distributed_ddl_* 服务器设置

<distributed_ddl>
    <!-- Path in ZooKeeper to queue with DDL queries -->
    <path>/clickhouse/task_queue/ddl</path>

    <!-- Settings from this profile will be used to execute DDL queries -->
    <profile>default</profile>

    <!-- Controls how much ON CLUSTER queries can be run simultaneously. -->
    <pool_size>1</pool_size>

    <!--
         Cleanup settings (active tasks will not be removed)
    -->

    <!-- Controls task TTL (default 1 week) -->
    <task_max_lifetime>604800</task_max_lifetime>

    <!-- Controls how often cleanup should be performed (in seconds) -->
    <cleanup_delay_period>60</cleanup_delay_period>

    <!-- Controls how many tasks could be in the queue -->
    <max_tasks_in_queue>1000</max_tasks_in_queue>
</distributed_ddl>