create big table for tests:
create table bigtab (
id number(12,6),
v1 varchar2(10),
padding varchar2(50)
)
nologging -- just to save a bit of time
/
insert /*+ append ordered full(s1) use_nl(s2) */
into bigtab
select
3000 + trunc((rownum-1)/500,6),
to_char(rownum),
rpad('x',50,'x')
from
all_objects s1, -- you'll need the privilege
all_objects s2
where
rownum <= 2000000
/
commit;
check size:
21:07:01 (1)[PDBORCL]c##bushmelev_aa@orcl> @seg_space bigtab
old 2: where segment_name=upper ('&1')
new 2: where segment_name=upper ('bigtab')
OWNER | SEGMENT_NAME | MB_SIZE
--------------- | ---------------------------------------- | -----------
C##BUSHMELEV_AA | BIGTAB | 152.0
Elapsed: 00:00:07.42
23:28:41 (1)[PDBORCL]c##bushmelev_aa@orcl> l
1 select owner,segment_name,sum(bytes)/1024/1024 as MB_SIZE from dba_extents
2 where segment_name=upper ('&1')
3* group by segment_name,owner
ps: describe of x$kcbwds ds, x$kcbwbpd pd you may find at oracle x$ tables
usefull links:
http://enkitec.tv/2012/05/19/oracle-full-table-scans-direct-path-reads-object-level-checkpoints-ora-8103s/
Direct path read and fast full index scans
lets_check:
select ksppinm as hidden_parameter , ksppstvl as value
from x$ksppi join x$ksppcv using (indx)
where ksppinm like '\_small_table_threshold%' escape '\' order by ksppinm;
HIDDEN_PARAMETER | VALUE
------------------------- | ----------
_small_table_threshold | 2203
00:25:34 (1)[PDBORCL]sys@orcl> select 2203*8192/1024/1024 from dual;
2203*8192/1024/1024
-------------------
17.2109375
so our 17 mb table should do DPR on full table scan
now lets full scan bigtable ( there is no indexes on table, so i use count (*) for minimize network influence on results ) an check sga info and session stats :
00:13:23 (1)[PDBORCL]c##bushmelev_aa@orcl> select count (*) from bigtab;
COUNT(*)
----------
2000000
using:
Script to Identify Objects and Amount of Blocks in the Buffer Pools – Default, Keep, Recycle, nK Cache (Doc ID 373472.1)
get info about sement in sga:
select BP_ID, decode(pd.bp_id,1,'KEEP',2,'RECYCLE',3,'DEFAULT',4,'2K SUBCACHE',5,'4K SUBCACHE',6,'8K SUBCACHE',7,'16K SUBCACHE',8,'32K SUBCACHE','UNKNOWN') subcache,
bh.object_name, round (sum(bh.blocks)* (select to_number(value) from v$parameter where name='db_block_size') /(1024*1024),2) used_mb
from x$kcbwds ds, x$kcbwbpd pd,
(select /*+ use_hash(x) */ set_ds, o.name object_name, count(*) BLOCKS from obj$ o, x$bh x where o.dataobj#=x.obj and x.state!=0 and o.owner#!=0 and upper(o.name)=upper('&mytable') group by set_ds, o.name) bh
where ds.set_id>=pd.bp_lo_sid and ds.set_id<=pd.bp_hi_sid and pd.bp_size!=0 and ds.addr=bh.set_ds group by pd.BP_ID, decode(pd.bp_id,1,'KEEP',2,'RECYCLE',3,'DEFAULT',4,'2K SUBCACHE',5,'4K SUBCACHE',6,'8K SUBCACHE',7,'16K SUBCACHE',8,'32K SUBCACHE','UNKNOWN') , bh.object_name; 00:50:04 (1)[PDBORCL]sys@orcl> @db_seg_cache2
Enter value for mytable: bigtab
BP_ID | SUBCACHE | OBJECT_NAME | USED_MB
---------- | ---------- | --------------- | ----------
3 | DEFAULT | BIGTAB | .01
now put full table (152mb) to keep_pool:
alter system set db_keep_cache_size=200m;
alter table bigtab storage (buffer_pool keep );
now fullscan works from cache:
now check buffer pools :
select BP_ID, decode(pd.bp_id,1,'KEEP',2,'RECYCLE',3,'DEFAULT',4,'2K SUBCACHE',5,'4K SUBCACHE',6,'8K SUBCACHE',7,'16K SUBCACHE',8,'32K SUBCACHE','UNKNOWN') subcache,
bh.object_name, round (sum(bh.blocks)* (select to_number(value) from v$parameter where name='db_block_size') /(1024*1024),2) used_mb
from x$kcbwds ds, x$kcbwbpd pd,
(select /*+ use_hash(x) */ set_ds, o.name object_name, count(*) BLOCKS from obj$ o, x$bh x where o.dataobj#=x.obj and x.state!=0 and o.owner#!=0 and upper(o.name)=upper('&mytable') group by set_ds, o.name) bh
where ds.set_id>=pd.bp_lo_sid and ds.set_id<=pd.bp_hi_sid and pd.bp_size!=0 and ds.addr=bh.set_ds
group by pd.BP_ID, decode(pd.bp_id,1,'KEEP',2,'RECYCLE',3,'DEFAULT',4,'2K SUBCACHE',5,'4K SUBCACHE',6,'8K SUBCACHE',7,'16K SUBCACHE',8,'32K SUBCACHE','UNKNOWN') , bh.object_name;
BP_ID | SUBCACHE | OBJECT_NAME | USED_MB
---------- | ---------- | --------------- | ----------
3 | DEFAULT | BIGTAB | .01
1 | KEEP | BIGTAB | 148.64
change keep pool to 140mb and repeat test:
alter system set db_keep_cache_size=120m;
00:39:38 (1)[PDBORCL]sys@orcl> /
Enter value for mytable: bigtab
BP_ID | SUBCACHE | OBJECT_NAME | USED_MB
---------- | ---------- | --------------- | ----------
3 | DEFAULT | BIGTAB | .01
1 | KEEP | BIGTAB | 133.27
even when 87% of table are in current mode in keep pool session do DPR
01:01:19 (1)[PDBORCL]sys@orcl> @db_seg_cache c##bushmelev_aa bigtab
AREA | STATUS | DISTINCT_BLOCK_COUNT | BLOCK_COUNT
------------ | ------------------------------ | ---------------------------------------- | ------------------------------------------------------------------------------------------------------------------------
BUFFER CACHE | free | 3 | 3
BUFFER CACHE | xcur | 17060 | 17060
BUFFER CACHE | summary | 17063 | 17063
DATABASE | db blocks | 19456 |
SGA | BUFFER CACHE of MAX SGA SIZE | 754,974,720 of 1,426,063,360 | (Resizeable)
Elapsed: 00:00:00.11
01:01:20 (1)[PDBORCL]sys@orcl> l
1 with OBJLIST as
2 (select DATA_OBJECT_ID
3 from dba_objects
4 where (owner, object_name) in ((upper(nvl('&&1', 'user')), upper('&&2')))
5 and DATA_OBJECT_ID is not null)
6 select 'BUFFER CACHE' as AREA,
7 nvl(status,'summary') as STATUS,
8 to_char(count(distinct(file# || '#' || block#))) as DISTINCT_BLOCK_COUNT,
9 to_char(count(*)) as BLOCK_COUNT
10 from V$BH, OBJLIST
11 where objd = OBJLIST.DATA_OBJECT_ID
12 group by rollup(status)
13 union all
14 select 'DATABASE',
15 'db blocks',
16 to_char(blocks),
17 '' as BH_COUNT
18 from dba_segments where (owner, segment_name) in ((upper(nvl('&&1', 'user')), upper('&&2')))
19 union all
20 select 'SGA',
21 'BUFFER CACHE of MAX SGA SIZE',
22 trim(to_char(s1.bytes, '999,999,999,999,999')) ||
23 ' of '||
24 trim(to_char(s2.bytes, '999,999,999,999,999')),
25 '(' || decode(s1.resizeable, 'Yes', 'Resizeable', 'Fixed') || ')'
26* from v$sgainfo s1, v$sgainfo s2 where s1.name = 'Buffer Cache Size' and s2.name = 'Maximum SGA Size'
and FTS goes by DRP:
