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What Query Cost is?

Antonios Chatzipavlis
Friday 21 July 2017

Συχνά δέχομαι την ερώτηση για το τι είναι το query cost. Η απάντηση είναι η παρακάτω

Το cost είναι κάτι που δεν έχει μονάδα μέτρησης ούτε έχει ορισθεί από το product group σαν κάτι συγκεκριμένο είναι σχετικιστικό

(ΛΕΞΙΚΟ ΤΗΣ ΝΕΑΣ ΕΛΛΗΝΙΚΗΣ ΓΛΩΣΣΑΣ του ΓΕΩΡΓΙΟΥ Δ. ΜΠΑΜΠΙΝΙΩΤΗ ΚΑΘΗΓΗΤΗ ΤΗΣ ΓΛΩΣΣΟΛΟΓΙΑΣ ΣΤΟ ΠΑΝΕΠΙΣΤΗΜΙΟ ΑΘΗΝΩΝ σελίδα 1726)

Με το όρο query cost εννοούμε το ποσό της προσπάθειας που χρειάστηκε ο query optimizer για να δημιουργήσει το query plan και το οποίο περιλαμβάνει CPU, Memory, I/O.

Αυτό το κόστος υπάρχει γιατί όπως είναι γνωστό ο query optimizer κάνει cost-based optimization δηλαδή ψάχνει και επιλέγει μέσα από όλους τους δυνατούς τρόπους εκτέλεσης ενός query αυτόν που έχει το χαμηλότερο κόστος.

Ο τρόπος υπολογισμού είναι ερετικός και προσεγγιστικός στο πόσο χρήση των CPU, Memory, I/O γίνεται.



//antonch


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Antonios Chatzipavlis

Antonios Chatzipavlis

Antonios is a Data Solutions Consultant and Trainer. He has been working in IT since 1988. In his career, he has worked as senior developer, IT Manager, Solutions Architect and IT Consultant. Since 1995 he has been devoted on new technologies and software development tools, mainly by Microsoft, either by training company staff and colleagues or assisting them in design, development and implementation as a consultant or chief developer. He has focused in Databases and Data Science since 1995. He specialized in Microsoft SQL Server since version 6.0 in areas like SQL Server Internals, Database Design and Development, Business Intelligence and in 2010 he has started working with Azure Data Platform, NoSQL databases, Big Data Technologies and Machine Learning. He is an active member of many IT communities in Greece, answering colleagues' questions and writing articles in his web site. He is the owner of SQLschool.gr which is a community portal with a lot of information about Microsoft SQL Server. He has been a Microsoft Certified Trainer (MCT) since 2000. Microsoft honored him as MVP on Data Platform due to his activities in SQL Server since 2010. He holds a large number of Microsoft Certifications and Microsoft SQL Server Certifications since version 6.5.

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