【clickhouse】Memory limit (for query) exceeded:would use 9.37 GiB (attempt to allocate chunk of 30198

1.概述

clickhouse报错：Memory limit (for query) exceeded:would use 9.37 GiB (attempt to allocate chunk of 301989888 bytes), maximum: 9.31 GiB

分析：

1. max_memory_usage指定单个SQL查询在该机器上面最大内存使用量
2. 除了些简单的SQL，空间复杂度是O(1) 如:
   select count(1) from table where column=value
   select column1, column2 from table where column=value
   凡是涉及group by, order by, distinct, join这样的SQL内存占用不再是O(1)

解决：

1： max_bytes_before_external_group_by
2： max_bytes_before_external_sort
3： uniq / uniqCombined / uniqHLL12
4： Join时小表放到右边， “右表广播” ^v