1. Introduction to Oracle Optimizer Heribertus Bramundito
2. Agenda Our Environment Heap Table Histogram Oracle Optimizer
Cost Selectivity & Cardinality Clustering Factor Access Method
Partitioning Join Method Sub-query Hint 3. Our Environment Heap
Table This is default table when we issue the CREATE TABLEstatement
Data is stored in random fashion, no specific sort of order (best
fit algorithm)3 4. Our Environment Heap Table (result from table
access)4 5. Our Environment Heap Table (result from index access)5
6. Our Environment IOT Table (a comparison)6 7. Our Environment
Histogram Collection of information about data distribution
inspecific column Oracle maintains 2 types of histogram: frequency
and height-balanced Oracle use histogram as additional information
when deciding whether to use index scan or table scan7 8. Our
Environment Histogram (column without histogram)8 9. Our
Environment Histogram (create histogram)9 10. Our Environment
Histogram (column with histogram)10 11. Agenda Our Environment Heap
Table Histogram Oracle Optimizer Cost Selectivity & Cardinality
Clustering Factor Access Method Partitioning Join Method Sub-query
Hint 12. Cost Jonathan Lewis: The cost represents the optimizers
best estimate of thetime it will take to execute the statement A
result of the calculation performed by optimizer Few conditions
that make CBO produces wrong result No statistics on the underlying
objects or statistics are obsolete Performance characteristics of
hardware or current workloads are not known Bug From Oracle
Performance Tuning Guide and ReferenceCost = (#SRds * sreadtim +
#MRds * mreadtim + #CPUCycles / cpuspeed) / sreadtim #SRds: number
of single data block reads #MRds: number of multi data block reads
#CPUCycle: number of CPU cycles sreadtim: single block read time
mreadtim: multi block read time cpuspeed: CPU cycles per second 12
13. Cost (the calculation) #SRds = blevel + #leaf_block * idx_sel +
clustering_factor * tbl_sel (index scan) #MRds = #tbl_blocks under
HWM / mbrc (full table/ index scan) sreadtim = ioseektim +
block_size * iotfrspeed (SREADTIM) mreadtim = ioseektim + mbrc *
block_size * iotfrspeed (MREADTIM) mbrc =
db_file_multiblock_read_count (MBRC) Going back to slide #4 (lets
try to calculate cost of tablescan) Cost = (24 / 16) * (10 + 16 *
8192 / 4092) / (10 + 8192 / 4092) = 1.5 * 42 /12 = 5.25 Slide #5
(index scan only) Cost = 0 + ceil(1 * 0.0588) = ceil(0.0588) = 1
Slide #10 (index scan with access to the table) Cost = 0 + 1 +
ceil((4 / 17) * 20) = 1 + ceil(4.705) = 1 + 5 = 6 13 14.
Selectivity & Cardinality Selectivity is what fraction of row
the predicate is supposed to fetch/ returnSelectivity = 1 /
num_distinct (no histogram)Selectivity = density (histogram)
Cardinality is total number of row the predicate is supposed to
returnCardinality = num_rows * density Going back to slide #4
Cardinality = num_rows = 20 Slide #5 Selectivity = (number of
bucket with data / number of total bucket) = 17 / 17 = 1Cardinality
= selectivity * num_rows = 1 * 20 = 20 Slide #10 14Selectivity = 4
/ 17 = 0.235Cardinality = 0.235 * 20 = 4.7 15. Clustering Factor
Represents the degree to which data is randomlydistributed through
a table Number of blocks
