SELECT 语句中分区键的数量有限制吗？

Question

执行选择时 Cassandra 分区键的数量是否有限制？

SELECT * FROM series_pfc_gas
  WHERE as_of='2024-07-25'
  AND name IN ('ZTP_FLX_B_LUX_ALL', 'ZTP_FLX_B_TCP_ALL', 'ZTP_FLX_B_EED_ALL', 'NCG_FLX_B_LUX_ALL', 'NCG_FLX_B_TCP_ALL', 'NCG_FLX_B_EED_ALL', 'PEG_FLX_B_LUX_ALL', 'PEG_FLX_B_TCP_ALL', 'PEG_FLX_B_EED_ALL', 'TTF_FLX_B_LUX_ALL', 'TTF_FLX_B_TCP_ALL', 'TTF_FLX_B_EED_ALL', 'ZTP_STC_B_LUX_ALL', 'ZTP_STC_B_TCP_ALL', 'ZTP_STC_B_EED_ALL', 'NCG_STC_B_LUX_ALL', 'NCG_STC_B_TCP_ALL', 'NCG_STC_B_EED_ALL', 'PEG_STC_B_LUX_ALL', 'PEG_STC_B_TCP_ALL', 'PEG_STC_B_EED_ALL', 'TTF_STC_B_LUX_ALL', 'TTF_STC_B_TCP_ALL', 'TTF_STC_B_EED_ALL')
  AND time >= '2024-01-01T00:00:00.000+01:00' AND time < '2029-01-01T00:00:00.000+01:00'

将返回以下错误消息：

InvalidRequest: Error from server: code=2200 [Invalid query] \
  message="Select query cannot be completed because it selects \
  24 partitions keys - more than the maximum allowed 20"

这是表格的结构：

CREATE TABLE py2api.series_op_gas (
    name text,
    as_of timestamp,
    time timestamp,
    day int,
    month int,
    quarter int,
    se_year int,
    season int,
    value double,
    week int,
    wk_year int,
    year int,
    PRIMARY KEY ((name, as_of), time)
) WITH CLUSTERING ORDER BY (time ASC)

一些行的示例：

 name              | as_of                           | time                            | day | month | quarter | se_year | season | value      | week | wk_year | year
-------------------+---------------------------------+---------------------------------+-----+-------+---------+---------+--------+------------+------+---------+------
 ZTP_FLX_L_LUX_PHY | 2023-09-11 00:00:00.000000+0000 | 2023-09-12 05:00:00.000000+0000 |  12 |     9 |       3 |    2023 |      1 |          0 |   37 |    2023 | 2023
 ZTP_FLX_L_LUX_PHY | 2023-09-11 00:00:00.000000+0000 | 2023-09-13 05:00:00.000000+0000 |  13 |     9 |       3 |    2023 |      1 | 4.4409e-16 |   37 |    2023 | 2023
 ZTP_FLX_L_LUX_PHY | 2023-09-11 00:00:00.000000+0000 | 2023-09-14 05:00:00.000000+0000 |  14 |     9 |       3 |    2023 |      1 | 4.4409e-16 |   37 |    2023 | 2023
 ZTP_FLX_L_LUX_PHY | 2023-09-11 00:00:00.000000+0000 | 2023-09-15 05:00:00.000000+0000 |  15 |     9 |       3 |    2023 |      1 |          0 |   37 |    2023 | 2023
 ZTP_FLX_L_LUX_PHY | 2023-09-11 00:00:00.000000+0000 | 2023-09-16 05:00:00.000000+0000 |  16 |     9 |       3 |    2023 |      1 | 4.4409e-16 |   37 |    2023 | 2023

有人想到了在 Cassandra 中编写密钥的另一种结构吗？

Answer 1

答案取决于版本和您所拥有的护栏：

partition_keys_in_select_warn_threshold: -1
partition_keys_in_select_fail_threshold: -1

https://cassandra.apache.org/_/blog/Apache-Cassandra-4.1-Features-Guardrails-Framework.html

如上所述，您在 IN 子句中添加太多分区键是在自找麻烦。协调器将负责收集您提供的所有分区键的数据，这可能会给协调器带来很大的负载。一般来说，我建议使用异步查询。从长远来看，它将防止随着负载增加而出现延迟问题和查询超时。

PS：由于按键太多，这可能取决于，但通常很低。如果你必须走这条路，我会把上限限制在 10 或 20 左右。