MariaDB ColumnStore:按计算值过滤SubQuery

Question

我有一个表，对于给定的(time, country, asn)元组，它存储了几个指标：

+----------+---------+-------+-------+--------+--------+
|   time   | country |  asn  |  rtt  |  rexb  |  reqs  |
+----------+---------+-------+-------+--------+--------+
| 10000000 |   US    | 12345 |  40   |  0.05  |  5000  |
| 10000000 |   US    | 54321 |  120  |  0.15  |  500   |
| 10000000 |   MX    | 12345 |  300  |  0.25  |  1000  |
| 10000000 |   MX    | 54321 |  160  |  0.10  |  200   |
|   ....   |   ...   |  ...  |  ...  |  ....  |  ....  |

在一般使用过程中，我将这些指标中的每个指标标准化为0到100之间的值，然后返回最大值，以便粗略估计与该国家/地区的ASN的连接有多好：

SELECT
    country,
    asn,
    least(
        -- least(100, greatest(0, ...)) = clip value between 0 and 100
        least(100, greatest(0,
            -- normalize and protect against null values
            -- sample normalization:
            --     0 ms RTT = "100% good"
            --     300 ms RTT = "0% good"
            coalesce((300 - rtt) / 3, 0)
        )),
        least(100, greatest(0,
            -- sample normalization:
            --     0% REXB = "100% good"
            --     50% REXB = "0% good"
            coalesce((0.5 - rexb) / 0.5, 0)
        )),
        -- Other metrics may follow
    ) as quality
FROM
    metrics
WHERE
    time = 10000000 -- "current time"

有时，我可能会使用reqs (对该country+ASN的请求数)列进行加权求平均值：

SELECT
    country,
    least(
        least(100, greatest(0,
            coalesce((300 - sum(rtt*reqs)/sum(reqs)) / 3, 0)
        )),
        least(100, greatest(0,
            coalesce((0.5 - sum(rexb*reqs)/sum(reqs)) / 0.5, 0)
        ))
    ) as avg_quality
FROM
    metrics
WHERE
    time = 10000000 -- "current time"
GROUP BY
    country

这个查询可以完美地工作。然而，当我试图在子查询中使用它时，我遇到了一个问题。

我的目标是找出有多少国家的“平均质量”低于某个阈值：

SELECT 
    count(*)
FROM (
    SELECT
        country,
        least(
            least(100, greatest(0,
                coalesce((300 - sum(rtt*reqs)/sum(reqs)) / 3, 0)
            )),
            least(100, greatest(0,
                coalesce((0.5 - sum(rexb*reqs)/sum(reqs)) / 0.5, 0)
            ))
        ) as avg_quality
    FROM
        metrics
    WHERE
        time = 10000000 -- "current time"
    GROUP BY
        time, country
) t1
WHERE t1.avg_quality < 50

这抛出了错误：

ERROR 1815 (HY000): Internal error: Lost connection to ExeMgr. Please contact your administrator

我可以毫无问题地执行更简单的子查询。为什么这个失败了，我该如何修复它？

我使用的是MariaDB，metrics表使用的是ColumnStore引擎。

快速更新

当我用WHERE country = "US"替换WHERE t1.avg_quality < 50时，查询执行起来没有问题。因此，它在执行子查询或过滤时没有问题。严格地说，当我尝试对计算列进行筛选时，它会失败。

Answer 1

我联系了我公司的DBA，看看他们是否有任何答案或建议。他们无法为这种行为提供解释，但他们能够提供一个变通办法：

SELECT 
    count(*)
FROM (
    SELECT
        country,
        least(
            least(100, greatest(0,
                coalesce((300 - sum(rtt*reqs)/sum(reqs)) / 3, 0)
            )),
            least(100, greatest(0,
                coalesce((0.5 - sum(rexb*reqs)/sum(reqs)) / 0.5, 0)
            ))
        ) as avg_quality
    FROM
        metrics
    WHERE
        time = 10000000 -- "current time"
    GROUP BY
        time, country
    HAVING
        avg_quality < 50
) t1