System statistics can be gathered in NOWORKLOAD or WORKLOAD mode. Different values will be set depending on that and the others will be calculated - derived from them. We can see defined values from SYS.AUX_STATS$ but here is a script that shows the calculated ones as well.
With no system statistics or NOWORKLOAD the values of IOSEEKTIM (latency in ms) and IOTFRSPEED (transfer in bytes/ms) are set and the SREADTIM (time to read 1 block in ms) and MREADTIM (for multiblock read) are calculated from them. MBRC depends on the defaults or the db_file_multiblock_read_count settings.
With WORKLOAD statistics, the SREADTIM and MREADTIM as well as MBRC are measured and those are the ones that are used by the optimizer.
Here is my script:
set echo off
set linesize 200 pagesize 1000
column pname format a30
column sname format a20
column pval2 format a20
select pname,pval1,calculated,formula from sys.aux_stats$ where sname='SYSSTATS_MAIN'
model
reference sga on (
select name,value from v$sga
) dimension by (name) measures(value)
reference parameter on (
select name,decode(type,3,to_number(value)) value from v$parameter where name='db_file_multiblock_read_count' and ismodified!='FALSE'
union all
select name,decode(type,3,to_number(value)) value from v$parameter where name='sessions'
union all
select name,decode(type,3,to_number(value)) value from v$parameter where name='db_block_size'
) dimension by (name) measures(value)
partition by (sname) dimension by (pname) measures (pval1,pval2,cast(null as number) as calculated,cast(null as varchar2(60)) as formula) rules(
calculated['MBRC']=coalesce(pval1['MBRC'],parameter.value['db_file_multiblock_read_count'],parameter.value['_db_file_optimizer_read_count'],8),
calculated['MREADTIM']=coalesce(pval1['MREADTIM'],pval1['IOSEEKTIM'] + (parameter.value['db_block_size'] * calculated['MBRC'] ) / pval1['IOTFRSPEED']),
calculated['SREADTIM']=coalesce(pval1['SREADTIM'],pval1['IOSEEKTIM'] + parameter.value['db_block_size'] / pval1['IOTFRSPEED']),
calculated[' multi block Cost per block']=round(1/calculated['MBRC']*calculated['MREADTIM']/calculated['SREADTIM'],4),
calculated[' single block Cost per block']=1,
formula['MBRC']=case when pval1['MBRC'] is not null then 'MBRC' when parameter.value['db_file_multiblock_read_count'] is not null then 'db_file_multiblock_read_count' when parameter.value['_db_file_optimizer_read_count'] is not null then '_db_file_optimizer_read_count' else '= _db_file_optimizer_read_count' end,
formula['MREADTIM']=case when pval1['MREADTIM'] is null then '= IOSEEKTIM + db_block_size * MBRC / IOTFRSPEED' end,
formula['SREADTIM']=case when pval1['SREADTIM'] is null then '= IOSEEKTIM + db_block_size / IOTFRSPEED' end,
formula[' multi block Cost per block']='= 1/MBRC * MREADTIM/SREADTIM',
formula[' single block Cost per block']='by definition',
calculated[' maximum mbrc']=sga.value['Database Buffers']/(parameter.value['db_block_size']*parameter.value['sessions']),
formula[' maximum mbrc']='= buffer cache size in blocks / sessions'
);
set echo on
Here is an exemple with default statistics:
PNAME PVAL1 CALCULATED FORMULA ------------------------------ ---------- ---------- -------------------------------------------------- CPUSPEEDNW 1519 IOSEEKTIM 10 IOTFRSPEED 4096 SREADTIM 12 = IOSEEKTIM + db_block_size / IOTFRSPEED MREADTIM 26 = IOSEEKTIM + db_block_size * MBRC / IOTFRSPEED CPUSPEED MBRC 8 = _db_file_optimizer_read_count MAXTHR SLAVETHR maximum mbrc 117.152542 = buffer cache size in blocks / sessions single block Cost per block 1 by definition multi block Cost per block .2708 = 1/MBRC * MREADTIM/SREADTIM
You see the calculated values for everything. Note the 'maximum mbrc' which limits the multiblock reads when the buffer cache is small. It divides the buffer cache size (at startup - can depend on ASMM and AMM settings) by the sessions parameter.
Here is an example with workload system statistics gathering:
PNAME PVAL1 CALCULATED FORMULA ------------------------------ ---------- ---------- -------------------------------------------------- CPUSPEEDNW 1511 IOSEEKTIM 15 IOTFRSPEED 4096 SREADTIM 1.178 1.178 MREADTIM .03 .03 CPUSPEED 3004 MBRC 8 8 MBRC MAXTHR 6861824 SLAVETHR maximum mbrc 114.983051 = buffer cache size in blocks / sessions single block Cost per block 1 by definition multi block Cost per block .0032 = 1/MBRC * MREADTIM/SREADTIM
here all values are explicitely set
And an example with exadata system statistics that defines noworkload values and sets also the MBRC (see Chris Antognini post about it)
PNAME PVAL1 CALCULATED FORMULA ------------------------------ ---------- ---------- -------------------------------------------------- CPUSPEEDNW 1539 IOSEEKTIM 16 IOTFRSPEED 204800 SREADTIM 16.04 = IOSEEKTIM + db_block_size / IOTFRSPEED MREADTIM 18.28 = IOSEEKTIM + db_block_size * MBRC / IOTFRSPEED CPUSPEED MBRC 57 57 MBRC MAXTHR SLAVETHR maximum mbrc 114.983051 = buffer cache size in blocks / sessions single block Cost per block 1 by definition multi block Cost per block .02 = 1/MBRC * MREADTIM/SREADTIM
And finaly here is a workload system statistics result but with explicitly setting the db_file_multiblock_read_count to 128:
PNAME PVAL1 CALCULATED FORMULA ------------------------------ ---------- ---------- -------------------------------------------------- CPUSPEEDNW 1539 IOSEEKTIM 15 IOTFRSPEED 4096 SREADTIM 17 = IOSEEKTIM + db_block_size / IOTFRSPEED MREADTIM 271 = IOSEEKTIM + db_block_size * MBRC / IOTFRSPEED CPUSPEED MBRC 128 db_file_multiblock_read_count MAXTHR SLAVETHR maximum mbrc 114.983051 = buffer cache size in blocks / sessions single block Cost per block 1 by definition multi block Cost per block .1245 = 1/MBRC * MREADTIM/SREADTIM
Here you see that the MBRC in noworkload is coming from the value which is set by the db_file_multiblock_read_count rather from the value 8 which is used by default by the optimizer when it is not set. And the MREADTIM is calculated from that i/o size
For more historical information about system statistics and how multiblock reads are costed (index vs. full table scan choice) see my article on latest OracleScene
As usual if you find anything to improve in that script, please share.