Streaming Database resultsets to client response chunk by chunk async way

Question

My goal is to fetch the data in chunks and pass it to response and append the data until I fetch all the records from data with async calls. So that client would keep on getting results until all the records are read from db.

I have an API which takes an id as input and fetch data from the data base, records for this id in database are more than 500000. After reading I am process each records to get desired fields/data and then returning the data in ResponseEntity. (current implementation using Springboot, RestAPI, posgresql)

Can we do this asynchronous way?

API:

@GetMapping(value = "/v1/batch/{batchId}")
public ResponseEntity<?> getBatchById(
        @PathVariable("batchId") long batchId,
) {
    log.info("received request for getting batch: {}", batchId);
    try {
        var resp = discountService.getBatchById(batchId);
            var fileInputStream = new InputStreamResource((InputStream) resp.data());
            DateFormat dateFormatter = new SimpleDateFormat("yyyy-MM-dd_HH-mm-ss");
            String currentDateTime = dateFormatter.format(new Date());

            return ResponseEntity.ok()
                                 .header(HttpHeaders.CONTENT_DISPOSITION,
                                         "attachment; filename=" + batchId + "_" + currentDateTime + ".csv")
                                 .contentType(new MediaType("application", "csv", StandardCharsets.UTF_8))
                                 .body(fileInputStream);
        
    } catch (Exception e) {
        log.error("caught exception while getting batch by id: ", e);
        return ResponseEntity.ok(new Resp<>(new Err("9009", "GENERAL EXCEPTION")));
    }
}

Method which gets data from database(postgresql):

public Data getBatchById(long batchId){
  final var pstmt = conn.prepareStatement(SELECT_BATCH_BY_ID)) {
        pstmt.setLong(1, batchId); 
  var rs = pstmt.executeQuery();  // execution time for this line is around 60 sec 
            if (!rs.next()) {
                log.warn("no batch: {} found", batchId);
                return null;
            }
            return processresult(rs);

}

• SELECT_BATCH_BY_ID is simple select query joined 2 tables.
• I have analyzed query it is taking hardly 150ns for execution on query console.
• But the same query from java method call using prepared statement to get the resultset is taking almost 50-60sec of time. (I have 500000 records in total)