2

假设我有一张非常大的表，有 250M 行：

create table example_customers as
select dbms_random.string('x', 100) as first_name
     , dbms_random.string('x', 100) as last_name
  from dual
connect by level <= 250000000;
begin
  dbms_stats.gather_table_stats(user, 'example_customers');
end;

我想用一个简单的查询对这个表进行全面扫描：select count(*) from example_customers

PLAN_TABLE_OUTPUT
Plan hash value: 2907982153
 
----------------------------------------------------------------------------------------
| Id  | Operation                  | Name              | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |                   |     1 |   384K  (1)| 00:00:16 |
|   1 |  SORT AGGREGATE            |                   |     1 |            |          |
|   2 |   TABLE ACCESS STORAGE FULL| EXAMPLE_CUSTOMERS |   250M|   384K  (1)| 00:00:16 |
----------------------------------------------------------------------------------------

然后我决定我想使用并行提示来尝试加快速度，使用select /*+parallel(10)*/ count(*) from example_customers

Plan hash value: 2126708148
 
-------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name              | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |                   |     1 | 42744   (1)| 00:00:02 |        |      |            |
|   1 |  SORT AGGREGATE                |                   |     1 |            |          |        |      |            |
|   2 |   PX COORDINATOR               |                   |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM)         | :TQ10000          |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE             |                   |     1 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR         |                   |   250M| 42744   (1)| 00:00:02 |  Q1,00 | PCWC |            |
|   6 |       TABLE ACCESS STORAGE FULL| EXAMPLE_CUSTOMERS |   250M| 42744   (1)| 00:00:02 |  Q1,00 | PCWP |            |
-------------------------------------------------------------------------------------------------------------------------

乍一看，这似乎是一个很大的改进——第一条语句的总成本是 384K，新语句的总成本是 43K，这表明并行执行语句的成本是最高的并行运行的 10 个单独操作中的一个，加上并行执行开销的一些额外成本。

然而，当我实际运行这些语句时，我看不到任何时间差异——它们都需要大约 35 秒。为什么第二个语句不快？

我有一些理论，但我不知道如何验证其中哪一个（如果有）是原因：

  • 10 个单独的并行操作中的每一个都在完整扫描表（如 xplan 所示，列出的行在整个过程中返回为 250m）
  • 第一个查询已经达到数据库在所有操作中读取数据的速度的限制，所以第二个查询仍然受到同样的限制
  • 并行操作被强制转换为串行操作（在这种情况下，我希望看到PX COORDINATOR FORCED SERIAL而不是PX COORDINATOR

附加信息

没有提示的查询的 SQL 监视器报告：

SQL Monitoring Report

SQL Text
------------------------------
select count(*) from example_customers

Global Information
------------------------------
 Status              :  DONE (ALL ROWS)                   
 Instance ID         :  1                                 
 Session             :  TESTING (2719:63368)              
 SQL ID              :  2v9j9nz9748xq                     
 SQL Execution ID    :  16777221                          
 Execution Started   :  07/27/2020 13:51:26               
 First Refresh Time  :  07/27/2020 13:51:30               
 Last Refresh Time   :  07/27/2020 13:52:01               
 Duration            :  35s                               
 Module/Action       :  PL/SQL Developer/SQL Window - New 
 Service             :  UCL                               
 Program             :  plsqldev.exe                      
 Fetch Calls         :  1                                 

Global Stats
=============================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Fetch | Buffer | Read  | Read  |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Calls |  Gets  | Reqs  | Bytes | Elig Bytes | Returned Bytes |
=============================================================================================================
|      36 |      34 |     1.39 |        0.00 |     1 |     1M | 11086 |  11GB |       11GB |           11GB |
=============================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=2907982153)
=======================================================================================================================================================================
| Id |          Operation           |       Name        |  Rows   | Cost |   Time    | Start  | Execs |   Rows   | Read  | Read  |  Mem  | Activity | Activity Detail |
|    |                              |                   | (Estim) |      | Active(s) | Active |       | (Actual) | Reqs  | Bytes | (Max) |   (%)    |   (# samples)   |
=======================================================================================================================================================================
|  0 | SELECT STATEMENT             |                   |         |      |        32 |     +4 |     1 |        1 |       |       |     . |          |                 |
|  1 |   SORT AGGREGATE             |                   |       1 |      |        32 |     +4 |     1 |        1 |       |       |     . |          |                 |
|  2 |    TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 385K |        36 |     +0 |     1 |     250M | 11086 |  11GB |   7MB |          |                 |
=======================================================================================================================================================================

带有提示的查询的 SQL 监视器报告：

SQL Monitoring Report

SQL Text
------------------------------
select /*+parallel(10)*/ count(*) from example_customers

Global Information
------------------------------
 Status              :  DONE (ALL ROWS)                   
 Instance ID         :  1                                 
 Session             :  TESTING (1937:31657)              
 SQL ID              :  882dpyfj3hx4m                     
 SQL Execution ID    :  16777216                          
 Execution Started   :  07/27/2020 13:51:20               
 First Refresh Time  :  07/27/2020 13:51:24               
 Last Refresh Time   :  07/27/2020 13:51:55               
 Duration            :  35s                               
 Module/Action       :  PL/SQL Developer/SQL Window - New 
 Service             :  UCL                               
 Program             :  plsqldev.exe                      
 DOP Downgrade       :  100%                              
 Fetch Calls         :  1                                 

Global Stats
=============================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Fetch | Buffer | Read  | Read  |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Calls |  Gets  | Reqs  | Bytes | Elig Bytes | Returned Bytes |
=============================================================================================================
|      36 |      35 |     1.32 |        0.00 |     1 |     1M | 11086 |  11GB |       11GB |           11GB |
=============================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=2126708148)
============================================================================================================================================================================
| Id |            Operation             |       Name        |  Rows   | Cost  |   Time    | Start  | Execs |   Rows   | Read  | Read  |  Mem  | Activity | Activity Detail |
|    |                                  |                   | (Estim) |       | Active(s) | Active |       | (Actual) | Reqs  | Bytes | (Max) |   (%)    |   (# samples)   |
============================================================================================================================================================================
|  0 | SELECT STATEMENT                 |                   |         |       |         1 |    +35 |     1 |        1 |       |       |     . |          |                 |
|  1 |   SORT AGGREGATE                 |                   |       1 |       |         1 |    +35 |     1 |        1 |       |       |     . |          |                 |
|  2 |    PX COORDINATOR                |                   |         |       |           |        |     1 |          |       |       |     . |          |                 |
|  3 |     PX SEND QC (RANDOM)          | :TQ10000          |       1 |       |         1 |    +35 |     1 |        0 |       |       |     . |          |                 |
|  4 |      SORT AGGREGATE              |                   |       1 |       |        32 |     +4 |     1 |        1 |       |       |     . |          |                 |
|  5 |       PX BLOCK ITERATOR          |                   |    250M | 42744 |        32 |     +4 |     1 |     250M |       |       |     . |          |                 |
|  6 |        TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 42744 |        36 |     +0 |     1 |     250M | 11086 |  11GB |   7MB |          |                 |
============================================================================================================================================================================

更多信息

如果禁用 exadata 智能扫描：

select /*+ OPT_PARAM('cell_offload_processing' 'false') PARALLEL(10) */ count(*) from example_customers;

SQL Monitoring Report

SQL Text
------------------------------
select /*+ OPT_PARAM('cell_offload_processing' 'false') PARALLEL(10) */ count(*) from example_customers                            

Global Stats
==========================================================================================
| Elapsed |   Cpu   |    IO    | Application |  Other   | Fetch | Buffer | Read  | Read  |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Waits(s) | Calls |  Gets  | Reqs  | Bytes |
==========================================================================================
|      38 |      36 |     2.04 |        0.00 |     0.11 |     1 |     1M | 11086 |  11GB |
==========================================================================================

SQL Plan Monitoring Details (Plan Hash Value=2126708148)
====================================================================================================================================================================
| Id |            Operation             |       Name        |  Rows   | Cost  |   Time    | Start  | Execs |   Rows   | Read  | Read  | Activity | Activity Detail |
|    |                                  |                   | (Estim) |       | Active(s) | Active |       | (Actual) | Reqs  | Bytes |   (%)    |   (# samples)   |
====================================================================================================================================================================
|  0 | SELECT STATEMENT                 |                   |         |       |         1 |    +38 |     1 |        1 |       |       |          |                 |
|  1 |   SORT AGGREGATE                 |                   |       1 |       |         1 |    +38 |     1 |        1 |       |       |          |                 |
|  2 |    PX COORDINATOR                |                   |         |       |           |        |     1 |          |       |       |          |                 |
|  3 |     PX SEND QC (RANDOM)          | :TQ10000          |       1 |       |         1 |    +38 |     1 |        0 |       |       |          |                 |
|  4 |      SORT AGGREGATE              |                   |       1 |       |        35 |     +4 |     1 |        1 |       |       |          |                 |
|  5 |       PX BLOCK ITERATOR          |                   |    250M | 42744 |        35 |     +4 |     1 |     250M |       |       |          |                 |
|  6 |        TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 42744 |        38 |     +1 |     1 |     250M | 11086 |  11GB |          |                 |
====================================================================================================================================================================

如果我对 group by 使用更复杂的查询，那么我会看到一些改进（从 71 秒到 61 秒），但仍然比我预期的要少得多：

SQL Monitoring Report

SQL Text
------------------------------
select substr(surname, 1, 1) , count(*) from example_customers group by substr(surname, 1, 1) order by substr(surname, 1, 1)                            

Global Stats
=============================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Fetch | Buffer | Read  | Read  |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Calls |  Gets  | Reqs  | Bytes | Elig Bytes | Returned Bytes |
=============================================================================================================
|      71 |      70 |     1.39 |        0.00 |     1 |     1M | 11086 |  11GB |       11GB |           11GB |
=============================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=525074000)
=======================================================================================================================================================================
| Id |          Operation           |       Name        |  Rows   | Cost |   Time    | Start  | Execs |   Rows   | Read  | Read  |  Mem  | Activity | Activity Detail |
|    |                              |                   | (Estim) |      | Active(s) | Active |       | (Actual) | Reqs  | Bytes | (Max) |   (%)    |   (# samples)   |
=======================================================================================================================================================================
|  0 | SELECT STATEMENT             |                   |         |      |        66 |     +6 |     1 |        5 |       |       |     . |          |                 |
|  1 |   SORT GROUP BY              |                   |       4 | 393K |        69 |     +3 |     1 |        5 |       |       |  2048 |          |                 |
|  2 |    TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 385K |        71 |     +1 |     1 |     250M | 11086 |  11GB |   7MB |          |                 |
=======================================================================================================================================================================

SQL Monitoring Report

SQL Text
------------------------------
select /*+ parallel(10) */substr(surname, 1, 1) , count(*) from example_customers group by substr(surname, 1, 1) order by substr(surname, 1, 1)                        

Global Stats
=============================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Fetch | Buffer | Read  | Read  |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Calls |  Gets  | Reqs  | Bytes | Elig Bytes | Returned Bytes |
=============================================================================================================
|      61 |      60 |     1.36 |        0.00 |     1 |     1M | 11086 |  11GB |       11GB |           11GB |
=============================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=3312522119)
==============================================================================================================================================================================
| Id |             Operation              |       Name        |  Rows   | Cost  |   Time    | Start  | Execs |   Rows   | Read  | Read  |  Mem  | Activity | Activity Detail |
|    |                                    |                   | (Estim) |       | Active(s) | Active |       | (Actual) | Reqs  | Bytes | (Max) |   (%)    |   (# samples)   |
==============================================================================================================================================================================
|  0 | SELECT STATEMENT                   |                   |         |       |           |        |     1 |          |       |       |     . |          |                 |
|  1 |   PX COORDINATOR                   |                   |         |       |           |        |     1 |          |       |       |     . |          |                 |
|  2 |    PX SEND QC (ORDER)              | :TQ10001          |       4 | 43519 |         1 |    +61 |     1 |        0 |       |       |     . |          |                 |
|  3 |     SORT GROUP BY                  |                   |       4 | 43519 |         1 |    +61 |     1 |        5 |       |       |  2048 |          |                 |
|  4 |      PX RECEIVE                    |                   |       4 | 43519 |           |        |     1 |          |       |       |     . |          |                 |
|  5 |       PX SEND RANGE                | :TQ10000          |       4 | 43519 |         1 |    +61 |     1 |        0 |       |       |     . |          |                 |
|  6 |        HASH GROUP BY               |                   |       4 | 43519 |        58 |     +4 |     1 |        5 |       |       |   3MB |          |                 |
|  7 |         PX BLOCK ITERATOR          |                   |    250M | 42771 |        58 |     +4 |     1 |     250M |       |       |     . |          |                 |
|  8 |          TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 42771 |        61 |     +1 |     1 |     250M | 11086 |  11GB |   7MB |          |                 |
==============================================================================================================================================================================

如果我们使用具有大量排序操作的查询，那么我们会看到并行执行实际上运行得更慢：

SQL Monitoring Report

SQL Text
------------------------------
select first_name , surname , row_number() over (partition by first_name order by surname asc) rn1 , row_number() over (partition by first_name order by surname desc) rn2 , row_number() over (partition by surname order by first_name asc) rn3 , row_number() over (partition by surname order by first_name desc) rn4 from example_customers                     

Global Stats
=============================================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Fetch | Buffer | Read  | Read  | Write | Write |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Calls |  Gets  | Reqs  | Bytes | Reqs  | Bytes | Elig Bytes | Returned Bytes |
=============================================================================================================================
|     838 |     688 |      150 |        0.00 |     1 |     1M | 59275 |  34GB | 58982 |  34GB |       11GB |           45GB |
=============================================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=3818639180)
==================================================================================================================================================================================================
| Id |            Operation            |       Name        |  Rows   | Cost |   Time    | Start  | Execs |   Rows   | Read  | Read  | Write | Write |  Mem  | Temp  | Activity | Activity Detail |
|    |                                 |                   | (Estim) |      | Active(s) | Active |       | (Actual) | Reqs  | Bytes | Reqs  | Bytes | (Max) | (Max) |   (%)    |   (# samples)   |
==================================================================================================================================================================================================
|  0 | SELECT STATEMENT                |                   |         |      |       246 |   +592 |     1 |      100 |       |       |       |       |     . |     . |          |                 |
|  1 |   WINDOW SORT                   |                   |    250M |   6M |       246 |   +592 |     1 |      100 |     1 |   1MB | 18233 |  11GB |   1GB |  11GB |          |                 |
|  2 |    WINDOW SORT                  |                   |    250M |   6M |       493 |   +345 |     1 |     250M | 19507 |  10GB | 14026 |  10GB |   1GB |  10GB |          |                 |
|  3 |     WINDOW SORT                 |                   |    250M |   6M |       473 |   +118 |     1 |     250M | 16086 |   8GB | 15230 |   8GB |   1GB |   8GB |          |                 |
|  4 |      WINDOW SORT                |                   |    250M |   6M |       346 |     +0 |     1 |     250M | 12595 |   6GB | 11493 |   6GB |   1GB |   6GB |          |                 |
|  5 |       TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 385K |       115 |     +2 |     1 |     250M | 11086 |  11GB |       |       |   7MB |     . |          |                 |
==================================================================================================================================================================================================

SQL Monitoring Report

SQL Text
------------------------------
select /*+ parallel(10) */ first_name , surname , row_number() over (partition by first_name order by surname asc) rn1 , row_number() over (partition by first_name order by surname desc) rn2 , row_number() over (partition by surname order by first_name asc) rn3 , row_number() over (partition by surname order by first_name desc) rn4 from example_customers                   

Global Stats
============================================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Fetch | Buffer | Read | Read  | Write | Write |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   | Calls |  Gets  | Reqs | Bytes | Reqs  | Bytes | Elig Bytes | Returned Bytes |
============================================================================================================================
|     919 |     747 |      172 |        0.00 |     1 |     1M | 116K |  40GB | 72314 |  40GB |       11GB |           51GB |
============================================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=2906577827)
============================================================================================================================================================================================================
| Id |                Operation                 |       Name        |  Rows   | Cost  |   Time    | Start  | Execs |   Rows   | Read  | Read  | Write | Write |  Mem  | Temp  | Activity | Activity Detail |
|    |                                          |                   | (Estim) |       | Active(s) | Active |       | (Actual) | Reqs  | Bytes | Reqs  | Bytes | (Max) | (Max) |   (%)    |   (# samples)   |
============================================================================================================================================================================================================
|  0 | SELECT STATEMENT                         |                   |         |       |         1 |   +435 |     1 |        0 |       |       |       |       |     . |     . |          |                 |
|  1 |   PX COORDINATOR                         |                   |         |       |           |        |     1 |          |       |       |       |       |     . |     . |          |                 |
|  2 |    PX SEND QC (RANDOM)                   | :TQ10003          |    250M |  672K |         1 |   +917 |     1 |        0 |       |       |       |       |     . |     . |          |                 |
|  3 |     WINDOW SORT                          |                   |    250M |  672K |       234 |   +684 |     1 |      100 |     1 |   1MB | 18234 |  11GB |   1GB |  11GB |          |                 |
|  4 |      WINDOW SORT                         |                   |    250M |  672K |       466 |   +452 |     1 |     250M | 19507 |  10GB | 16146 |  10GB |   1GB |  10GB |          |                 |
|  5 |       PX RECEIVE                         |                   |    250M |  672K |           |        |     1 |          |       |       |       |       |     . |     . |          |                 |
|  6 |        PX SEND HASH                      | :TQ10002          |    250M |  672K |       236 |   +447 |     1 |        0 |       |       |       |       |     . |     . |          |                 |
|  7 |         WINDOW SORT                      |                   |    250M |  672K |       480 |   +203 |     1 |     250M | 16086 |   8GB | 16015 |   8GB |   1GB |   8GB |          |                 |
|  8 |          PX RECEIVE                      |                   |    250M |  672K |           |        |     1 |          |       |       |       |       |     . |     . |          |                 |
|  9 |           PX SEND HASH                   | :TQ10001          |    250M |  672K |       245 |   +203 |     1 |        0 |       |       |       |       |     . |     . |          |                 |
| 10 |            WINDOW SORT                   |                   |    250M |  672K |       447 |     +1 |     1 |     250M | 69191 |  12GB | 21919 |  12GB |   1GB |   6GB |          |                 |
| 11 |             PX RECEIVE                   |                   |    250M | 42771 |           |        |     1 |          |       |       |       |       |     . |     . |          |                 |
| 12 |              PX SEND RANGE               | :TQ10000          |    250M | 42771 |       115 |     +4 |     1 |        0 |       |       |       |       |     . |     . |          |                 |
| 13 |               PX BLOCK ITERATOR          |                   |    250M | 42771 |       115 |     +4 |     1 |     250M |       |       |       |       |     . |     . |          |                 |
| 14 |                TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 42771 |       115 |     +4 |     1 |     250M | 11086 |  11GB |       |       |   7MB |     . |          |                 |
============================================================================================================================================================================================================```
4

2 回答 2

1

我测试你的场景，所以让我告诉你。由于内存不足，无法使用CONNECT BY LEVEL，但是我插入了50M条记录，足以进行比较

SQL> select count(*) from test_perf.example_customers ;

  COUNT(*)
----------
   5000000

Elapsed: 00:00:02.15
SQL> desc test_perf.example_customers
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 C1                                                 VARCHAR2(120 CHAR)
 C2                                                 VARCHAR2(120 CHAR)

SQL>
SQL>
SQL> alter table test_perf.example_customers noparallel ;

Table altered.

Elapsed: 00:00:00.01
SQL>

现在，让我们看看两个计划，有和没有平行

SQL> set autotrace traceonly explain
SQL> select count(*) from test_perf.example_customers ;
Elapsed: 00:00:00.00

Execution Plan
----------------------------------------------------------
Plan hash value: 2907982153

--------------------------------------------------------------------------------
| Id  | Operation          | Name              | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                   |     1 | 24110   (1)| 00:00:02 |
|   1 |  SORT AGGREGATE    |                   |     1 |            |          |
|   2 |   TABLE ACCESS FULL| EXAMPLE_CUSTOMERS |  5000K| 24110   (1)| 00:00:02 |
--------------------------------------------------------------------------------

SQL> select /*+parallel(a,10) */ count(*) from test_perf.example_customers a
Elapsed: 00:00:00.00

Execution Plan
----------------------------------------------------------
Plan hash value: 2126708148



 -----------------------------------------------------------------------------------------------------------------
    | Id  | Operation              | Name              | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
    -----------------------------------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT       |                   |     1 |  2677   (1)| 00:00:01 |        |      |            |
    |   1 |  SORT AGGREGATE        |                   |     1 |            |          |        |      |            |
    |   2 |   PX COORDINATOR       |                   |       |            |          |        |      |            |
    |   3 |    PX SEND QC (RANDOM) | :TQ10000          |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
    |   4 |     SORT AGGREGATE     |                   |     1 |            |          |  Q1,00 | PCWP |            |
    |   5 |      PX BLOCK ITERATOR |                   |  5000K|  2677   (1)| 00:00:01 |  Q1,00 | PCWC |            |
    |   6 |       TABLE ACCESS FULL| EXAMPLE_CUSTOMERS |  5000K|  2677   (1)| 00:00:01 |  Q1,00 | PCWP |            |
    -----------------------------------------------------------------------------------------------------------------
    
    Note
    -----
       - Degree of Parallelism is 10 because of table property
    
    SQL>

现在让我们看看实际发生了什么

SQL> set autotrace traceonly
SQL> select count(*) from test_perf.example_customers ;

Elapsed: 00:00:00.45

Execution Plan
----------------------------------------------------------
Plan hash value: 2907982153

--------------------------------------------------------------------------------
| Id  | Operation          | Name              | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                   |     1 | 24110   (1)| 00:00:02 |
|   1 |  SORT AGGREGATE    |                   |     1 |            |          |
|   2 |   TABLE ACCESS FULL| EXAMPLE_CUSTOMERS |  5000K| 24110   (1)| 00:00:02 |
--------------------------------------------------------------------------------


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      68305  consistent gets
          0  physical reads
     358820  redo size
        542  bytes sent via SQL*Net to client
        608  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> set autotrace traceonly
SQL> select /*+parallel(a,10) */ count(*) from test_perf.example_customers a
  2  ;

Elapsed: 00:00:01.55

Execution Plan
----------------------------------------------------------
Plan hash value: 2126708148

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name              | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                   |     1 |  2677   (1)| 00:00:01 |        |      |            |
|   1 |  SORT AGGREGATE        |                   |     1 |            |          |        |      |            |
|   2 |   PX COORDINATOR       |                   |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM) | :TQ10000          |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE     |                   |     1 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR |                   |  5000K|  2677   (1)| 00:00:01 |  Q1,00 | PCWC |            |
|   6 |       TABLE ACCESS FULL| EXAMPLE_CUSTOMERS |  5000K|  2677   (1)| 00:00:01 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------------

Note
-----
   - Degree of Parallelism is 10 because of table property


Statistics
----------------------------------------------------------
         31  recursive calls
          0  db block gets
      91667  consistent gets
      64207  physical reads
          0  redo size
        542  bytes sent via SQL*Net to client
        608  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

让我们刷新缓冲区缓存和共享池，然后再试一次

SQL> select count(*) from test_perf.example_customers ;

  COUNT(*)
----------
   5000000

Elapsed: 00:00:01.44
SQL> select /*+parallel(a,10) */ count(*) from test_perf.example_customers a
  2  ;

  COUNT(*)
----------
   5000000

Elapsed: 00:00:00.66
SQL>

拥有 1 亿条记录

SQL> select count(*) from test_perf.example_customers ;

  COUNT(*)
----------
  10000000

Elapsed: 00:00:11.00
SQL> select /*+parallel(a,16) */  count(*) from test_perf.example_customers a ;

  COUNT(*)
----------
  10000000

Elapsed: 00:00:02.71
SQL>

** 更新 **

将 PARALLEL_DEGREE_POLICY 设置为 AUTO

SQL> alter table test_perf.example_customers noparallel ;

Table altered.

SQL> alter session set parallel_degree_policy=auto ;

Session altered.

SQL> alter session enable parallel query;

Session altered.

SQL> set autotrace traceonly explain
SQL> select /*+parallel */ count(*) from test_perf.example_customers a ;

Execution Plan
----------------------------------------------------------
Plan hash value: 2126708148

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name              | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                   |     1 | 13387   (1)| 00:00:02 |        |      |            |
|   1 |  SORT AGGREGATE        |                   |     1 |            |          |        |      |            |
|   2 |   PX COORDINATOR       |                   |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM) | :TQ10000          |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE     |                   |     1 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR |                   |  5000K| 13387   (1)| 00:00:02 |  Q1,00 | PCWC |            |
|   6 |       TABLE ACCESS FULL| EXAMPLE_CUSTOMERS |  5000K| 13387   (1)| 00:00:02 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------------

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 2
   - parallel scans affinitized for buffer cache

如您所见，使用 50M 行时您会注意到差异，而使用 100M 行时差异更大。我可以向您保证，在正常情况下，与可序列化查询相比，表越大，并行执行速度会更快。Exadata 是另一个世界。Exadata 的智能扫描和单元卸载功能在非常快的磁盘中运行，由于高速 I/O，您不会注意到差异。Key Osborne 有一篇精彩的文章，其中描述了 Exadata 中存储服务器的惊人功能以及存储索引。

http://kerryosborne.oracle-guy.com/2010/08/10/oracle-exadata-storage-indexes/

在 250M 行中，您可能不会注意到，因为 Oracle 花费在创建 QC 和从属设备上的时间，以及它们之间的通信（即内存）或多或少与 Exadata 存储层在读取查询中花费的时间相同.

抱歉，我无法在 Exadata 中进行测试；)

于 2020-07-27T15:35:12.680 回答
0

我找到了根本问题，涉及的步骤是：

  • sql 监视器报告报告了 100% DOP 降级以尝试使用并行执行
  • 使用 dbms_sqltune.report_sql_detail，我能够获得有关执行计划中步骤的更多信息
  • 使用它，我能够获得 PX COORDINATOR`操作的 DOP 降级原因 (352)
  • 352有时意味着进程数不足，但有很多可用的px进程
  • 352 有时意味着资源管理器具有最大 DOP 限制
  • 资源管理器有几个计划，其中一些计划的并行度限制为 1 indba_rsrc_plan_directives
  • 通过为会话交换消费者组，并行提示按预期工作
  • 并行执行查询的执行时间从 35 秒缩短到不到 3 秒

使用的代码：

begin
  dbms_resource_manager.switch_consumer_group_for_sess(session_id => 2690
                                                      ,session_serial => 25388
                                                      ,consumer_group => 'HIGH'
                                                      );
  
end;
select /*+ parallel */ count(*)
  from EXAMPLE_CUSTOMERS

生成的sql报告：

SQL Monitoring Report

SQL Text
------------------------------
select /*+ parallel */ count(*) from EXAMPLE_CUSTOMERS

Global Information
------------------------------
 Status              :  DONE (ALL ROWS)                   
 Instance ID         :  1                                 
 Session             :  TESTING (2690:25388)              
 SQL ID              :  cr2th8jckamab                     
 SQL Execution ID    :  16777219                          
 Execution Started   :  07/28/2020 00:05:19               
 First Refresh Time  :  07/28/2020 00:05:18               
 Last Refresh Time   :  07/28/2020 00:05:21               
 Duration            :  2s                                
 Module/Action       :  PL/SQL Developer/SQL Window - New 
 Service             :  UCL                               
 Program             :  plsqldev.exe                      
 Fetch Calls         :  1                                 

Global Stats
======================================================================================================================================
| Elapsed |   Cpu   |    IO    | Application | Concurrency |  Other   | Fetch | Buffer | Read  | Read  |  Offload   |    Offload     |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   |  Waits(s)   | Waits(s) | Calls |  Gets  | Reqs  | Bytes | Elig Bytes | Returned Bytes |
======================================================================================================================================
|      51 |      36 |     3.08 |        0.00 |        0.00 |       12 |     1 |     1M | 11137 |  11GB |       11GB |           11GB |
======================================================================================================================================

Parallel Execution Details (DOP=25 , Servers Allocated=25)
 Instances  : 2 

============================================================================================================================================================================================
| Instance |      Name      | Type  | Server# | Elapsed |   Cpu   |    IO    | Application | Concurrency |  Other   | Buffer | Read | Read  |  Offload   |    Offload     | Wait Events |  |
|          |                |       |         | Time(s) | Time(s) | Waits(s) |  Waits(s)   |  Waits(s)   | Waits(s) |  Gets  | Reqs | Bytes | Elig Bytes | Returned Bytes | (sample #)  |  |
============================================================================================================================================================================================
| 1        | PX Coordinator | QC    |         |    0.48 |    0.05 |          |        0.00 |             |     0.43 |      7 |      |     . |          . |              . |        NaN% |  |
| 1        | p000           | Set 1 |       1 |    1.97 |    1.39 |     0.12 |             |        0.00 |     0.47 |  56158 |  440 | 437MB |      437MB |          437MB |        NaN% |  |
| 1        | p001           | Set 1 |       2 |    2.09 |    1.45 |     0.13 |             |             |     0.51 |  56563 |  443 | 440MB |      440MB |          440MB |        NaN% |  |
| 1        | p002           | Set 1 |       3 |    1.94 |    1.38 |     0.11 |             |             |     0.45 |  54215 |  424 | 422MB |      422MB |          422MB |        NaN% |  |
| 1        | p003           | Set 1 |       4 |    2.05 |    1.19 |     0.11 |             |        0.00 |     0.74 |  44952 |  355 | 350MB |      350MB |          350MB |        NaN% |  |
| 1        | p004           | Set 1 |       5 |    2.09 |    1.47 |     0.13 |             |             |     0.49 |  57279 |  448 | 446MB |      446MB |          446MB |        NaN% |  |
| 1        | p005           | Set 1 |       6 |    2.09 |    1.41 |     0.11 |             |        0.00 |     0.57 |  54826 |  428 | 427MB |      427MB |          427MB |        NaN% |  |
| 1        | p006           | Set 1 |       7 |    2.13 |    1.16 |     0.10 |             |             |     0.87 |  46373 |  363 | 361MB |      361MB |          361MB |        NaN% |  |
| 1        | p007           | Set 1 |       8 |    2.18 |    1.18 |     0.13 |             |             |     0.87 |  46237 |  361 | 360MB |      360MB |          360MB |        NaN% |  |
| 1        | p008           | Set 1 |       9 |    2.10 |    1.39 |     0.13 |             |             |     0.59 |  55364 |  433 | 431MB |      431MB |          431MB |        NaN% |  |
| 1        | p009           | Set 1 |      10 |    2.11 |    1.48 |     0.12 |             |        0.00 |     0.51 |  59575 |  465 | 464MB |      464MB |          464MB |        NaN% |  |
| 1        | p00a           | Set 1 |      11 |    2.02 |    1.21 |     0.10 |             |             |     0.72 |  45981 |  360 | 358MB |      358MB |          358MB |        NaN% |  |
| 1        | p00b           | Set 1 |      12 |    1.98 |    1.20 |     0.11 |             |             |     0.67 |  48583 |  382 | 378MB |      378MB |          378MB |        NaN% |  |
| 1        | p00c           | Set 1 |      13 |    2.03 |    1.36 |     0.13 |             |             |     0.54 |  53580 |  421 | 417MB |      417MB |          417MB |        NaN% |  |
| 1        | p00d           | Set 1 |      14 |    2.17 |    1.22 |     0.10 |             |             |     0.85 |  48413 |  380 | 377MB |      377MB |          377MB |        NaN% |  |
| 1        | p00e           | Set 1 |      15 |    2.04 |    1.17 |     0.10 |             |             |     0.77 |  47261 |  370 | 368MB |      368MB |          368MB |        NaN% |  |
| 1        | p00f           | Set 1 |      16 |    2.07 |    1.18 |     0.11 |             |             |     0.79 |  46365 |  363 | 361MB |      361MB |          361MB |        NaN% |  |
| 2        | p000           | Set 1 |      17 |    1.89 |    1.52 |     0.12 |             |             |     0.25 |  61265 |  481 | 477MB |      477MB |          477MB |        NaN% |  |
| 2        | p001           | Set 1 |      18 |    1.91 |    1.68 |     0.14 |             |        0.00 |     0.09 |  66176 |  519 | 515MB |      515MB |          515MB |        NaN% |  |
| 2        | p002           | Set 1 |      19 |    1.87 |    1.51 |     0.14 |             |             |     0.22 |  66672 |  522 | 519MB |      519MB |          519MB |        NaN% |  |
| 2        | p003           | Set 1 |      20 |    1.99 |    1.66 |     0.14 |             |        0.00 |     0.20 |  64478 |  503 | 502MB |      502MB |          502MB |        NaN% |  |
| 2        | p004           | Set 1 |      21 |    1.96 |    1.70 |     0.15 |             |             |     0.12 |  70023 |  551 | 545MB |      545MB |          545MB |        NaN% |  |
| 2        | p005           | Set 1 |      22 |    1.94 |    1.68 |     0.15 |             |             |     0.11 |  63342 |  496 | 493MB |      493MB |          493MB |        NaN% |  |
| 2        | p006           | Set 1 |      23 |    1.89 |    1.68 |     0.14 |             |             |     0.08 |  72179 |  565 | 562MB |      562MB |          562MB |        NaN% |  |
| 2        | p007           | Set 1 |      24 |    2.07 |    1.74 |     0.14 |             |             |     0.19 |  71812 |  562 | 559MB |      559MB |          559MB |        NaN% |  |
| 2        | p008           | Set 1 |      25 |    1.97 |    1.57 |     0.14 |             |             |     0.26 |  64243 |  502 | 500MB |      500MB |          500MB |        NaN% |  |
============================================================================================================================================================================================

Instance Drill-Down
===========================================================================================================================================================================================================================================
| Instance | Process Names                                                                      | Elapsed |   Cpu   |    IO    | Application | Concurrency |  Other   | Buffer | Read | Read  |  Offload   |    Offload     | Wait Events |
|          |                                                                                    | Time(s) | Time(s) | Waits(s) |  Waits(s)   |  Waits(s)   | Waits(s) |  Gets  | Reqs | Bytes | Elig Bytes | Returned Bytes |             |
===========================================================================================================================================================================================================================================
|    1     | QC p000 p001 p002 p003 p004 p005 p006 p007 p008 p009 p00a p00b p00c p00d p00e p00f |      34 |      21 |     1.82 |        0.00 |        0.00 |       11 |   822K | 6436 |   6GB |        6GB |            6GB |             |
|    2     | p000 p001 p002 p003 p004 p005 p006 p007 p008                                       |      18 |      15 |     1.26 |             |        0.00 |     1.52 |   600K | 4701 |   5GB |        5GB |            5GB |             |
===========================================================================================================================================================================================================================================

SQL Plan Monitoring Details (Plan Hash Value=2126708148)
============================================================================================================================================================================
| Id |            Operation             |       Name        |  Rows   | Cost  |   Time    | Start  | Execs |   Rows   | Read  | Read  |  Mem  | Activity | Activity Detail |
|    |                                  |                   | (Estim) |       | Active(s) | Active |       | (Actual) | Reqs  | Bytes | (Max) |   (%)    |   (# samples)   |
============================================================================================================================================================================
|  0 | SELECT STATEMENT                 |                   |         |       |         1 |     +2 |     1 |        1 |       |       |     . |          |                 |
|  1 |   SORT AGGREGATE                 |                   |       1 |       |         1 |     +2 |     1 |        1 |       |       |     . |          |                 |
|  2 |    PX COORDINATOR                |                   |         |       |         2 |     +1 |    26 |       25 |       |       |     . |          |                 |
|  3 |     PX SEND QC (RANDOM)          | :TQ10000          |       1 |       |         2 |     +1 |    25 |       25 |       |       |     . |          |                 |
|  4 |      SORT AGGREGATE              |                   |       1 |       |         2 |     +1 |    25 |       25 |       |       |     . |          |                 |
|  5 |       PX BLOCK ITERATOR          |                   |    250M | 17097 |         2 |     +1 |    25 |     250M |       |       |     . |          |                 |
|  6 |        TABLE ACCESS STORAGE FULL | EXAMPLE_CUSTOMERS |    250M | 17097 |         3 |     +0 |   379 |     250M | 11137 |  11GB | 165MB |          |                 |
============================================================================================================================================================================
于 2020-07-27T23:19:28.400 回答