#!/usr/bin/python3
import asyncio
import asyncpg
from time import time
try:
	import uvloop
	asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
except:
	pass  # it will just be slower, no harm done

async def await_save(query, block, pool):
	async with pool.acquire() as conn:
		await conn.executemany(query, block)

async def process_db(reader_params, reader_query, writer_params, writer_query, block_size=100, block_count=10):
	loop = asyncio.get_event_loop()
	total_count = 0
	reader = await asyncpg.connect(*reader_params)
	writers = await asyncpg.create_pool(*writer_params)
	pending = set()
	async with reader.transaction():
		cursor = await reader.cursor(reader_query)
		while 1:
			while len(pending) > block_count:
				completed, pending = await asyncio.wait(pending, return_when=asyncio.FIRST_COMPLETED)
				total_count += len(completed)
				if total_count % 100 < len(completed):
					print(total_count * block_size, "rows")
			block = await cursor.fetch(block_size)
			if not block:
				break
			pending.add(loop.create_task(await_save(writer_query, block, writers)))
	await asyncio.wait(pending)

params = {
	"reader_params": {host="1.1.1.1", port=..., database=..., user=..., password=...},
	"writer_params": {host="2.2.2.2", port=..., database=..., user=..., password=...},
	"reader_query": "select column_a, column_b from table where column_c = 42;",
	"writer_query": "insert into copied_table (col_a, col_b) values ($1, $2);",
}
loop = asyncio.get_event_loop()
loop.run_until_complete(process_db(**params))