通过案例学调优之--IndexFULLSCAN和IndexFASTFULLSCAN
程序员文章站
2022-04-14 12:27:30
...
通过案例学调优之--Index FULL SCAN和Index FAST FULL SCAN Index FULL SCAN 和 ndex FAST FULL SCAN工作原理： Index FULL SCAN 和I ndex FAST FULL SCAN 的适用情况：适用于我们想选择的列都包含在索引里边时，这时候就可以使用IFS或者FFS来代替全表扫描来
通过案例学调优之--Index FULL SCAN和Index FAST FULL SCAN
Index FULL SCAN 和ndex FAST FULL SCAN工作原理：
Index FULL SCAN 和Index FAST FULL SCAN的适用情况：适用于我们想选择的列都包含在索引里边时，这时候就可以使用IFS或者FFS来代替全表扫描来得到想要的结果。
INDEX FULL SCAN：
HINT写法：INDEX(表名 索引名)
原理：ORACLE定位到索引的ROOT BLOCK，然后到BRANCH BLOCK（如果有的话），再定位到第一个LEAF BLOCK, 然后根据LEAF BLOCK的双向链表顺序读取。它所读取的块都是有顺序的，也是经过排序的。
INDEX FAST FULL SCAN：
HINT写法：INDEX_FFS(表名 索引名)
原理：从段头开始，读取包含位图块，ROOT BLOCK，所有的BRANCH BLOCK，LEAF BLOCK，读取的顺序完全有物理存储位置决定，并采取多块读，每次读取DB_FILE_MULTIBLOCK_READ_COUNT个块。查询某个表记录总数的时候，往往基于PRIMARY KEY的INDEX FAST FULL SCAN是最有效的。
Fast Full Index Scans :
Fast full index scans are an alternative to a full table scan when the index contains all the columns that are needed for the query, and at least one column in the index key has the NOT NULL constraint. A fast full scan accesses the data in the index itself, without accessing the table. It cannot be used to eliminate a sort operation, because the data is not ordered by the index key. It reads the entire index using multiblock reads, unlike a full index scan, and can be parallelized.
Fast full scan is available only with the CBO. You can specify it with the initialization parameter OPTIMIZER_FEATURES_ENABLE or the INDEX_FFS hint. Fast full index scans cannot be performed against bitmap indexes.
A fast full scan is faster than a normal full index scan in that it can use multiblock I/O and can be parallelized just like a table scan.
http://download-west.oracle.com/doc…imops.htm#51111
Full Table Scans :
This type of scan reads all rows from a table and filters out those that do not meet the selection criteria. During a full table scan, all blocks in the table that are under the high water mark are scanned. Each row is examined to determine whether it satisfies the statement’s WHERE clause.
When Oracle performs a full table scan, the blocks are read sequentially. Because the blocks are adjacent, I/O calls larger than a single block can be used to speed up the process. The size of the read calls range from one block to the number of blocks indicated by the initialization parameter DB_FILE_MULTIBLOCK_READ_COUNT. Using multiblock reads means a full table scan can be performed very efficiently. Each block is read only once.
http://download-west.oracle.com/doc…imops.htm#44852
案例分析：
1、创建表和索引
16:02:10 SYS@ prod >create table t as select * from dba_objects where 1=2;
Table created.
16:05:43 SYS@ prod >insert into t select * from dba_objects where object_id is not null;
73025 rows created.
16:06:46 SYS@ prod >select count(*) from t;
COUNT(*)
----------
73025
16:06:56 SYS@ prod >commit;
Commit complete.
16:13:48 SYS@ prod >exec dbms_stats.gather_table_stats('SYS','T',cascade=>true);
PL/SQL procedure successfully completed.
16:14:33 SYS@ prod >set autotrace trace
16:15:32 SYS@ prod >select object_id from t;
73025 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 356K| 284 (1)| 00:00:04 |
| 1 | TABLE ACCESS FULL| T | 73025 | 356K| 284 (1)| 00:00:04 |
--------------------------------------------------------------------------
Statistics
----------------------------------------------------------
141 recursive calls
0 db block gets
5857 consistent gets
1038 physical reads
0 redo size
1060958 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
73025 rows processed
从上面的执行计划中可知，此时走了全表扫描。
--由于我们需要查询的列为object_id，因此理论上只需要读取索引就应该可以返回所有数据，而此时为什么是全表扫描呢？
--这是因为NULL值与索引的特性所决定的。即null值不会被存储到B树索引。因此应该为表 t 的列 object_id 添加 not null 约束。
16:16:14 SYS@ prod >desc t;
Name Null? Type
----------------------------------------------------------------- -------- --------------------------------------------
OWNER VARCHAR2(30)
OBJECT_NAME VARCHAR2(128)
SUBOBJECT_NAME VARCHAR2(30)
OBJECT_ID NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(19)
CREATED DATE
LAST_DDL_TIME DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)
NAMESPACE NUMBER
EDITION_NAME VARCHAR2(30)
在object_id上添加not null约束
16:16:42 SYS@ prod >alter table t modify(object_id not null);
Table altered.
Elapsed: 00:00:00.34
16:16:46 SYS@ prod >desc t
Name Null? Type
----------------------------------------------------------------- -------- --------------------------------------------
OWNER VARCHAR2(30)
OBJECT_NAME VARCHAR2(128)
SUBOBJECT_NAME VARCHAR2(30)
OBJECT_ID NOT NULL NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(19)
CREATED DATE
LAST_DDL_TIME DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)
NAMESPACE NUMBER
EDITION_NAME VARCHAR2(30)
2、对Index_FS和Index_FFS对比
16:16:49 SYS@ prod >select object_id from t;
73025 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1220328745
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 356K| 46 (0)| 00:00:01 |
| 1 | INDEX FAST FULL SCAN| T_ID | 73025 | 356K| 46 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
5028 consistent gets
0 physical reads
0 redo size
1060958 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
73025 rows processed
16:17:20 SYS@ prod >select * from t;
73025 rows selected.
Elapsed: 00:00:01.99
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 6917K| 284 (1)| 00:00:04 |
| 1 | TABLE ACCESS FULL| T | 73025 | 6917K| 284 (1)| 00:00:04 |
--------------------------------------------------------------------------
Statistics
----------------------------------------------------------
284 recursive calls
0 db block gets
5885 consistent gets
27 physical reads
0 redo size
8096826 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
73025 rows processed
16:20:19 SYS@ prod >select /*+ index(t t_id) */ object_id from t;
73025 rows selected.
Elapsed: 00:00:00.56
Execution Plan
----------------------------------------------------------
Plan hash value: 2842924753
-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 356K| 163 (0)| 00:00:02 |
| 1 | INDEX FULL SCAN | T_ID | 73025 | 356K| 163 (0)| 00:00:02 |
-------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
5021 consistent gets
0 physical reads
0 redo size
1060958 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
73025 rows processed
从以上（full table，index full scan，index fast full scan）付出的cost进行比较，index_ffs的cost最小(46)
3、在对查询做排序时对比
16:20:48 SYS@ prod >select object_id from t order by object_id ;
73025 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2842924753
-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 356K| 163 (0)| 00:00:02 |
| 1 | INDEX FULL SCAN | T_ID | 73025 | 356K| 163 (0)| 00:00:02 |
-------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
5021 consistent gets
0 physical reads
0 redo size
1060958 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
73025 rows processed
16:21:28 SYS@ prod >select /*+ index_ffs(t t_id) */ object_id from t order by object_id;
73025 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2317820129
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 356K| | 271 (2)| 00:00:04 |
| 1 | SORT ORDER BY | | 73025 | 356K| 872K| 271 (2)| 00:00:04 |
| 2 | INDEX FAST FULL SCAN| T_ID | 73025 | 356K| | 46 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
170 consistent gets
0 physical reads
0 redo size
1060958 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
73025 rows processed
16:23:02 SYS@ prod >select /*+ full(t) */ object_id from t order by object_id;
73025 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 961378228
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 73025 | 356K| | 508 (1)| 00:00:07 |
| 1 | SORT ORDER BY | | 73025 | 356K| 872K| 508 (1)| 00:00:07 |
| 2 | TABLE ACCESS FULL| T | 73025 | 356K| | 284 (1)| 00:00:04 |
-----------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
1043 consistent gets
32 physical reads
0 redo size
1060958 bytes sent via SQL*Net to client
53963 bytes received via SQL*Net from client
4870 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
73025 rows processed
从上面的执行计划中可以看出，只要是涉及到排序操作，Oracle会毫不犹豫地选择INDEX FULL SCAN，因为INDEX FULL SCAN方式扫描一定是 按创建索引是的方式来排序的。
4、对index_fs 和 index_ffs查看trace
INDEX_FS:
16:45:24 sys@ prod >alter session set events '10046 trace name context forever,level 12';
16:32:34 sys@ prod >set autotrace trace
16:31:42 sys@ prod >select /*+ index (t t_id) */ object_id from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 2842924753
-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 19 | 247 | 1 (0)| 00:00:01 |
| 1 | INDEX FULL SCAN | T_ID | 19 | 247 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
753 bytes sent via SQL*Net to client
426 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
19 rows processed
16:33:00 sys@ prod >alter session set events '10046 trace name context off';
Session altered.
查看trace文件内容（节选）
select /* index(t t_id) */ object_id from t
END OF STMT
PARSE #4:c=5000,e=5235,p=7,cr=9,cu=0,mis=1,r=0,dep=0,og=1,plh=2842924753,tim=1416818316519023
EXEC #4:c=0,e=25,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=2842924753,tim=1416818316519139
WAIT #4: nam='SQL*Net message to client' ela= 3 driver id=1650815232 #bytes=1 p3=0 obj#=76546 tim=1416818316519211
WAIT #4: nam='db file sequential read' ela= 0 file#=4 block#=139 blocks=1 obj#=76547 tim=1416818316519280
FETCH #4:c=999,e=58,p=1,cr=1,cu=0,mis=0,r=1,dep=0,og=1,plh=2842924753,tim=1416818316519303
16:44:09 SYS@ prod >select object_name,object_id,object_type from dba_objects
16:44:30 2 where object_id='76547';
OBJECT_NAME OBJECT_ID OBJECT_TYPE
-------------------- ---------- -------------------
T_ID 76547 INDEX
WAIT #4: nam='db file sequential read'，在T_ID的index上，产生了单块读得wait。
INDEX_FFS:
16:45:24 sys@ prod >alter session set events '10046 trace name context forever,level 12';
Session altered.
16:46:10 SCOTT@ prod >set autotrace trace
16:46:16 SCOTT@ prod >select /*+ index_ffs(t t_id) */ object_id from t
19 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1220328745
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 19 | 247 | 2 (0)| 00:00:01 |
| 1 | INDEX FAST FULL SCAN| T_ID | 19 | 247 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
6 consistent gets
0 physical reads
0 redo size
753 bytes sent via SQL*Net to client
426 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
19 rows processed
16:46:17 SCOTT@ prod >alter session set events '10046 trace name context off';
Session altered.
查看trace文件内容（节选）
select /*+ index_ffs(t t_id) */ object_id from t
END OF STMT
PARSE #19:c=1000,e=1050,p=0,cr=0,cu=0,mis=1,r=0,dep=0,og=1,plh=1220328745,tim=1416818962627696
EXEC #19:c=0,e=28,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=1220328745,tim=1416818962627788
WAIT #19: nam='SQL*Net message to client' ela= 3 driver id=1650815232 #bytes=1 p3=0 obj#=19 tim=1416818962627824
WAIT #19: nam='db file sequential read' ela= 10 file#=1 block#=91000 blocks=1 obj#=76545 tim=1416818962627888
WAIT #19: nam='db file scattered read' ela= 20 file#=1 block#=91001 blocks=7 obj#=76545 tim=1416818962627977
FETCH #19:c=0,e=181,p=8,cr=8,cu=0,mis=0,r=1,dep=0,og=1,plh=1220328745,tim=1416818962628030
WAIT #19: nam='SQL*Net message from client' ela= 235 driver id=1650815232 #bytes=1 p3=0 obj#=76545 tim=1416818962630284
16:53:45 SYS@ prod >select object_name,object_type,object_id from dba_objects
16:54:02 2 where object_id=76545;
OBJECT_NAME OBJECT_TYPE OBJECT_ID
-------------------- ------------------- ----------
T_ID INDEX 76545
WAIT #19: nam='db file sequential read' block#=91000 blocks=1 ,对索引段的头部块，做了单块读
WAIT #19: nam='db file scattered read' block#=91001 blocks=7，对index的其余的块，做了多块的读取
进一步验证：
1）查看T_ID索引段分配的block，其中block#91000为段头块
16:55:12 SYS@ prod >col segment_name for a20
16:55:18 SYS@ prod >select segment_name,segment_type,file_id,block_id,blocks from dba_extents
16:55:50 2 where segment_name='T_ID' and owner='SYS';
SEGMENT_NAME SEGMENT_TYPE FILE_ID BLOCK_ID BLOCKS
-------------------- ------------------ ---------- ---------- ----------
T_ID INDEX 1 91000 8
T_ID INDEX 1 92032 8
T_ID INDEX 1 92040 8
T_ID INDEX 1 92048 8
T_ID INDEX 1 92056 8
T_ID INDEX 1 92064 8
T_ID INDEX 1 92072 8
T_ID INDEX 1 92080 8
T_ID INDEX 1 92088 8
T_ID INDEX 1 92096 8
T_ID INDEX 1 92104 8
T_ID INDEX 1 92112 8
T_ID INDEX 1 92120 8
T_ID INDEX 1 92128 8
T_ID INDEX 1 92136 8
T_ID INDEX 1 92144 8
T_ID INDEX 1 92160 128
17 rows selected.
2）对block#91000做dump
16:56:19 SYS@ prod >alter system dump datafile 1 block 91000;
System altered.
[oracle@RH6 ~]$ more /u01/app/oracle/diag/rdbms/prod/prod/trace/prod_ora_3415.trc
Trace file /u01/app/oracle/diag/rdbms/prod/prod/trace/prod_ora_3415.trc
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
ORACLE_HOME = /u01/app/oracle/product/11.2.0/db_1
System name: Linux
Node name: RH6
Release: 2.6.32-71.el6.i686
Version: #1 SMP Wed Sep 1 01:26:34 EDT 2010
Machine: i686
Instance name: prod
Redo thread mounted by this instance: 1
Oracle process number: 17
Unix process pid: 3415, image: oracle@RH6 (TNS V1-V3)
*** 2014-11-24 16:57:55.182
*** SESSION ID:(45.143) 2014-11-24 16:57:55.182
*** CLIENT ID:() 2014-11-24 16:57:55.182
*** SERVICE NAME:(SYS$USERS) 2014-11-24 16:57:55.182
*** MODULE NAME:(sqlplus@RH6 (TNS V1-V3)) 2014-11-24 16:57:55.182
*** ACTION NAME:() 2014-11-24 16:57:55.182
Start dump data blocks tsn: 0 file#:1 minblk 91000 maxblk 91000
Block dump from cache:
Dump of buffer cache at level 4 for tsn=0, rdba=4285304
BH (0x28beb940) file#: 1 rdba: 0x00416378 (1/91000) class: 4 ba: 0x28974000
set: 3 pool 3 bsz: 8192 bsi: 0 sflg: 1 pwc: 0,19
dbwrid: 0 obj: 76545 objn: 76545 tsn: 0 afn: 1 hint: f
hash: [0x32a97bd0,0x32a97bd0] lru: [0x27fe9a74,0x287ef23c]
ckptq: [NULL] fileq: [NULL] objq: [0x30baa69c,0x287ef254]
st: XCURRENT md: NULL tch: 3
flags:
LRBA: [0x0.0.0] LSCN: [0x0.0] HSCN: [0xffff.ffffffff] HSUB: [65535]
cr pin refcnt: 0 sh pin refcnt: 0
Block dump from disk:
buffer tsn: 0 rdba: 0x00416378 (1/91000)
scn: 0x0000.00811496 seq: 0x02 flg: 0x04 tail: 0x14961002
frmt: 0x02 chkval: 0xaa58 type: 0x10=DATA SEGMENT HEADER - UNLIMITED
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x00E49600 to 0x00E4B600
E49600 0000A210 00416378 00811496 04020000 [....xcA.........]
E49610 0000AA58 00000000 00000000 00000000 [X...............]
E49620 00000000 00000011 000000FF 00001020 [............ ...]
E49630 00000010 00000024 00000080 00416824 [....$.......$hA.]
E49640 00000000 00000010 00000000 000000A3 [................]
E49650 00000000 00000000 00000000 00000011 [................]
E49660 00000000 00012B01 40000000 00416379 [.....+.....@ycA.]
E49670 00000007 00416780 00000008 00416788 [.....gA......gA.]
E49680 00000008 00416790 00000008 00416798 [.....gA......gA.]
E49690 00000008 004167A0 00000008 004167A8 [.....gA......gA.]
E496A0 00000008 004167B0 00000008 004167B8 [.....gA......gA.]
E496B0 00000008 004167C0 00000008 004167C8 [.....gA......gA.]
E496C0 00000008 004167D0 00000008 004167D8 [.....gA......gA.]
E496D0 00000008 004167E0 00000008 004167E8 [.....gA......gA.]
E496E0 00000008 004167F0 00000008 00416800 [.....gA......hA.]
E496F0 00000080 00000000 00000000 00000000 [................]
E49700 00000000 00000000 00000000 00000000 [................]
Repeat 242 times
E4A630 00000000 00010000 00020001 00000000 [................]
E4A640 00000000 00000000 00000000 00000000 [................]
Repeat 250 times
E4B5F0 00000000 00000000 00000000 14961002 [................]
Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 17 #blocks: 255
last map 0x00000000 #maps: 0 offset: 4128
Highwater:: 0x00416824 ext#: 16 blk#: 36 ext size: 128
#blocks in seg. hdr's freelists: 0
#blocks below: 163
mapblk 0x00000000 offset: 16
Unlocked
Map Header:: next 0x00000000 #extents: 17 obj#: 76545 flag: 0x40000000
Extent Map
-----------------------------------------------------------------
0x00416379 length: 7
0x00416780 length: 8
0x00416788 length: 8
0x00416790 length: 8
0x00