如何选择ClickHouse中的shard数量？

在确定了选择ClickHouse中的分片数量的准则之后（如数据大小、查询特征等），可以参考以下代码示例计算出最佳分片数量。

1.使用系统函数获取表大小和块大小

SELECT MAX(block_size) AS max_block_size, sum(rows) AS total_rows, database, table FROM system.parts WHERE active AND database = 'YOUR_DATABASE' AND table = 'YOUR_TABLE' GROUP BY database, table;

2.通过计算获得最佳的shard数量

SELECT database, table, total_rows, max_block_size, ceil(total_rows / 1000000) as shard_number FROM ( SELECT MAX(block_size) AS max_block_size, sum(rows) AS total_rows, database, table FROM system.parts WHERE active AND database = 'YOUR_DATABASE' AND table = 'YOUR_TABLE' GROUP BY database, table ) base_info;

3.根据情况查看shard的分布

SELECT partition, count() FROM YOUR_TABLE_all GROUP BY partition ORDER BY count() DESC LIMIT 10;

4.根据查询模式选择shard数量

当查询主要以单shard为主时，可以减少shard数量，利用分区键平均负载每个shard。

SELECT database, table, total_rows, max_block_size, ceil(total_rows / 1000000 / 2) as shard_number FROM ( SELECT MAX(block_size) AS max_block_size, sum(rows) AS total_rows, database, table FROM system.parts WHERE active AND database = 'YOUR_DATABASE' AND table = 'YOUR_TABLE' GROUP BY database, table ) base_info;

总之，需要在实际情况下结合数据大小、查询负载、查询模式等综合考虑，选择合适的shard数量。