Query

DuckDB-Wasm provides functions for querying data. Queries are run sequentially.

First, a connection needs to be created by calling connect. Then, queries can be run by calling query or send.

Query Execution

// 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 to 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();

Export Parquet

// 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();