DuckDB-Wasm 提供了用于查询数据的函数。查询是按顺序运行的。
首先，需要通过调用connect来创建一个连接。然后，可以通过调用query或send来运行查询。
查询执行
// 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, 10000) AS 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();
导出 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();