原SQLselect rcv_idmm_id rcvIdmmId ,work_item_id workItemId,crm_order_id crmOrderId,work_order_id workOrderId, service_no serviceNo,id_no idNo,group_id groupId,order_type orderType,op_time opTime, prpty_name_value prptyNameValue,step_id ,stepId,,ret_info retInfo,remarks from ( select rcv_idmm_id ,work_item_id,crm_order_id,work_order_id,service_no,id_no,group_id,order_type,op_time,prpty_name_value,step_id,ret_idmm_id,ret_info,remarks from cms_worktask_his_202111 union all select rcv_idmm_id ,work_item_id,crm_order_id,work_order_id,service_no,id_no,group_id,order_type,op_time,prpty_name_value,step_id,ret_idmm_id,ret_info,remarks from cms_worktask_his_202112 ) u WHERE u.crm_order_id = '17211942';

执行完以后需要7秒钟，数据在cms_worktask_his_202111表中，单独查询只需要0.3秒，

第一种：不推荐，将多余的列名去掉，只查询需要的列名，我只查询一个列的数据，降到2秒多，

select crm_order_id from ( select crm_order_id from cms_worktask_his_202111 union all select crm_order_id from cms_worktask_his_202112 ) u WHERE u.crm_order_id = '17211942';

第二种：推荐，将where条件放在每一个子查询中，和单表查询差不多，只需要0.4秒，如下：

select rcv_idmm_id rcvIdmmId ,work_item_id workItemId,crm_order_id crmOrderId,work_order_id workOrderId, service_no serviceNo,id_no idNo,group_id groupId,order_type orderType,op_time opTime, prpty_name_value prptyNameValue,step_id stepId,ret_info retInfo,remarks from ( select rcv_idmm_id ,work_item_id,crm_order_id,work_order_id,service_no,id_no,group_id,order_type,op_time,prpty_name_value,step_id,ret_idmm_id,ret_info,remarks from cms_worktask_his_202111 WHERE crm_order_id = '1721315942' union all select rcv_idmm_id ,work_item_id,crm_order_id,work_order_id,service_no,id_no,group_id,order_type,op_time,prpty_name_value,step_id,ret_idmm_id,ret_info,remarks from cms_worktask_his_202112 WHERE crm_order_id = '17211942' ) u