问题描述
我们正在处理一些财务数据，需要按特定顺序选择交易，这也取决于交易类型（买入或卖出）。我们正在将Postgresql 12与TimescaleDB插件1.7.2一起使用。交易存储在超表中。
最初，我们虽然打算在
<form action="{% url 'likepost' post.pk %}" method="POST">
{% csrf_token %}
<button type="submit" name="like_id",value="{{post.post_id}}" class="btn btn-primary btn-sm">Like</button>
</form>
中使用CASE WHEN，但是一旦查询命中ORDER BY部分，就会返回所有返回的行。我们的表格中没有没有重复项。这是我们初始查询的示例：
CASE WHEN每行重复的结果示例：
SELECT ticker_id,timestamp,microseconds,price,amount,type FROM tickers
WHERE timestamp >= '2017-08-01T16:00Z' AND timestamp < '2017-08-02T00:00Z' AND fk_exchange = 4 AND fk_Trade_pair = 29
ORDER BY timestamp DESC,microseconds DESC,type,CASE WHEN type='BUY' THEN price END DESC,CASE WHEN type='SELL' THEN price END ASC,id DESC;
编辑：查询计划：
TICKER_ID TIMESTAMP MICROS PRICE AMOUNT TYPE
...
6abdca2a-884e-0f8c-86ee-4afa71435b97 2017-08-01 23:59:00+00 58812000 117.00000000 16910 SELL
6abdca2a-884e-0f8c-86ee-4afa71435b97 2017-08-01 23:59:00+00 58812000 117.00000000 16910 SELL
7f7cfe72-43ca-a153-21a9-e71ddd17ec62 2017-08-01 23:59:00+00 57367000 117.00000000 2085 SELL
7f7cfe72-43ca-a153-21a9-e71ddd17ec62 2017-08-01 23:59:00+00 57367000 117.00000000 2085 SELL
...
我们最终使用了以下查询，但效率较低，感觉像是一种骇人听闻的方式来获得所需的结果：
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|QUERY PLAN |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|Gather Merge (cost=5091.03..9691.03 rows=40000 width=115) |
| Workers Planned: 1 |
| -> Sort (cost=4091.02..4191.02 rows=40000 width=115) |
| Sort Key: _hyper_14_1591_chunk."timestamp" DESC,_hyper_14_1591_chunk.microseconds DESC,_hyper_14_1591_chunk.type,(CASE WHEN (_hyper_14_1591_chunk.type = 'BUY'::Trade_type) THEN _hyper_14_1591_chunk.price ELSE NULL::numeric END) DESC,(CASE WHEN (_hyper_14_1591_chunk.type = 'SELL'::Trade_type) THEN _hyper_14_1591_chunk.price ELSE NULL::numeric END),_hyper_14_1591_chunk.id DESC|
| -> Result (cost=6.37..1033.48 rows=40000 width=115) |
| -> Custom Scan (DecompressChunk) on _hyper_14_1591_chunk (cost=6.37..433.48 rows=68000 width=51) |
| Filter: (("timestamp" >= '2020-08-01 16:00:00+00'::timestamp with time zone) AND ("timestamp" < '2020-08-02 00:00:00+00'::timestamp with time zone)) |
| -> Bitmap Heap Scan on compress_hyper_17_1684_chunk (cost=45.26..433.48 rows=68 width=252) |
| Recheck Cond: (fk_Trade_pair = 29) |
| Filter: ((_ts_Meta_max_1 >= '2020-08-01 16:00:00+00'::timestamp with time zone) AND (_ts_Meta_min_1 < '2020-08-02 00:00:00+00'::timestamp with time zone) AND (fk_exchange = 4)) |
| -> Bitmap Index Scan on compress_hyper_17_1684_chunk__compressed_hypertable_17_fk_Trade (cost=0.00..45.24 rows=2261 width=0) |
| Index Cond: (fk_Trade_pair = 29) |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
如果我们从初始查询中删除WITH sorted_tickers AS (
SELECT row_number()
OVER (ORDER BY timestamp DESC,CASE WHEN type = 'BUY' THEN price END DESC,CASE WHEN type = 'SELL' THEN price END ASC,id DESC)
as row_nr,type
FROM tickers
WHERE timestamp >= '2017-08-01T16:00Z'
AND timestamp < '2017-08-02T00:00Z')
SELECT disTINCT row_nr,type
FROM sorted_tickers
ORDER BY row_nr ASC;
部分，尽管没有正确的顺序，但我们获得了没有重复的预期结果。我们是否遗漏了某些东西/使用错误？有什么想法会导致条件CASE WHEN返回重复的行？
解决方法
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