Add sql.describe()

README.md

@@ -392,6 +392,20 @@ sql.file(path.join(__dirname, 'query.sql'), [], {
 
 ```
 
+## Describe `sql.describe(stmt) -> Promise`
+
+Describe the parameters and output columns of the given SQL statement.
+
+```js
+
+const { params, columns } = await sql.describe('select * from users where id = $1 and name like $2')
+
+```
+
+The resulting `params` is an array of Postgres data type ids (OIDs) for parameters in order `$1`, `$2`,
+etc. `columns` is an array of objects `{ name, type, parser }`, where `name` is the column name, `type`
+is the type OID, and `parser` is a function used to parse the value to JavaScript.
+
 ## Transactions
 
 

lib/backend.js

@@ -155,7 +155,10 @@ function Backend({
       : console.log(parseError(x)) // eslint-disable-line
   }
 
-  function NoData() { /* No handling needed */ }
+  function NoData() {
+    if (backend.query.describe)
+      backend.query.result.columns = []
+  }
 
   function Authentication(x) {
     const type = x.readInt32BE(5)
@@ -174,8 +177,17 @@ function Backend({
   }
 
   /* c8 ignore next 3 */
-  function ParameterDescription() {
-    backend.error = errors.notSupported('ParameterDescription')
+  function ParameterDescription(x) {
+    const length = x.readInt16BE(5)
+    let index = 7
+
+    backend.query.statement.params = Array(length)
+
+    for (let i = 0; i < length; ++i) {
+      backend.query.statement.params[i] = x.readInt32BE(index)
+      index += 4
+    }
+    backend.query.result.params = backend.query.statement.params
   }
 
   function RowDescription(x) {

lib/connection.js

@@ -119,6 +119,7 @@ function Connection(options = {}) {
       query.args = args
       query.result = []
       query.result.count = null
+      if (!query.describe) query.result.count = null
       idle_timeout && clearTimeout(timer)
 
       typeof options.debug === 'function' && options.debug(id, str, args)
@@ -168,7 +169,9 @@ function Connection(options = {}) {
     query.statement = { name: sig ? 'p' + uid + statement_id++ : '', sig }
     return Buffer.concat([
       frontend.Parse(query.statement.name, str, args),
-      bind(query, args)
+      query.describe
+        ? frontend.Describe(query.statement.name)
+        : bind(query, args)
     ])
   }
 

lib/frontend.js

@@ -5,11 +5,14 @@ const { errors } = require('./errors.js')
 
 const N = String.fromCharCode(0)
 const empty = Buffer.alloc(0)
+const flushSync = Buffer.concat([
+  bytes.H().end(),
+  bytes.S().end()
+])
 const execute = Buffer.concat([
   bytes.D().str('P').str(N).end(),
   bytes.E().str(N).i32(0).end(),
-  bytes.H().end(),
-  bytes.S().end()
+  flushSync,
 ])
 
 const authNames = {
@@ -39,6 +42,7 @@ module.exports = {
   auth,
   Bind,
   Parse,
+  Describe,
   Query,
   Close,
   Execute
@@ -190,6 +194,13 @@ function Parse(name, str, args) {
   return bytes.end()
 }
 
+function Describe(name) {
+  return Buffer.concat([
+    bytes.D().str('S').str(name).str(N).end(),
+    flushSync,
+  ])
+}
+
 function Execute(rows) {
   return Buffer.concat([
     bytes.E().str(N).i32(rows).end(),

lib/index.js

@@ -275,7 +275,8 @@ function Postgres(a, b) {
       unsafe,
       array,
       file,
-      json
+      json,
+      describe,
     })
 
     function notify(channel, payload) {
@@ -316,6 +317,10 @@ function Postgres(a, b) {
       return promise
     }
 
+    function describe(stmt) {
+      return query({ raw: true, describe: true }, connection || getConnection(), stmt)
+    }
+
     options.types && entries(options.types).forEach(([name, type]) => {
       sql.types[name] = (x) => ({ type: type.to, value: x })
     })

tests/index.js

@@ -1101,3 +1101,33 @@ t('Catches query format errors', async() => [
   'wat',
   await sql.unsafe({ toString: () => { throw new Error('wat') } }).catch((e) => e.message)
 ])
+
+t('Describe a statement', async() => {
+  await sql`create table tester (name text, age int)`
+  const r = await sql.describe('select name, age from tester where name like $1 and age > $2')
+  return [
+    '25,23/name:25,age:23',
+    `${r.params.join(',')}/${r.columns.map(c => `${c.name}:${c.type}`).join(',')}`,
+    await sql`drop table tester`
+  ]
+})
+
+t('Describe a statement without parameters', async() => {
+  await sql`create table tester (name text, age int)`
+  const r = await sql.describe('select name, age from tester')
+  return [
+    '0,2',
+    `${r.params.length},${r.columns.length}`,
+    await sql`drop table tester`
+  ]
+})
+
+t('Describe a statement without columns', async () => {
+  await sql`create table tester (name text, age int)`
+  const r = await sql.describe('insert into tester (name, age) values ($1, $2)')
+  return [
+    '2,0',
+    `${r.params.length},${r.columns.length}`,
+    await sql`drop table tester`
+  ]
+})

types/index.d.ts

@@ -271,6 +271,11 @@ declare namespace postgres {
     columns: ColumnList<U>;
   }
 
+  interface DescribeResult {
+    params: number[]
+    columns: ColumnList<string>
+  }
+
   type ExecutionResult<T> = [] & ResultQueryMeta<number, T>;
   type RowList<T extends readonly Row[]> = T & ResultQueryMeta<T['length'], keyof T[number]>;
 
@@ -318,6 +323,7 @@ declare namespace postgres {
     PostgresError: typeof PostgresError;
 
     array<T extends SerializableParameter[] = SerializableParameter[]>(value: T): ArrayParameter<T>;
+    describe(stmt: string): Promise<DescribeResult>
     begin<T>(cb: (sql: TransactionSql<TTypes>) => T | Promise<T>): Promise<UnwrapPromiseArray<T>>;
     begin<T>(options: string, cb: (sql: TransactionSql<TTypes>) => T | Promise<T>): Promise<UnwrapPromiseArray<T>>;
     end(options?: { timeout?: number }): Promise<void>;