Clickhouse：DB::Exception：超出内存限制（用于查询）

Question

当 Clickhouse 查询内存不足时，我该怎么办？你不能只是调高内存，对吧？内存也有限制，硬盘怎么配置？

SELECT
    UserID,
    Title
FROM 
(
    SELECT
        L.UserID,
        L.Title
    FROM tutorial.hits_v1 AS L
    INNER JOIN tutorial.hits_v2 AS R ON L.UserID = R.UserID
) AS T
ORDER BY UserID ASC
LIMIT 10

#user.d/abc.xml


<?xml version="1.0"?>
<yandex>
    <!-- Profiles of settings. -->
    <profiles>
        <!-- Default settings. -->
        <default>
            <!-- Maximum memory usage for processing single query, in bytes. -->
            <max_memory_usage>350000000</max_memory_usage>
            <max_memory_usage_for_user>350000000</max_memory_usage_for_user>
            <max_bytes_before_external_group_by>100000000</max_bytes_before_external_group_by>
            <max_bytes_before_external_sort>100000000</max_bytes_before_external_sort>
        </default>
    </profiles>
</yandex>

Answer 1

• 避免使用巨大的表作为 JOIN 的右表：“ClickHouse 获取 <right_table> 并在 RAM 中为其创建哈希表”
• 将查询限制应用于子查询，而不是外部查询

试试这个：

SELECT L.UserID, L.Title
FROM tutorial.hits_v1 AS L
INNER JOIN (
  SELECT UserID
  FROM tutorial.hits_v2
  /* WHERE .. */
  LIMIT 10) AS R ON L.UserID = R.UserID
ORDER BY UserID

或者

SELECT UserID, Title
FROM tutorial.hits_v1
WHERE UserID IN (SELECT UserID FROM tutorial.hits_v2 /* WHERE .. */ LIMIT 10)
ORDER BY UserID

Answer 2

如果有余量，请在执行查询之前增加最大内存：

SET max_memory_usage = 8000000000;

就我而言，将其设置为 8 GB 解决了这个问题。