database_mcp_postgres/tools/
explain_query.rs

1//! MCP tool: `explain_query`.
2
3use std::borrow::Cow;
4
5use database_mcp_server::AppError;
6use database_mcp_server::types::{ExplainQueryRequest, QueryResponse};
7use database_mcp_sql::Connection as _;
8use database_mcp_sql::validation::validate_read_only_with_dialect;
9use rmcp::handler::server::router::tool::{AsyncTool, ToolBase};
10use rmcp::model::{ErrorData, ToolAnnotations};
11use serde_json::Value;
12
13use crate::PostgresHandler;
14
15/// Marker type for the `explain_query` MCP tool.
16pub(crate) struct ExplainQueryTool;
17
18impl ExplainQueryTool {
19    const NAME: &'static str = "explain_query";
20    const DESCRIPTION: &'static str = r#"Return the execution plan for a SQL query to diagnose performance. Use this tool instead of running EXPLAIN directly through read_query — it provides structured JSON output. Accepts an optional `database_name` to explain queries against a different database.
21
22<usecase>
23Use when:
24- A query runs slowly and you need to understand why
25- Investigating performance bottlenecks
26- Planning index creation to optimize queries
27- Analyzing join methods, table scan strategies, and sort operations
28</usecase>
29
30<when_not_to_use>
31- Running actual queries → use read_query or write_query
32- Checking table structure → use get_table_schema
33</when_not_to_use>
34
35<examples>
36✓ "Why is my SELECT on orders slow?" → explain_query(query="SELECT ...")
37✓ "Should I add an index?" → explain_query with analyze=true
38✗ "Run this SELECT" → use read_query
39</examples>
40
41<safety>
42Set `analyze` to true for actual execution statistics (EXPLAIN ANALYZE).
43IMPORTANT: EXPLAIN ANALYZE actually executes the query! In read-only mode, only read-only statements are allowed with analyze.
44When analyze is false, returns EXPLAIN (FORMAT JSON) output without executing.
45</safety>
46
47<what_it_returns>
48A JSON array of execution plan rows showing access methods, join types, row estimates, and costs.
49</what_it_returns>"#;
50}
51
52impl ToolBase for ExplainQueryTool {
53    type Parameter = ExplainQueryRequest;
54    type Output = QueryResponse;
55    type Error = ErrorData;
56
57    fn name() -> Cow<'static, str> {
58        Self::NAME.into()
59    }
60
61    fn description() -> Option<Cow<'static, str>> {
62        Some(Self::DESCRIPTION.into())
63    }
64
65    fn annotations() -> Option<ToolAnnotations> {
66        Some(
67            ToolAnnotations::new()
68                .read_only(true)
69                .destructive(false)
70                .idempotent(true)
71                .open_world(true),
72        )
73    }
74}
75
76impl AsyncTool<PostgresHandler> for ExplainQueryTool {
77    async fn invoke(handler: &PostgresHandler, params: Self::Parameter) -> Result<Self::Output, Self::Error> {
78        Ok(handler.explain_query(&params).await?)
79    }
80}
81
82impl PostgresHandler {
83    /// Returns the execution plan for a query.
84    ///
85    /// When `analyze` is true and read-only mode is enabled, the inner
86    /// query is validated to be read-only before executing.
87    ///
88    /// # Errors
89    ///
90    /// Returns [`AppError::ReadOnlyViolation`] if `analyze` is true,
91    /// read-only mode is enabled, and the query is a write statement.
92    /// Returns [`AppError::Query`] if the backend reports an error.
93    pub async fn explain_query(&self, request: &ExplainQueryRequest) -> Result<QueryResponse, AppError> {
94        if request.analyze && self.config.read_only {
95            validate_read_only_with_dialect(&request.query, &sqlparser::dialect::PostgreSqlDialect {})?;
96        }
97
98        let explain_sql = if request.analyze {
99            format!("EXPLAIN (ANALYZE, FORMAT JSON) {}", request.query)
100        } else {
101            format!("EXPLAIN (FORMAT JSON) {}", request.query)
102        };
103
104        let rows = self
105            .connection
106            .fetch(&explain_sql, Some(&request.database_name))
107            .await?;
108
109        Ok(QueryResponse {
110            rows: Value::Array(rows),
111        })
112    }
113}
database_mcp_postgres/tools/explain_query.rs

database_mcp_postgres/tools/
explain_query.rs
