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dblinick

Postgres MCP Pro

by dblinick

explain_query

Read-only

Analyze SQL query execution plans with detailed cost estimates, simulate hypothetical indexes, and optionally run queries for real execution statistics.

Instructions

Explains the execution plan for a SQL query, showing how the database will execute it and provides detailed cost estimates.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
sqlYesSQL query to explain
analyzeNoWhen True, actually runs the query to show real execution statistics instead of estimates. Takes longer but provides more accurate information.
hypothetical_indexesNoA list of hypothetical indexes to simulate. Each index must be a dictionary with these keys: - 'table': The table name to add the index to (e.g., 'users') - 'columns': List of column names to include in the index (e.g., ['email'] or ['last_name', 'first_name']) - 'using': Optional index method (default: 'btree', other options include 'hash', 'gist', etc.) Examples: [ {"table": "users", "columns": ["email"], "using": "btree"}, {"table": "orders", "columns": ["user_id", "created_at"]} ] If there is no hypothetical index, you can pass an empty list.

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
resultYes
Behavior3/5

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

Annotations declare readOnlyHint=true, so the tool is known to be a safe read operation. However, the description does not disclose that the 'analyze' parameter actually executes the query, which is an important behavioral nuance. No contradiction with annotations, but little added context beyond purpose.

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?

A single, clear sentence that immediately states what the tool does. No unnecessary words or repetition.

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?

Given the rich schema (100% param coverage) and existing output schema, the description is mostly sufficient. However, it could mention the optional analyze and hypothetical_indexes capabilities in the main description for quicker agent awareness, though these are covered in the schema.

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?

Schema description coverage is 100%, with detailed parameter descriptions including examples for hypothetical_indexes. The description itself adds no parameter-level information, so baseline 3 applies.

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?

Description clearly states the tool explains the execution plan for a SQL query, with specific verb 'explains' and resource 'execution plan'. It also mentions cost estimates, distinguishing it from execute_sql and analyze_query_indexes.

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 context is implied ('explains the execution plan') but no explicit guidance on when to use this tool versus alternatives like analyze_query_indexes or execute_sql. No exclusions or prerequisites are mentioned.

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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