dbmcp_postgres/tools/
explain_query.rs

1//! MCP tool: `explainQuery`.
2
3use std::borrow::Cow;
4
5use dbmcp_server::types::{ExplainQueryRequest, QueryResponse};
6use dbmcp_sql::Connection as _;
7use dbmcp_sql::SqlError;
8use dbmcp_sql::validation::validate_read_only;
9use rmcp::handler::server::router::tool::{AsyncTool, ToolBase};
10use rmcp::model::{ErrorData, ToolAnnotations};
11
12use crate::PostgresHandler;
13
14/// Marker type for the `explainQuery` MCP tool.
15pub(crate) struct ExplainQueryTool;
16
17impl ExplainQueryTool {
18    const NAME: &'static str = "explainQuery";
19    const TITLE: &'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 readQuery — it provides structured JSON output.
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 readQuery or writeQuery
32- Checking table structure → use listTables(detailed=true)
33</when_not_to_use>
34
35<examples>
36✓ "Why is my SELECT on orders slow?" → explainQuery(query="SELECT ...")
37✓ "Should I add an index?" → explainQuery with analyze=true
38✗ "Run this SELECT" → use readQuery
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 title() -> Option<String> {
62        Some(Self::TITLE.into())
63    }
64
65    fn description() -> Option<Cow<'static, str>> {
66        Some(Self::DESCRIPTION.into())
67    }
68
69    fn annotations() -> Option<ToolAnnotations> {
70        Some(
71            ToolAnnotations::new()
72                .read_only(true)
73                .destructive(false)
74                .idempotent(true)
75                .open_world(true),
76        )
77    }
78}
79
80impl AsyncTool<PostgresHandler> for ExplainQueryTool {
81    async fn invoke(handler: &PostgresHandler, params: Self::Parameter) -> Result<Self::Output, Self::Error> {
82        Ok(handler.explain_query(params).await?)
83    }
84}
85
86impl PostgresHandler {
87    /// Returns the execution plan for a query.
88    ///
89    /// When `analyze` is true and read-only mode is enabled, the inner
90    /// query is validated to be read-only before executing.
91    ///
92    /// # Errors
93    ///
94    /// Returns [`SqlError::ReadOnlyViolation`] if `analyze` is true,
95    /// read-only mode is enabled, and the query is a write statement.
96    /// Returns [`SqlError::Query`] if the backend reports an error.
97    pub async fn explain_query(
98        &self,
99        ExplainQueryRequest {
100            database,
101            query,
102            analyze,
103        }: ExplainQueryRequest,
104    ) -> Result<QueryResponse, SqlError> {
105        if analyze && self.config.read_only {
106            let _ = validate_read_only(&query, &sqlparser::dialect::PostgreSqlDialect {})?;
107        }
108
109        let database = database.as_deref().map(str::trim).filter(|s| !s.is_empty());
110
111        let explain_sql = if analyze {
112            format!("EXPLAIN (ANALYZE, FORMAT JSON) {query}")
113        } else {
114            format!("EXPLAIN (FORMAT JSON) {query}")
115        };
116
117        let rows = self.connection.fetch_json(explain_sql.as_str(), database).await?;
118
119        Ok(QueryResponse { rows })
120    }
121}
dbmcp_postgres/tools/explain_query.rs

dbmcp_postgres/tools/
explain_query.rs
