Oracle 中的解释计划 - 如何检测最有效的查询？
Explain Plan in Oracle - how do I detect the most efficient query?
简介
我正在编写一个查询来比较包含散列的 2 个不同的 Oracle 数据库表。
我想找出位置 1 中的哪些哈希尚未迁移到位置 2。
我有三个不同的查询，并为它们编写了 explain plan for 语句。
但是，结果并没有告诉我太多信息。
问题
我如何找到最有效和最快的方法？
当前猜测
然而，我怀疑第一个查询是最快的，因为它一次性使用了远程 link。但这只是猜测，并没有实际结果支持。
代码
--------------------------
EXPLAIN PLAN
SET statement_id = 'ex_plan1' FOR
select* from document doc left outer join migrated_document@V2_PROD migrated on doc.hash = migrated.document_hash AND migrated.document_hash is null ;
SELECT PLAN_TABLE_OUTPUT
FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan1','BASIC'));
---------------------------------------------------
| Id | Operation | Name |
---------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN RIGHT OUTER| |
| 2 | REMOTE | MIGRATED_DOCUMENT |
| 3 | TABLE ACCESS FULL | DOCUMENT |
---------------------------------------------------
--------------------------
EXPLAIN PLAN
SET statement_id = 'ex_plan2' FOR
select* from document doc where not exists( select 1 from migrated_document@V2_PROD migrated where migrated.document_hash = doc.HASH ) ;
SELECT PLAN_TABLE_OUTPUT
FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan2','BASIC'));
------------------------------------------------
| Id | Operation | Name |
------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | FILTER | |
| 2 | TABLE ACCESS FULL| DOCUMENT |
| 3 | REMOTE | MIGRATED_DOCUMENT |
------------------------------------------------
--------------------------
EXPLAIN PLAN
SET statement_id = 'ex_plan3' FOR
select* from document doc where doc.hash not in ( select migrated.document_hash from migrated_document@V2_PROD migrated) ;
SELECT PLAN_TABLE_OUTPUT
FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan3','BASIC'));
------------------------------------------------
| Id | Operation | Name |
------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS ANTI | |
| 2 | TABLE ACCESS FULL| DOCUMENT |
| 3 | REMOTE | MIGRATED_DOCUMENT |
------------------------------------------------
--------------------------
更新
我更新了 explain plan 语句以获得更多结果。
由于远程操作...是否可以降低成本但更慢？
如果我正确读取数据，那么选项 2 似乎是最好的。
但我仍然认为选项 1 更快。
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Inst |IN-OUT|
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 105K| 51M| 194 | | |
| 1 | HASH JOIN RIGHT OUTER| | 105K| 51M| 194 | | |
| 2 | REMOTE | MIGRATED_DOCUMENT | 1 | 275 | 2 | V2_MN~ | R->S |
| 3 | TABLE ACCESS FULL | DOCUMENT | 105K| 23M| 192 | | |
-------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Inst |IN-OUT|
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 105K| 23M| 104K| | |
| 1 | FILTER | | | | | | |
| 2 | TABLE ACCESS FULL| DOCUMENT | 105K| 23M| 192 | | |
| 3 | REMOTE | MIGRATED_DOCUMENT | 1 | 50 | 1 | V2_MN~ | R->S |
----------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Inst |IN-OUT|
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 105K| 29M| 526 | | |
| 1 | NESTED LOOPS ANTI | | 105K| 29M| 526 | | |
| 2 | TABLE ACCESS FULL| DOCUMENT | 105K| 23M| 192 | | |
| 3 | REMOTE | MIGRATED_DOCUMENT | 1 | 50 | 0 | V2_MN~ | R->S |
----------------------------------------------------------------------------------------
在完美世界中，您会选择成本最低的计划。但是我认为您的第一个查询不会满足您的要求。在我看来，没有行会加入；您应该使用过滤谓词而不是连接。
而不是
select * from document doc
left outer join migrated_document@V2_PROD migrated
on doc.hash = migrated.document_hash
AND migrated.document_hash is null
应该是
select * from document doc
left outer join migrated_document@V2_PROD migrated
on doc.hash = migrated.document_hash
WHERE migrated.document_hash is null
简介
我正在编写一个查询来比较包含散列的 2 个不同的 Oracle 数据库表。
我想找出位置 1 中的哪些哈希尚未迁移到位置 2。
我有三个不同的查询，并为它们编写了 explain plan for 语句。
但是，结果并没有告诉我太多信息。
问题
我如何找到最有效和最快的方法？
当前猜测
然而，我怀疑第一个查询是最快的，因为它一次性使用了远程 link。但这只是猜测，并没有实际结果支持。
代码
--------------------------
EXPLAIN PLAN
SET statement_id = 'ex_plan1' FOR
select* from document doc left outer join migrated_document@V2_PROD migrated on doc.hash = migrated.document_hash AND migrated.document_hash is null ;
SELECT PLAN_TABLE_OUTPUT
FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan1','BASIC'));
---------------------------------------------------
| Id | Operation | Name |
---------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN RIGHT OUTER| |
| 2 | REMOTE | MIGRATED_DOCUMENT |
| 3 | TABLE ACCESS FULL | DOCUMENT |
---------------------------------------------------
--------------------------
EXPLAIN PLAN
SET statement_id = 'ex_plan2' FOR
select* from document doc where not exists( select 1 from migrated_document@V2_PROD migrated where migrated.document_hash = doc.HASH ) ;
SELECT PLAN_TABLE_OUTPUT
FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan2','BASIC'));
------------------------------------------------
| Id | Operation | Name |
------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | FILTER | |
| 2 | TABLE ACCESS FULL| DOCUMENT |
| 3 | REMOTE | MIGRATED_DOCUMENT |
------------------------------------------------
--------------------------
EXPLAIN PLAN
SET statement_id = 'ex_plan3' FOR
select* from document doc where doc.hash not in ( select migrated.document_hash from migrated_document@V2_PROD migrated) ;
SELECT PLAN_TABLE_OUTPUT
FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan3','BASIC'));
------------------------------------------------
| Id | Operation | Name |
------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS ANTI | |
| 2 | TABLE ACCESS FULL| DOCUMENT |
| 3 | REMOTE | MIGRATED_DOCUMENT |
------------------------------------------------
--------------------------
更新
我更新了 explain plan 语句以获得更多结果。
由于远程操作...是否可以降低成本但更慢？
如果我正确读取数据，那么选项 2 似乎是最好的。 但我仍然认为选项 1 更快。
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Inst |IN-OUT|
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 105K| 51M| 194 | | |
| 1 | HASH JOIN RIGHT OUTER| | 105K| 51M| 194 | | |
| 2 | REMOTE | MIGRATED_DOCUMENT | 1 | 275 | 2 | V2_MN~ | R->S |
| 3 | TABLE ACCESS FULL | DOCUMENT | 105K| 23M| 192 | | |
-------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Inst |IN-OUT|
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 105K| 23M| 104K| | |
| 1 | FILTER | | | | | | |
| 2 | TABLE ACCESS FULL| DOCUMENT | 105K| 23M| 192 | | |
| 3 | REMOTE | MIGRATED_DOCUMENT | 1 | 50 | 1 | V2_MN~ | R->S |
----------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Inst |IN-OUT|
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 105K| 29M| 526 | | |
| 1 | NESTED LOOPS ANTI | | 105K| 29M| 526 | | |
| 2 | TABLE ACCESS FULL| DOCUMENT | 105K| 23M| 192 | | |
| 3 | REMOTE | MIGRATED_DOCUMENT | 1 | 50 | 0 | V2_MN~ | R->S |
----------------------------------------------------------------------------------------
在完美世界中，您会选择成本最低的计划。但是我认为您的第一个查询不会满足您的要求。在我看来，没有行会加入；您应该使用过滤谓词而不是连接。
而不是
select * from document doc
left outer join migrated_document@V2_PROD migrated
on doc.hash = migrated.document_hash
AND migrated.document_hash is null
应该是
select * from document doc
left outer join migrated_document@V2_PROD migrated
on doc.hash = migrated.document_hash
WHERE migrated.document_hash is null