# FastAPI — StreamingResponse + async generator + chunked
from fastapi import FastAPI
from fastapi.responses import StreamingResponse

app = FastAPI()

async def csv_iter(query):
    yield "id,name,total\n"
    # курсор по БД, batch 1000, async
    async for batch in db.stream(query, batch_size=1000):
        buf = []
        for row in batch:
            buf.append(f"{row.id},{row.name},{row.total}\n")
        yield "".join(buf)

@app.get("/export")
async def export(filter: str):
    q = build_query(filter)
    return StreamingResponse(
        csv_iter(q),
        media_type="text/csv",
        headers={
            "Content-Disposition": 'attachment; filename="export.csv"',
            "X-Accel-Buffering": "no",   # nginx: не буферизовать
        },
    )

# Загружаем всё в память и возвращаем
@app.get("/export")
def export(filter: str):
    rows = db.execute(query).fetchall()   # 100k tuples в памяти
    csv = "id,name,total\n"
    for r in rows:                        # Python string + new string каждый раз
        csv += f"{r.id},{r.name},{r.total}\n"
    return Response(csv, media_type="text/csv")