decimal.InvalidOperation exception is thrown with AWS Redshift.

Dieses Issue hat noch niemand übernommen.

* **asyncpg version**: 0.23.0
* **PostgreSQL version**: PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.27314
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: AWS Redshift.
* **Python version**: 3.6.13
* **Platform**: Linux 5.0.0-23-generic #24~18.04.1-Ubuntu SMP Mon Jul 29 16:12:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**: no
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: no

I am trying to use asyncpg with AWS Redshift. I got an decimal.InvalidOperation exception when I try to run a simple query as follows. Is there any way to make asyncpg work with AWS Redshift? psycopg2 is working with AWS Redshift without major issue so I expect asyncpg could work with AWS Redshift also. Thank you in advance.

```
import asyncio
import asyncpg

async def run():
conn = await asyncpg.connect(
user='user', password='password', database='database', host='host')
values = await conn.fetch('SELECT 1.0 / 0.5')
await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())
```

```
$ python asyncpg_redshift.py
Traceback (most recent call last):
File "asyncpg_redshift.py", line 11, in
loop.run_until_complete(run())
File "/home/taeho/.pyenv/versions/3.6.13/lib/python3.6/asyncio/base_events.py", line 488, in run_until_complete
return future.result()
File "asyncpg_redshift.py", line 7, in run
values = await conn.fetch('SELECT 1.0 / 0.5')
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 588, in fetch
record_class=record_class,
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 1632, in _execute
ignore_custom_codec=ignore_custom_codec,
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 1655, in __execute
ignore_custom_codec=ignore_custom_codec,
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 1697, in _do_execute
result = await executor(stmt, None)
File "asyncpg/protocol/protocol.pyx", line 201, in bind_execute
File "asyncpg/protocol/coreproto.pyx", line 70, in asyncpg.protocol.protocol.CoreProtocol._read_server_messages
File "asyncpg/protocol/coreproto.pyx", line 212, in asyncpg.protocol.protocol.CoreProtocol._process__bind_execute
File "asyncpg/protocol/coreproto.pyx", line 497, in asyncpg.protocol.protocol.CoreProtocol._parse_data_msgs
File "asyncpg/protocol/protocol.pyx", line 781, in asyncpg.protocol.protocol.BaseProtocol._decode_row
File "asyncpg/protocol/prepared_stmt.pyx", line 295, in asyncpg.protocol.protocol.PreparedStatementState._decode_row
File "asyncpg/protocol/codecs/base.pyx", line 289, in asyncpg.protocol.protocol.Codec.decode
File "asyncpg/protocol/codecs/base.pyx", line 196, in asyncpg.protocol.protocol.Codec.decode_scalar
File "asyncpg/pgproto/./codecs/numeric.pyx", line 287, in asyncpg.pgproto.pgproto.numeric_decode_binary
File "asyncpg/pgproto/./codecs/numeric.pyx", line 279, in asyncpg.pgproto.pgproto.numeric_decode_binary_ex
decimal.InvalidOperation: []
$
```

I think the strange result of the following query could be also related.

```
import asyncio
import asyncpg

async def run():
conn = await asyncpg.connect(
user='user', password='password', database='database', host='host')
values = await conn.fetch('SELECT CAST(1.0 AS DECIMAL(10, 1))')
print(values)
await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())
```

```
$ python asyncpg_redshift2.py
[]
$
```

Führen Sie zunächst die beiden bereitgestellten Python-Abfragen gegen AWS Redshift aus und vergleichen Sie ihre Ergebnisse mit dem Traceback. Untersuchen Sie asyncpg/pgproto/./codecs/numeric.pyx, wo der Fehler während der numerischen Dekodierung auftritt, und ermitteln Sie, wie sich die von Redshift zurückgegebenen numerischen Daten von der erwarteten Darstellung von PostgreSQL unterscheiden. Als abgeschlossen gilt die Aufgabe, wenn die Inkompatibilität reproduziert und ein unterstütztes Verhalten oder eine eng begrenzte Fehlerbehebung festgelegt wurde.

Vom Indexierungsmodell aus dem Issue-Text verfasst.