import { Connection, DuckDB, RowResultFormat } from "node-duckdb";
import { createWriteStream } from "fs";
import { Transform } from "stream";
class ArrayToCsvTransform extends Transform {
constructor() {
super({objectMode: true})
}
_transform(chunk: any[], _encoding: string, callback: any) {
this.push(chunk.join(",") + '\n');
callback();
}
}
async function outputToFileAsCsv() {
// create new database in memory
const db = new DuckDB();
// create a new connection to the database
// note you can execute only one streaming query at a time per one connection
const connection = new Connection(db);
// by default a query is a streaming one, as opposed to being materialized
await connection.execute("CREATE TABLE people(id INTEGER, name VARCHAR);");
await connection.execute("INSERT INTO people VALUES (1, 'Mark'), (2, 'Hannes'), (3, 'Bob');");
// result is a stream of arrays
const resultStream = await connection.execute("SELECT * FROM people;", {rowResultFormat: RowResultFormat.Array});
const transformToCsvStream = new ArrayToCsvTransform();
const writeStream = createWriteStream("my-people-output");
// objects -> csv strings -> file
resultStream.pipe(transformToCsvStream).pipe(writeStream);
}
outputToFileAsCsv();
