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High-Performance Parallel Database Processing and Grid Databases- P7

High-Performance Parallel Database Processing and Grid Databases- P7: Parallel databases are database systems that are implemented on parallel computing platforms. Therefore, high-performance query processing focuses on query processing, including database queries and transactions, that makes use of parallelism techniques applied to an underlying parallel computing platform in order to achieve high performance. | 280 Chapter 9 Parallel Query Scheduling and Optimization The constant y can be used as a design parameter to determine the operations that will be corrected. When y takes a large value the operations with large potential estimation errors will be involved in the plan correction. A small value of y implies that the plan correction is toiitt d to dieoperationswhose result sizes can be estimated more accurately. In fact when y 0 the APC method becomes the PPC method while for suffioieoOy largo y teeOOC nsedioO aocomen the OPC method. 9.6.2 Migratiun Subquery migration isbasudonup-io ateleedinformatiooavadableat theirno when the query planiheorrected. Migration process is activated by a high load processing node when it finds at least one low load processing node from the load table. The process interacts withselected low load processing nodes and if successful some ready-to-run subqueries are migrated. Two decisions need to be made on which node s shouldbe probed hcdwlnch subenery sd is tobe aesUocatoi. Alternatives may be eayoested from simole rrndemselectionioOiasedseeectionin terms of certain benafitipenioty moesures. eh bsaseOmioeatioe strategy isusedthat attempts to minimizethe adaitioltalecslof Oremieratten. In the migration proaeos neseribo0inFseore9.14 eachsuUquohyin -in serdy queue is checked insuruiofindocerreatlow lbadp9boessmgaede mtgratSonto which incurs the smallesSconil IfdOeeorttt ereotesUan ahossoant Utaeshoin a the subquery is marked as nonmigoatabln andwfflnutbeconrAeteOl orU t. Oitor subqueries will be aUi n enid one oiutimefos migtotiontn ea tan eeiOn toder of the additional costs. Tho proceio iinpo wOco eioiershtoode iseo longer i tM e load level or no low tone nodeiniound. The threshold a detenu nieswlnre eshquery ismigraatbiem iermsoSadaitionrl data transfer requiredelongwiSO miuoSioo. Suchpptatedeuferimpuses tworldoad on the original subqeetyeiunier -eat mitiatesthemtosaiioo and ttns reduces or evoo negates the perfonnaecogainooo rSechirae-. .