CBO Cost Formulas成本计算公式大全:
成本模型：连接方式Join method
注意 连接基数(Join Cardinality)不受到连接方式(join method) 的影响， oracle中主要的三种join method HASH JOIN、Nested Loops、Sort Merge:
- Nested Loops嵌套循环成本公式：
- Cost(outer)+Cost(inner))*cardinality(outer)
- Sort merge 合并排序连接成本公式:
- Cost(outer) + Cost(inner) + Sort(outer) + Sort(inner)
- Hash Join 哈希连接公式:
- Cost(outer) + Cost(inner) + Build(outer) + Probe(inner)
Index Unique Scan Cost成本计算
INDEX UNIQUE SCAN COST = (BLEVEL (1-(OIC/100)) + 1) * (OICA/100)
Index Range Scan Cost成本计算
INDEX RANGE SCAN COST = (BLEVEL + FF*LFBL)*(1-(OIC/100))+ FF*CLUF)* (OICA/100)
formula does not include the CPU cost
- BLEVEL = number of branch levels in index
- add +1 for leaf block
- FF = filtering factor – selectivity
- LFBL = number of leaf blocks
- CLUF = index clustering factor
- OIC = optimizer_index_caching(default 0)
- OICA = optimizer_index_cost_adj parameter(default=100)
CPU costing启用的情况下：
mreadtime -Average time , in milliseconds, for a multi-block read (according to sys.aux_stats$)
sreadtime - Average time , in milliseconds, for a single-block read (according to sys.aux_stats$)
MBRC - Average number of blocks to be read in a multi-block read (according to sys.aux_stats$
#SRDs - number of single block reads
#MRDs - number of multi block reads
#CPUCycles - number of CPU Cycles
sreadtime = ioseektim + db_block_size/iotfrspeed
mreadtim = ioseektim + db_file_multiblock_read_count * db_block_size / iotrfspeed
#MRds = #Blks/MBRC
Cost 成本本身 =(#SRds * sreadtim +#MRds * mreadtim + #CPUCycles/cpuspeed)/sreadtim ,
Cost成本的单位 为 single-block read time=sreadtim