[20180730]exadata与行链接.txt
--//最近一段时间在看<expert oracle exadata>,智能扫描的三大优化方法是:字段投影,谓词过滤,存储索引.大多数智能扫描
--//优化方法的主要目标就是在执行扫描时减少需要传输回数据块服务端的数据量.
--//而智能扫描的先决条件,满足三个基本条件:
--//1.必须要是对象上的全扫描.
--//2.扫描必须要使用oracle直接路径读取机制.
--//3.对象必须存储在exadata存储中.
--//有几种情况转换为块传输模式:
--//1.一致性读取,
--//2,行链接(注意:不是行迁移)
--//3.存储服务器CPU太忙.
--//正好生产系统有1张大表存在大量行链接.自己测试看看.
1.环境:
xxxx> @ &r/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
2.测试:
xxxx> select bytes/1024/1024/1024 GB ,BLOCKS from dba_segments where owner='XXXX_YYY' and segment_name='BIG_TAB';
GB BLOCKS
---------- ----------
207.549805 27203968
--//表占用207G.
xxxx> set timing on
xxxx> @ &r/viewsess 'table fetch continued row'
NAME STATISTIC# VALUE SID
-------------------------- ---------- ---------- ----------
table fetch continued row 417 0 2323
Elapsed: 00:00:00.00
xxxx> select /*+ full(a) */ count(*) from xxxxxx_yyy.big_tab a;
COUNT(*)
----------
44669678
Elapsed: 00:01:38.73
xxxx> @ &r/viewsess 'table fetch continued row'
NAME STATISTIC# VALUE SID
------------------------- ---------- ---------- ----------
table fetch continued row 417 0 2323
Elapsed: 00:00:00.01
--//需要大约98秒完成查询.table fetch continued row的记数没有变化.
xxxx> @ &r/desc xxxxxx_yyy.big_tab;
Name Null? Type
------------- -------- ----------------------------
YZBXH NOT NULL NUMBER(18)
BRID NOT NULL NUMBER(18)
ZYH NOT NULL NUMBER(18)
YEPB NOT NULL NUMBER(1)
BRKS NOT NULL NUMBER(4)
BRBQ NUMBER(4)
BRCH VARCHAR2(20)
YDYZLB NOT NULL NUMBER(4)
XMLB NOT NULL NUMBER(8)
...
CAKEY VARCHAR2(2000)
YZCA VARCHAR2(3000)
TZ_CAKEY VARCHAR2(2000)
TZCA VARCHAR2(3000)
HSCAKEY VARCHAR2(2000)
HSCA VARCHAR2(3000)
TZ_HSCAKEY VARCHAR2(2000)
TZ_HSCA VARCHAR2(3000)
DYSJ DATE
YZZXSJ VARCHAR2(80)
ZXTZSJ DATE
ZYDY NUMBER(1)
FZLJ NUMBER(8)
BRZH NUMBER(8)
TZBRZH NUMBER(8)
TJSJ DATE
TQMXBZ NUMBER(1)
--//顺便找靠前的可以为NULL的字段BRBQ. 注意看那些XXkey的字段,正是这些字段导致了大量的行链接与行迁移.
xxxx> select sysdate from dual;
SYSDATE
-------------------
2018-07-30 08:49:54
Elapsed: 00:00:00.01
xxxx> select /*+ full(a) */ count(BRBQ) from xxxxxx_yyy.big_tab a;
select /*+ full(a) */ count(BRBQ) from xxxxxx_yyy.big_tab a
*
ERROR at line 1:
ORA-01013: user requested cancel of current operation
Elapsed: 00:03:09.84
xxxx> select sysdate from dual;
SYSDATE
-------------------
2018-07-30 08:53:37
Elapsed: 00:00:00.00
xxxx> @ &r/viewsess 'table fetch continued row'
NAME STATISTIC# VALUE SID
------------------------- ---------- ---------- ----------
table fetch continued row 417 53476 2323
Elapsed: 00:00:00.00
--//时间太长仅仅测试不到4分钟,放弃!!(注上个星期测试4个小时也没有出来报ora-01555错误,当时选择靠后的字段).说明可以大量的行
--//链接导致无法充分发挥exadata智能扫描的优势,转为块传输模式.
--//如果在服务器上观察可以发现:
# perf top -k /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
PerfTop: 5431 irqs/sec kernel:51.3% exact: 0.0% [1000Hz cycles], (all, 24 CPUs)
----------------------------------------------------------------------------------------------------------
samples pcnt function DSO
_______ _____ _______________________ _______________________________________________________
1125.00 5.3% kafger /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
763.00 3.6% qertbFetchByRowID /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
~~~~~~~~~~~~~~~~
740.00 3.5% kafgex1 /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
740.00 3.5% kcbgtcr /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
607.00 2.8% kdifxs1 /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
477.00 2.2% qerixtFetch /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
451.00 2.1% expepr /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
386.00 1.8% kdsgrp /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/oracle
--//出现许多qertbFetchByRowID函数调用.
3.顺便测试exadata的存储索引:
--//重新登录:
xxxx> @ &r/viewsess 'storage'
NAME STATISTIC# VALUE SID
---------------------------------------------------------------------- ---------- ---------- ----------
cell physical IO bytes saved by storage index 274 0 4821
xxxx> @ &r/viewsess 'table fetch continued row'
NAME STATISTIC# VALUE SID
---------------------------------------------------------------------- ---------- ---------- ----------
table fetch continued row 417 0 4821
xxxx> set timing on
xxxx> select /*+ full(a) */ count(*) from xxxxxx_yyy.big_tab a where ZXTZSJ between trunc(sysdate)-1 and trunc(sysdate)-1+1/86400;
COUNT(*)
----------
5
Elapsed: 00:47:05.16
--//仅仅取1秒的日期范围数据,竟然需要47分钟.
xxxx> @ &r/viewsess 'table fetch continued row'
NAME STATISTIC# VALUE SID
---------------------------------------------------------------------- ---------- ---------- ----------
table fetch continued row 417 3461046 4821
Elapsed: 00:00:00.00
xxxx> @ &r/viewsess 'storage'
NAME STATISTIC# VALUE SID
---------------------------------------------------------------------- ------------ ------------ ------------
cell physical IO bytes saved by storage index 274 15562743808 4821
--//15562743808/1024/1024/1024 = 14.49393463134765625
--//15562743808/1024/1024 = 14841.7890625
--//15562743808/8192 = 1899749块
--//竟然存储索引可以使用.我以为会失效.也许前面的部分数据是使用expdp导入的(我当时记忆5XG),可能发生行链接或者迁移的量很少.估计开始
--//实施也不规范,XXkey的字段都是NULL.我估计仅仅没有发生行迁移或者链接的存储单元1M的数据才有存储索引.(我的理解)
--//看看与cell相关的记数变化情况:
xxxx> @ &r/viewsess 'cell'
NAME STATISTIC# VALUE SID
---------------------------------------------------------------------- ------------ ------------ ------------
cell writes to flash cache 58 0 4821
cell overwrites in flash cache 59 0 4821
cell partial writes in flash cache 60 0 4821
cell physical IO interconnect bytes 64 33219425400 4821
cell physical IO bytes saved during optimized file creation 271 0 4821
cell physical IO bytes saved during optimized RMAN file restore 272 0 4821
cell physical IO bytes eligible for predicate offload 273 222633656320 4821
cell physical IO bytes saved by storage index 274 15562743808 4821
cell physical IO bytes sent directly to DB node to balance CPU 275 0 4821
cell smart IO session cache lookups 276 0 4821
cell smart IO session cache hits 277 0 4821
cell smart IO session cache soft misses 278 0 4821
cell smart IO session cache hard misses 279 0 4821
cell smart IO session cache hwm 280 0 4821
cell num smart IO sessions in rdbms block IO due to user 281 0 4821
cell num smart IO sessions in rdbms block IO due to open fail 282 0 4821
cell num smart IO sessions in rdbms block IO due to no cell mem 283 0 4821
cell num smart IO sessions in rdbms block IO due to big payload 284 0 4821
cell num smart IO sessions using passthru mode due to user 285 0 4821
cell num smart IO sessions using passthru mode due to cellsrv 286 0 4821
cell num smart IO sessions using passthru mode due to timezone 287 0 4821
cell num smart file creation sessions using rdbms block IO mode 288 0 4821
cell num block IOs due to a file instant restore in progress 289 0 4821
cell physical IO interconnect bytes returned by smart scan 290 6552450168 4821
cell num bytes in passthru during predicate offload 291 0 4821
cell num bytes in block IO during predicate offload 292 0 4821
cell num fast response sessions 293 0 4821
cell num fast response sessions continuing to smart scan 294 0 4821
cell num smartio automem buffer allocation attempts 295 1 4821
cell num smartio automem buffer allocation failures 296 0 4821
cell statistics spare1 297 0 4821
cell statistics spare2 298 0 4821
cell statistics spare3 299 0 4821
cell statistics spare4 300 0 4821
cell statistics spare5 301 0 4821
cell statistics spare6 302 0 4821
cell scans 421 1 4821
cell blocks processed by cache layer 422 25661755 4821
cell blocks processed by txn layer 423 25661095 4821
cell blocks processed by data layer 424 25282464 4821
cell blocks processed by index layer 425 0 4821
cell commit cache queries 426 0 4821
cell transactions found in commit cache 427 0 4821
cell blocks helped by commit cache 428 0 4821
cell blocks helped by minscn optimization 429 25647939 4821
chained rows skipped by cell 430 3467111 4821
chained rows processed by cell 431 1116529 4821
chained rows rejected by cell 432 3467111 4821
cell simulated physical IO bytes eligible for predicate offload 433 0 4821
cell simulated physical IO bytes returned by predicate offload 434 0 4821
cell CUs sent uncompressed 435 0 4821
cell CUs sent compressed 436 0 4821
cell CUs sent head piece 437 0 4821
cell CUs processed for uncompressed 438 0 4821
cell CUs processed for compressed 439 0 4821
cell IO uncompressed bytes 440 207119351808 4821
cell index scans 457 0 4821
cell flash cache read hits 646 3138149 4821
58 rows selected.
--//对于许多记数不是很熟悉,仅仅做一个记录.
--//附上viewsess.sql脚本:
set verify off
column name format a70
SELECT b.NAME, a.statistic#, a.VALUE,a.sid
FROM v$mystat a, v$statname b
WHERE lower(b.NAME) like lower('%&1%') AND a.statistic# = b.statistic# ;
--and a.value>0;