SQL：Oracle：优化左外连接查询

Question

我有 1 个本地表（除了一个之外，所有列名都与远程表不同）和 2 个远程表（具有相同的列名），我需要为其合并数据。

以下是我使用 LEFT OUTER JOIN 和 UNION 编写的查询，但性能很慢。

谁能帮助优化这个查询？

select
"CONTROL_M_SERVER",
"HOST",
CASE
WHEN "AGSTAT" = 'V' THEN 'Available'
WHEN "AGSTAT" = 'U' THEN 'Unavailable'
WHEN "AGSTAT" = 'R' THEN 'Discovering'
ELSE 'Not Defined in Control-M'
END as Agent_Status,
T1.VERSION,
"PORTS",
"MANAGEMENT_IP",
"OPERATING_SYSTEM",
"CLUSTER_ALIAS",
"NODEGROUP",
"APPLICATION_ID",
"DATE_CONFIGURED",
"CONFIGURED_BY"
from "CTMAGENTAUDIT" T1
left outer join (select NODEID,AGSTAT from CMR_NODES@SPDB UNION ALL select NODEID,AGSTAT from CMR_NODES@DEVDB) T2 on T2.NODEID = T1.HOST;

Answer 1

我在您的查询中看到的主要问题是CTMAGENTAUDIT 和包含联合的子查询之间的最外层左连接。该子查询的问题在于，正如所写，Oracle 不可能使用任何索引进行连接。这意味着 Oracle 在加入时可能不得不采用较慢的方法，可能是全扫描。

这里的一种方法是创建一个包含联合查询的物化视图，然后对其进行索引：

CREATE MATERIALIZED VIEW T2 AS
SELECT NODEID, AGSTAT FROM CMR_NODES@SPDB
UNION ALL
SELECT NODEID, AGSTAT FROM CMR_NODES@DEVDB;

CREATE INDEX mv_node_idx ON T2 (NODEID);

有了这个索引物化视图，我希望您的查询现在执行得更好：

SELECT
    CONTROL_M_SERVER,
    HOST,
    CASE WHEN AGSTAT = 'V' THEN 'Available'
         WHEN AGSTAT = 'U' THEN 'Unavailable'
         WHEN AGSTAT = 'R' THEN 'Discovering'
         ELSE 'Not Defined in Control-M' END AS Agent_Status,
    T1.VERSION,
    PORTS,
    MANAGEMENT_IP,
    OPERATING_SYSTEM,
    CLUSTER_ALIAS,
    NODEGROUP,
    APPLICATION_ID,
    DATE_CONFIGURED,
    CONFIGURED_BY
FROM CTMAGENTAUDIT T1
LEFT OUTER JOIN T2
    ON T2.NODEID = T1.HOST;

Answer 2

我会这样做：

select
"CONTROL_M_SERVER",
"HOST",
CASE
WHEN "AGSTAT" = 'V' THEN 'Available'
WHEN "AGSTAT" = 'U' THEN 'Unavailable'
WHEN "AGSTAT" = 'R' THEN 'Discovering'
ELSE 'Not Defined in Control-M'
END as Agent_Status,
T1.VERSION,
"PORTS",
"MANAGEMENT_IP",
"OPERATING_SYSTEM",
"CLUSTER_ALIAS",
"NODEGROUP",
"APPLICATION_ID",
"DATE_CONFIGURED",
"CONFIGURED_BY",
(select t2.NODEID, t2.AGSTAT from CMR_NODES@SPDB t2 where t1.host = t2.nodeid),  
(select t3.NODEID, t3.AGSTAT from CMR_NODES@DEVDB t3 where T1.HOST = T3.NODEID) from t1;

Answer 3

您的查询基本上是：

select . . .
from CTMAGENTAUDIT T1 left outer join
     (select NODEID, AGSTAT
      from CMR_NODES@SPDB UNION ALL
      select NODEID, AGSTAT
      from CMR_NODES@DEVDB
     ) T2
     on T2.NODEID = T1.HOST;

假设NODEID/AGSTAT 组合在每个CMR_NODE 表中都是唯一的，我会这样写：

select . . .,
       coalesce(s1.AGSTAT, s2.AGSTAT) as AGSTAT,
       (case coalesce(s1.AGSTAT, s2.AGSTAT) 
            when 'V' then 'Available'
            when 'U' then 'Unavailable'
            when 'R' then 'Discovering'
            else 'Not Defined in Control-M'
        end) as Agent_Status
from CTMAGENTAUDIT T1 left outer join
     CMR_NODES@SPDB s1
     on s1.NODEID = T1.HOST left outer join
     CMR_NODES@DEVDB s2
     on s1.NODEID = T1.HOST

这将至少允许单独优化每个表 - 这应该会有所帮助。

显然，如果您具有设置跨服务器物化视图的权限和愿望，则使用物化视图的解决方案会更快。物化视图存在额外的维护问题，尤其是当您有多个此类视图并假设它们同时更新时。