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Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
Are Indexes Unnecessary in Exadata
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Are Indexes Unnecessary in Exadata

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You heard it - drop indexes in Exadata. But should you? Learn why that is true in some cases and, more important, when it is not.

You heard it - drop indexes in Exadata. But should you? Learn why that is true in some cases and, more important, when it is not.

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  • 1. Are Indexes Unnecessary in Exadata? Arup Nanda Longtime Oracle DBA (and now DMA)
  • 2. Disclaimer If you downloaded this slide deck, please note: These slides are designed merely as props to help my presentation. They are not intended to stand as independent sources of information. Therefore the contents of the slide deck are not meant to be exhaustive in any way of the content delivered at the session. Are Indexes Unnecessary in Exadata 2
  • 3. Are Indexes Unnecessary in Exadata 3 Quotes Why? Because there is a storage index. “ Drop all the indexes and reclaim space.“ ” You don’t need indexes on Exadata. “ ”
  • 4. 3 Questions for “Best Practices” Are Indexes Unnecessary in Exadata 4 1. Why it is better than the rest? 2. What happens if it is not followed? 3. When are they not applicable?
  • 5. Are Indexes Unnecessary in Exadata 5 Storage Index
  • 6. Instances and Databases 6 Storage datafile1 datafile2 SELECT NAME FROM CUSTOMERS WHERE STATUS ='ANGRY' Instance Combination of •Memory Areas •Background Processes Are Indexes Unnecessary in Exadata
  • 7. Query Processing 7 Storage datafile1 datafile2 SELECT NAME FROM CUSTOMERS WHERE STATUS ='ANGRY' Database Block JILL Are Indexes Unnecessary in Exadata
  • 8. Are Indexes Unnecessary in Exadata 8 Index Table
  • 9. Components for Performance 9 CPU Memory Network I/O Controller Disk Less I/O = better performance Are Indexes Unnecessary in Exadata
  • 10. The Solution • A typical query may: – Select 10% of the entire storage – Use only 1% of the data it gets • To gain performance, the DB needs to shed weight • It has to get less from the storage  Filtering at the storage level  The storage must be cognizant of the data 10 SELECT NAME FROM CUSTOMERS WHERE STATUS ='ANGRY' CPU Memory Network I/O Controller Disk Filtering should be Applied Here Are Indexes Unnecessary in Exadata
  • 11. 11 CPU Memory Network I/O Controller Disk iDB Are Indexes Unnecessary in Exadata
  • 12. Are Indexes Unnecessary in Exadata 12 Disk1 Disk2 Disk3 iDB
  • 13. 13 Disk4 MIN = 3 MAX = 5 MIN = 4 MAX = 5 MIN = 3 MAX = 5 MIN = 1 MAX = 3 Disk1 Disk2 Disk3 Storage Index SELECT … FROM TABLE WHERE COL1 = 2 Are Indexes Unnecessary in Exadata
  • 14. Storage Indexes • Do not point to the database blocks • Merely stores for a Storage “Unit” – Max/Min Values – Whether nulls are present – For some columns • Is on Memory of Cells; not disk – Disappears when the cell is down Are Indexes Unnecessary in Exadata 14
  • 15. Checking Storage Index Use select name, value/1024/1024 as stat_value from v$mystat s, v$statname n where s.statistic# = n.statistic# and n.name in ( 'cell physical IO bytes saved by storage index', 'cell physical IO interconnect bytes returned by smart scan’) 15 Are Indexes Unnecessary in Exadata Output STAT_NAME STAT_VALUE ---------- ---------- SI Savings 5120.45 Smart Scan 1034.00
  • 16. Offloading and Smart Scan Offloading Processing to storage cells Smart Scan Reduction in I/O Are Indexes Unnecessary in Exadata 16
  • 17. Offloading • Column Projection select cust_id, sale_amt from sales • Predicate Filtering where status = ‘ANGRY’ • Function Offloading select min(sale_amt) • Virtual Columns Are Indexes Unnecessary in Exadata 17
  • 18. Smart Scan Benefits • Less I/O means – Faster disk access time – Less data from storage to DB – Less buffers – Less CPU – Less data between compute nodes Are Indexes Unnecessary in Exadata 18
  • 19. Why Not? • Pre-requisite for Smart Scan – Direct Path – Full Table or Full Index Scan – > 0 Predicates – Simple Comparison Operators • Other Reasons – Cell is not offload capable • The diskgroup attribute cell.smart_scan_capable set to FALSE; – Not on clustered tables, IOTs, etc. 19 Are Indexes Unnecessary in Exadata
  • 20. Impact of Data Distribution Are Indexes Unnecessary in Exadata 20 Disk4 MIN = 3 MAX = 5 MIN = 4 MAX = 5 MIN = 3 MAX = 5 MIN = 1 MAX = 2 MIN = 1 MAX = 5 MIN = 1 MAX = 5 MIN = 1 MAX = 5 MIN = 1 MAX = 5 SELECT … FROM TABLE WHERE COL1 = 2
  • 21. 8 Columns Are Indexes Unnecessary in Exadata 21 C1 C2 C3 C4 C5 C6 C7 C8 C2 C3 C4 C5 C6 C7 C8 C9 C3 C4 C5 C6 C7 C8 C9 C10 Table T1
  • 22. No Predicate • Aggregations select sum(sale_amt) from sales • Sorting select ... from sales order by sale_amt; Are Indexes Unnecessary in Exadata 22 Index on SALE_AMT
  • 23. Function Based Indexes • Traditional Indexes can’t work select … from sales where to_char(sale_dt,’YY’) = ‘13’ • Function Based Indexes help • SI indexes will not be useful Are Indexes Unnecessary in Exadata 23
  • 24. IOTs • Index Organized Tables • PK-based rows • Secondary Indexes built on the other columns Are Indexes Unnecessary in Exadata 24 PK Index Index
  • 25. Clustered Tables Are Indexes Unnecessary in Exadata 25
  • 26. Exclusion for SIs • Not for non-equality select sale_amt from sales where status != ‘SHIPPED’ • No Wildcards select sale_amt from sales where city like ‘NEW YORK%’ Are Indexes Unnecessary in Exadata 26
  • 27. Virtual Columns • Example alter table EMP add ( tot_sal number(13) generated always as sal+comm) ) • Implication – Do not actually exists in the table – Computed at runtime Are Indexes Unnecessary in Exadata 27
  • 28. Indexes on Small Tables • Small table – Parameter _small_table_threshold • Indexes still help small table http://richardfoote.wordpress.com/2009/04/16/indexes-on-small-tables- part-i-one-of-the-few/ – Less latching Are Indexes Unnecessary in Exadata 28
  • 29. Summary of SI Limitations • Direct Path not used • No Predicate No SI • No Inequality (!=) • ≤ 8 columns • No Virtual Columns • No wildcard match (LIKE ‘..%’) • No IOT, Clustered Table • Latching on small tables • First-timer Penalty – Only subsequent queries benefit Are Indexes Unnecessary in Exadata 29
  • 30. Flash Cache 30 Storage datafile1 datafile2 These are flash cards presented as disks; not memory to the Storage Cells. They are similar to SAN cache; but Oracle controls what goes on there and how long it stays. Are Indexes Unnecessary in Exadata alter table person storage (cell_flash_cache keep)
  • 31. Flash Trick for Indexes • Pin Oft-Used Objects in Flash SQL> alter index in_t2 storage (cell_flash_cache keep); • Check flash CellCLI> list flashcachecontent attributes – > cachedKeepSize, cachedSize, hitCount, - > hoursToExpiration, missCount - > where objectnumber = 382380; • Or, partitions Are Indexes Unnecessary in Exadata 31
  • 32. Drop the Index? • Make the indexes invisible SQL> alter index i1 invisible; – Maintains the index; but optimizer ignores it • See the performance impact. • Selectively see the impact SQL> alter session set optimizer_use_invisible_indexes = true; • See the performance impact. Are Indexes Unnecessary in Exadata 32
  • 33. Disable • Two parameters – Could be session level • To disable offloading cell_offload_processing = false; • To disable storage indexes alone _kcfis_storageidx_disabled = true; Are Indexes Unnecessary in Exadata 33
  • 34. In Conclusion • Full table scans in Exadata – may be faster compared to non-Exadata – may not be faster than index scans in Exadata – may benefit from Storage Indexes • Storage Indexes are not same as DB Indexes • No DB Indexes helps in some cases – But not all • Test by making DB Indexes invisible • Force FTS in those cases where index hurts Are Indexes Unnecessary in Exadata 34
  • 35. Thank You!Blog: arup.blogspot.com | Twitter: arupnanda Are Indexes Unnecessary in Exadata 35

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