Abfrage

DuckDB-Wasm stellt Funktionen zum Abfragen von Daten bereit. Abfragen werden nacheinander ausgeführt.

Zuerst muss eine Verbindung durch Aufruf von connect erstellt werden. Anschließend können Abfragen durch Aufruf von query oder send ausgeführt werden.

Abfrageausführung

// Create a new connection
const conn = await db.connect();

// Either materialize the query result
await conn.query<{ v: arrow.Int }>(`
    SELECT * FROM generate_series(1, 100) t(v)
`);
// ..., or fetch the result chunks lazily
for await (const batch of await conn.send<{ v: arrow.Int }>(`
    SELECT * FROM generate_series(1, 100) t(v)
`)) {
    // ...
}

// Close the connection to release memory
await conn.close();

Prepared Statements

// Create a new connection
const conn = await db.connect();
// Prepare query
const stmt = await conn.prepare(`SELECT v + ? FROM generate_series(0, 10_000) t(v);`);
// ... and run the query with materialized results
await stmt.query(234);
// ... or result chunks
for await (const batch of await stmt.send(234)) {
    // ...
}
// Close the statement to release memory
await stmt.close();
// Closing the connection will release statements as well
await conn.close();

Arrow Table nach JSON

// Create a new connection
const conn = await db.connect();

// Query
const arrowResult = await conn.query<{ v: arrow.Int }>(`
    SELECT * FROM generate_series(1, 100) t(v)
`);

// Convert arrow table to json
const result = arrowResult.toArray().map((row) => row.toJSON());

// Close the connection to release memory
await conn.close();

Parquet exportieren

// Create a new connection
const conn = await db.connect();

// Export Parquet
conn.send(`COPY (SELECT * FROM tbl) TO 'result-snappy.parquet' (FORMAT parquet);`);
const parquet_buffer = await this._db.copyFileToBuffer('result-snappy.parquet');

// Generate a download link
const link = URL.createObjectURL(new Blob([parquet_buffer]));

// Close the connection to release memory
await conn.close();