List database schemas visible to this server, honoring the configured schema allowlist. Cheap. Call this first to see what exists; then use list_objects to look inside a schema.

List tables, views, materialized views, sequences, and foreign tables in a schema, with comments and approximate row counts. Cheap. Filter by object_type and/or a SQL LIKE name_pattern (e.g. 'order%'). Prefer describe_object for the full shape of a single relation.

Describe one relation: columns (name, type, nullable, default, comment), primary key, foreign keys in both directions, indexes, check and unique constraints, and an approximate row count from pg_class.reltuples. Cheap. Prefer this over querying catalogs by hand; use it before writing queries against unfamiliar tables.

Run a single read-only SQL statement inside a READ ONLY transaction and return {columns, rows, row_count, truncated, notice?}. Exactly one statement per call. Use %s placeholders with the params list for user-supplied values. Results are capped at the configured row and byte limits with an explicit truncation notice when more data exists — no LIMIT is ever injected into your SQL. Cost depends entirely on the query; run explain_query first for anything potentially expensive.

Return the PostgreSQL execution plan (EXPLAIN, FORMAT JSON) for a single statement. Cheap without analyze. With analyze=true the statement is actually executed to collect real timing — the transaction is always rolled back, so even in read-write mode nothing persists, but the statement's runtime cost is fully paid. Parameters are not supported here; inline literal values instead.

List installed and available PostgreSQL extensions with versions and descriptions. Cheap. Check here whether an extension (pg_stat_statements, postgis, hypopg, ...) exists before writing queries that depend on it.

Report server version, current role and database, effective access mode, and key settings (timeouts, search_path, recovery state). Cheap. Useful for orientation and for diagnosing permission or timeout surprises.