inspect blocking locks

pg_inspect_locks

Read-onlyIdempotent

Identify blocked sessions and their blockers to pinpoint lock contention causing hangs or stuck queries. Shows blocked and blocking PIDs, lock types, contested relations, and queries for cluster-wide diagnosis.

Instructions

Show current lock contention: which sessions are blocked and who is blocking them. Returns blocked PID, blocking PID, lock types, relation being contested, and the queries involved. Use this first when a tool call hangs or the app feels stuck - it's the fastest way to identify a long-held transaction holding a lock. Row shape: one row per (blocked_pid, blocking_pid) pair. A session waiting on multiple blockers appears on multiple rows -- group/deduplicate by blocked_pid if you want a per-blocked-session count. Scope: CLUSTER-WIDE, unlike pg_health active_queries, which is filtered to the database in DATABASE_URL. Lock waits cross databases (a shared catalog, a long transaction in a sibling database), so the blocker is not always somewhere this connection could see -- but it does mean a row here may name a pid in another database entirely. relation is only resolved for locks in the CURRENT database or on a cluster-shared catalog (pg_authid, pg_database, ...); it is NULL for a lock held in another database, because a pg_class OID is only meaningful within its own database. A NULL relation has TWO unrelated causes, and lock_type is what tells them apart. When the wait is ON a relation (lock_type relation / extend / page / tuple), NULL means the lock is held in another database and cannot be named from here. When the wait is on something else (transactionid / virtualxid / advisory), relation was only ever a best-effort hint -- an alphabetical guess among the blocker's held write-intent locks, not authoritative -- so NULL there means the guess found no candidate, and the blocker is very likely LOCAL. Do not read that second case as 'somewhere else': use blocking_pid and blocking_query, which are populated either way. Note also that pg_cancel_backend / pg_terminate_backend signal by PID across the whole cluster, so even a genuinely cross-database blocker is actionable via pg_kill. Use the blocked/blocking query text to disambiguate which table is actually contested.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`limit`	No	Max blocked/blocker pairs (default 50).

Output Schema

TableJSON Schema

Name	Required	Description	Default
`rows`	Yes

Schema Changelog

Changes observed during successful MCP inspections. Dates show when Glama detected each change.

Changed17 schema fields changedv0.12.1Sep 4, 2026

removedOutput schema / properties / rows / items / properties / blocked_duration_seconds / anyOf
```
Removed value: -[
-  {
-    "type": "number"
-  },
-  {
-    "type": "null"
-  }
-]
```
addedOutput schema / properties / rows / items / properties / blocked_duration_seconds / type
```
Added value: +[
+  "number",
+  "null"
+]
```

removedOutput schema / properties / rows / items / properties / blocked_query / anyOf

Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "null"
-  }
-]

addedOutput schema / properties / rows / items / properties / blocked_query / type
```
Added value: +[
+  "string",
+  "null"
+]
```

removedOutput schema / properties / rows / items / properties / blocked_user / anyOf

Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "null"
-  }
-]

addedOutput schema / properties / rows / items / properties / blocked_user / type
```
Added value: +[
+  "string",
+  "null"
+]
```
removedOutput schema / properties / rows / items / properties / blocking_duration_seconds / anyOf
```
Removed value: -[
-  {
-    "type": "number"
-  },
-  {
-    "type": "null"
-  }
-]
```
addedOutput schema / properties / rows / items / properties / blocking_duration_seconds / type
```
Added value: +[
+  "number",
+  "null"
+]
```

removedOutput schema / properties / rows / items / properties / blocking_query / anyOf

Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "null"
-  }
-]

addedOutput schema / properties / rows / items / properties / blocking_query / type
```
Added value: +[
+  "string",
+  "null"
+]
```

removedOutput schema / properties / rows / items / properties / blocking_state / anyOf

Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "null"
-  }
-]

addedOutput schema / properties / rows / items / properties / blocking_state / type
```
Added value: +[
+  "string",
+  "null"
+]
```

removedOutput schema / properties / rows / items / properties / blocking_user / anyOf

Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "null"
-  }
-]

addedOutput schema / properties / rows / items / properties / blocking_user / type
```
Added value: +[
+  "string",
+  "null"
+]
```

removedOutput schema / properties / rows / items / properties / relation / anyOf

Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "null"
-  }
-]

changedOutput schema / properties / rows / items / properties / relation / description

Previous value: -"schema.table. For transactionid / virtualxid waits this is a best-effort GUESS among the blocker's held write-intent locks, not authoritative -- disambiguate with the query text."New value: +"schema.table. NULL has two causes, told apart by `lock_type`: on a relation wait (relation / extend / page / tuple) NULL means the lock is held in ANOTHER database and cannot be named from here; on a transactionid / virtualxid / advisory wait this field was only ever a best-effort GUESS among the blocker's held write-intent locks, so NULL means the guess found nothing and the blocker is very likely LOCAL. Never authoritative on the guess path -- disambiguate with the query text."

addedOutput schema / properties / rows / items / properties / relation / type
```
Added value: +[
+  "string",
+  "null"
+]
```

Changed2 schema fields changedv0.12.0Aug 28, 2026

changedInput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "additionalProperties": false,
+  "properties": {
+    "rows": {
+      "items": {
+        "additionalProperties": false,
+        "properties": {
+          "blocked_duration_seconds": {
+            "anyOf": [
+              {
+                "type": "number"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "description": "Since query_start."
+          },
+          "blocked_pid": {
+            "type": "number"
+          },
+          "blocked_query": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "blocked_user": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "blocking_duration_seconds": {
+            "anyOf": [
+              {
+                "type": "number"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "blocking_pid": {
+            "type": "number"
+          },
+          "blocking_query": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "blocking_state": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "blocking_user": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "lock_type": {
+            "description": "pg_locks.locktype: relation, transactionid, virtualxid, tuple, ...",
+            "type": "string"
+          },
+          "relation": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "description": "schema.table. For transactionid / virtualxid waits this is a best-effort GUESS among the blocker's held write-intent locks, not authoritative -- disambiguate with the query text."
+          }
+        },
+        "required": [
+          "blocked_pid",
+          "blocked_user",
+          "blocked_query",
+          "blocked_duration_seconds",
+          "blocking_pid",
+          "blocking_user",
+          "blocking_query",
+          "blocking_state",
+          "blocking_duration_seconds",
+          "relation",
+          "lock_type"
+        ],
+        "type": "object"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "rows"
+  ],
+  "type": "object"
+}

First observedv0.7.0Jul 29, 2026

TDQS

A4.6/5.0

Behavior5/5

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

Beyond the readOnly/idempotent annotations, the description discloses cluster-wide scope, the two distinct causes of NULL relation values, how lock_type disambiguates them, and that relation is only a best-effort hint for non-relation lock types. It also explains row shape and deduplication behavior, substantially exceeding annotation coverage.

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?

The description is front-loaded with purpose and usage, then structured into scope, row-shape, and NULL-semantics sections. It is longer than typical and has some redundancy (e.g., repeating cross-database caveats), but every sentence contributes meaningful information for correct use and interpretation.

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?

The description is thoroughly complete for a lock-inspection tool: it covers output row shape, deduplication, cluster-wide scope, NULL relation semantics, lock_type disambiguation, and cross-database actionability via pg_kill. The output schema handles return-value structure, so nothing needed for correct selection and invocation is missing.

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

Parameters3/5

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

The only parameter, limit, is fully described in the input schema with default, minimum, and maximum values. The description adds no parameter-specific meaning; with 100% schema coverage, the baseline score of 3 is appropriate.

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?

The description clearly states the tool's purpose: 'Show current lock contention: which sessions are blocked and who is blocking them.' It lists specific returned columns and explicitly distinguishes itself from pg_health via the cluster-wide versus database-scoped contrast, making its role among siblings unambiguous.

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

Usage Guidelines5/5

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

It provides explicit when-to-use guidance: 'Use this first when a tool call hangs or the app feels stuck.' It also names the alternative pg_health and its scope limitation, explains cross-database lock waits, and notes that blockers remain actionable via pg_kill, giving an agent clear decision context.

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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