[20220121]Hash Aggregation.txt
[20220121]Hash Aggregation.txt
--//重複裡面的測試:
1.環境:
SCOTT@book> @ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
create table t1
as
with generator as (
select
rownum id
from dual
connect by
level <= 1e4 -- > comment to avoid wordpress format issue
)
select
trunc(dbms_random.value(0,262144)) n_256K,
trunc(dbms_random.value(0,131072)) n_128K,
trunc(dbms_random.value(0,8192)) n_8k
from
generator v1,
generator v2
where
rownum <= 8 * 1048576 -- > comment to avoid wordpress format issue
;
set arraysize 1000
set timing on
set autotrace traceonly
prompt ===========
prompt No Order by
prompt ===========
select n_8K, count(*) ct from t1 group by n_8k ;
prompt =============
prompt With Order by
prompt =============
select n_8K, count(*) ct from t1 group by n_8k order by 1 ;
--//顯示的執行計劃如下:
SCOTT@book> select n_8K, count(*) ct from t1 group by n_8k ;
8192 rows selected.
Elapsed: 00:00:01.47
Execution Plan
----------------------------------------------------------
Plan hash value: 136660032
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8192 | 32768 | 6261 (5)| 00:01:16 |
| 1 | HASH GROUP BY | | 8192 | 32768 | 6261 (5)| 00:01:16 |
| 2 | TABLE ACCESS FULL| T1 | 8388K| 32M| 6024 (1)| 00:01:13 |
---------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
21876 consistent gets
21873 physical reads
0 redo size
79540 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
10 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8192 rows processed
SCOTT@book> select n_8K, count(*) ct from t1 group by n_8k order by 1 ;
8192 rows selected.
Elapsed: 00:00:03.58
Execution Plan
----------------------------------------------------------
Plan hash value: 3946799371
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8192 | 32768 | 6261 (5)| 00:01:16 |
| 1 | SORT GROUP BY | | 8192 | 32768 | 6261 (5)| 00:01:16 |
| 2 | TABLE ACCESS FULL| T1 | 8388K| 32M| 6024 (1)| 00:01:13 |
---------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
21876 consistent gets
21873 physical reads
0 redo size
79540 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
10 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
8192 rows processed
--//你可以發現走SORT GROUP BY需要將近4秒,HASH GROUP BY快一些.
SCOTT@book> @ sl all
alter session set statistics_level = all;
Session altered.
select n_8K, count(*) ct from t1 group by n_8k ;
SCOTT@book> @ dpc '' '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 92vdmp2bp6nt8, child number 1
-------------------------------------
select n_8K, count(*) ct from t1 group by n_8k
Plan hash value: 136660032
---------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 6261 (100)| | 8192 |00:00:02.08 | 21876 | 21873 | | | |
| 1 | HASH GROUP BY | | 1 | 8192 | 32768 | 6261 (5)| 00:01:16 | 8192 |00:00:02.08 | 21876 | 21873 | 415M| 16M| 5521K (0)|
| 2 | TABLE ACCESS FULL| T1 | 1 | 8388K| 32M| 6024 (1)| 00:01:13 | 8388K|00:00:00.60 | 21876 | 21873 | | | |
---------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$1 / T1@SEL$1
select n_8K, count(*) ct from t1 group by n_8k order by 1;
SCOTT@book> @ dpc '' '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 00uj4uh3nzz1y, child number 1
-------------------------------------
select n_8K, count(*) ct from t1 group by n_8k order by 1
Plan hash value: 3946799371
---------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 6261 (100)| | 8192 |00:00:03.89 | 21876 | 21873 | | | |
| 1 | SORT GROUP BY | | 1 | 8192 | 32768 | 6261 (5)| 00:01:16 | 8192 |00:00:03.89 | 21876 | 21873 | 549K| 549K| 487K (0)|
| 2 | TABLE ACCESS FULL| T1 | 1 | 8388K| 32M| 6024 (1)| 00:01:13 | 8388K|00:00:00.57 | 21876 | 21873 | | | |
---------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$1 / T1@SEL$1
--//如果上下比較可以發現除了執行計劃不同外,差別在於Omem,1mem,Used-Mem的使用上,SORT GROUP BY消耗要小許多,但是如果排序的記
--//錄多有點慢.
--//作者提供提示來最佳化該語句,實際上這個僅僅在排序很大的情況下才建議使用走HASH GROUP BY,但是資源消耗也大一些.
select dist_8k, ct from ( select /*+ no_merge */ n_8K dist_8k, count(*) ct from t1 group by n_8k) order by dist_8k ;
select /*+ use_hash_aggregation */ n_8K, count(*) ct from t1 group by n_8k order by 1 ;
--//執行計劃如下:
SCOTT@book> @ dpc '' '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID d5nc55qsjnszm, child number 0
-------------------------------------
select dist_8k, ct from ( select /*+ no_merge */ n_8K dist_8k,
count(*) ct from t1 group by n_8k) order by dist_8k
Plan hash value: 1705136228
-----------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 6261 (100)| | 8192 |00:00:01.92 | 21876 | 21873 | | | |
| 1 | SORT ORDER BY | | 1 | 8192 | 208K| 6261 (5)| 00:01:16 | 8192 |00:00:01.92 | 21876 | 21873 | 407K| 407K| 361K (0)|
| 2 | VIEW | | 1 | 8192 | 208K| 6261 (5)| 00:01:16 | 8192 |00:00:01.91 | 21876 | 21873 | | | |
| 3 | HASH GROUP BY | | 1 | 8192 | 32768 | 6261 (5)| 00:01:16 | 8192 |00:00:01.91 | 21876 | 21873 | 415M| 16M| 5527K (0)|
| 4 | TABLE ACCESS FULL| T1 | 1 | 8388K| 32M| 6024 (1)| 00:01:13 | 8388K|00:00:00.55 | 21876 | 21873 | | | |
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$2 / from$_subquery$_001@SEL$1
3 - SEL$2
4 - SEL$2 / T1@SEL$2
SCOTT@book> @ dpc '' '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID cny0n0cbpsy00, child number 0
-------------------------------------
select /*+ use_hash_aggregation */ n_8K, count(*) ct from t1 group by
n_8k order by 1
Plan hash value: 2808104874
----------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 6261 (100)| | 8192 |00:00:01.92 | 21876 | 21873 | | | |
| 1 | SORT ORDER BY | | 1 | 8192 | 32768 | 6261 (5)| 00:01:16 | 8192 |00:00:01.92 | 21876 | 21873 | 407K| 407K| 361K (0)|
| 2 | HASH GROUP BY | | 1 | 8192 | 32768 | 6261 (5)| 00:01:16 | 8192 |00:00:01.92 | 21876 | 21873 | 415M| 16M| 5526K (0)|
| 3 | TABLE ACCESS FULL| T1 | 1 | 8388K| 32M| 6024 (1)| 00:01:13 | 8388K|00:00:00.56 | 21876 | 21873 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
3 - SEL$1 / T1@SEL$1
--//作者還給了一個如何加入提示的例子.
SCOTT@book> create table t2 as select * from all_objects ;
Table created.
SCOTT@book> create table t3 as select * from all_objects ;
Table created.
--//分析略.
select
t2.owner, count(*)
from
t2
where
t2.object_id in (
select t3.object_id
from t3
where t3.object_type = 'TABLE'
)
group by
t2.owner
order by
t2.owner
/
SCOTT@book> @ dpc '' outline ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 6pcbhwhh1659s, child number 0
-------------------------------------
select t2.owner, count(*) from t2 where
t2.object_id in ( select t3.object_id
from t3 where t3.object_type = 'TABLE' )
group by t2.owner order by t2.owner
Plan hash value: 2646727453
------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 677 (100)| | 23 |00:00:00.07 | 2428 | 2424 | | | |
| 1 | SORT GROUP BY | | 1 | 29 | 725 | 677 (1)| 00:00:09 | 23 |00:00:00.07 | 2428 | 2424 | 2048 | 2048 | 2048 (0)|
|* 2 | HASH JOIN RIGHT SEMI| | 1 | 2121 | 53025 | 676 (1)| 00:00:09 | 2978 |00:00:00.07 | 2428 | 2424 | 2293K| 2293K| 1487K (0)|
|* 3 | TABLE ACCESS FULL | T3 | 1 | 2121 | 29694 | 338 (1)| 00:00:05 | 2979 |00:00:00.01 | 1214 | 1212 | | | |
| 4 | TABLE ACCESS FULL | T2 | 1 | 84838 | 911K| 338 (1)| 00:00:05 | 84838 |00:00:00.02 | 1214 | 1212 | | | |
------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$5DA710D3
~~~~~~~~~~~~~~~~~~
3 - SEL$5DA710D3 / T3@SEL$2
4 - SEL$5DA710D3 / T2@SEL$1
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
DB_VERSION('11.2.0.4')
ALL_ROWS
OUTLINE_LEAF(@"SEL$5DA710D3")
UNNEST(@"SEL$2")
OUTLINE(@"SEL$1")
OUTLINE(@"SEL$2")
FULL(@"SEL$5DA710D3" "T2"@"SEL$1")
FULL(@"SEL$5DA710D3" "T3"@"SEL$2")
LEADING(@"SEL$5DA710D3" "T2"@"SEL$1" "T3"@"SEL$2")
USE_HASH(@"SEL$5DA710D3" "T3"@"SEL$2")
SWAP_JOIN_INPUTS(@"SEL$5DA710D3" "T3"@"SEL$2")
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")
3 - filter("T3"."OBJECT_TYPE"='TABLE')
select /*+ use_hash_aggregation(@SEL$5DA710D3) */
t2.owner, count(*)
from
t2
where
t2.object_id in (
select t3.object_id
from t3
where t3.object_type = 'TABLE'
)
group by
t2.owner
order by
t2.owner
/
SCOTT@book> @ dpc '' outline ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 3p240auatqfbf, child number 0
-------------------------------------
select /*+ use_hash_aggregation(@SEL$5DA710D3) */ t2.owner,
count(*) from t2 where t2.object_id in (
select t3.object_id from t3 where
t3.object_type = 'TABLE' ) group by t2.owner order by
t2.owner
Plan hash value: 4018836393
-------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 677 (100)| | 23 |00:00:00.07 | 2428 | 2424 | | | |
| 1 | SORT ORDER BY | | 1 | 29 | 725 | 677 (1)| 00:00:09 | 23 |00:00:00.07 | 2428 | 2424 | 2048 | 2048 | 2048 (0)|
| 2 | HASH GROUP BY | | 1 | 29 | 725 | 677 (1)| 00:00:09 | 23 |00:00:00.07 | 2428 | 2424 | 1322K| 1322K| 1117K (0)|
|* 3 | HASH JOIN RIGHT SEMI| | 1 | 2121 | 53025 | 676 (1)| 00:00:09 | 2978 |00:00:00.07 | 2428 | 2424 | 2293K| 2293K| 1531K (0)|
|* 4 | TABLE ACCESS FULL | T3 | 1 | 2121 | 29694 | 338 (1)| 00:00:05 | 2979 |00:00:00.01 | 1214 | 1212 | | | |
| 5 | TABLE ACCESS FULL | T2 | 1 | 84838 | 911K| 338 (1)| 00:00:05 | 84838 |00:00:00.02 | 1214 | 1212 | | | |
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$5DA710D3
4 - SEL$5DA710D3 / T3@SEL$2
5 - SEL$5DA710D3 / T2@SEL$1
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
DB_VERSION('11.2.0.4')
ALL_ROWS
OUTLINE_LEAF(@"SEL$5DA710D3")
UNNEST(@"SEL$2")
OUTLINE(@"SEL$1")
OUTLINE(@"SEL$2")
FULL(@"SEL$5DA710D3" "T2"@"SEL$1")
FULL(@"SEL$5DA710D3" "T3"@"SEL$2")
LEADING(@"SEL$5DA710D3" "T2"@"SEL$1" "T3"@"SEL$2")
USE_HASH(@"SEL$5DA710D3" "T3"@"SEL$2")
SWAP_JOIN_INPUTS(@"SEL$5DA710D3" "T3"@"SEL$2")
USE_HASH_AGGREGATION(@"SEL$5DA710D3")
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")
4 - filter("T3"."OBJECT_TYPE"='TABLE')
--//除非你遇到大結果集合的排序,可能這樣的最佳化才有意義.
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/267265/viewspace-2853434/，如需轉載，請註明出處，否則將追究法律責任。
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