SAG_Indexing and Query Optimization
- 1. SAG – Indexing and Query Optimization Vaibhav Jain vjain44@csc.com Ext : 706224
- 2. Indexing – A Refresher • Introducing Indexes What are Indexes? What are the types of Indexes? What do these terms inside SQL Server mean? – Pages, Extents – Heap – Clustered Indexes – Non-Clustered Indexes
- 3. Page, Extents • The fundamental unit of data storage in SQL Server is the PAGE (8k) This means SQL Server databases have 128 pages per megabyte Each page begins with a 96-byte header Maximum amount of data contained in a single row on a page is 8,060 bytes • Extents are a collection of eight physically contiguous pages
- 4. HEAP • A heap is a table without a clustered index • The data rows are not stored in any particular order • Data pages are not linked in a linked list
- 5. Clustered Index • The data rows are stored in order based on the clustered index key • The clustered index is implemented as a B-tree index structure • Data pages in the leaf level, are linked in a doubly-linked list • Clustered indexes have one row in sys.partitions, with index_id = 1
- 6. Clustered Index B - Tree
- 7. Non-Clustered Index • Nonclustered indexes have a B-tree index structure similar to the one in clustered indexes • The difference is that nonclustered indexes do not affect the order of the data rows. • Each index row contains the nonclustered key value, a row locator and any included, or nonkey, columns
- 8. Query Optimization Simple flow chart Real pie chart
- 9. Query Processing and Optimization Parsing Optimization Execution
- 10. Query Processing and Optimization • Cost Based Optimizer • Disk IO Cost • CPU Cost • Memory Cost – Very insignificant • Indexes • Selects optimal indexes to be used • Query Algorithms • Converts Logical operation into physical operations
- 11. Execution Plan Operators • When • Good • Bad • Action Item 80% - 20% Rule
- 12. Table Scan • When Table without clustered index is accessed • Good or Bad* Can’t decide • Action Item Create Clustered Index
- 13. Clustered Index Scan • When Table with clustered index is accessed but query doesn’t uses the clustered index key • Good or Bad* Bad unless large data with most columns and rows retrieved • Action Item Evaluate Clustered Index Keys
- 14. Clustered Index Seek • When Table with clustered index is accessed and query locates specific rows in B+ tree • Good or Bad* Good • Action Item Evaluate possibility of non-clustered index
- 15. Non-Clustered Index Scan • When Columns part of non-clustered index accessed in query • Good or Bad* Bad unless large data with most columns and rows retrieved • Action Item Create more refined non-clustered index
- 16. Non-Clustered Index Seek • When Columns part of non-clustered index accessed in query and rows located in B+ tree • Good or Bad* Good • Action Item Further evaluate other operators
- 17. Lookups • When Query Optimizer uses non-clustered index to search few column data and base table for other columns data • Good or Bad* Good • Action Item Included Index or Covered Index