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Graded Discussion Board (GDB)

Data Warehousing (CS614) 

Dear Students,

GDB of the subject Data Warehousing (CS614) is going to open in a couple of days. Before submitting the GDB, please read all instructions thoroughly.

The GDB will remain open for two days (48 hours).

You may submit your GDB from “August 2, 2018 To August 3, 2018, 11:59 PM”

GDB Topic: 

The task of optimizer is to decide which join technique should be used for a particular search query.  In a decision support system, Sort Merge Join and Hash Based join are some important join techniques which can be used by the optimizer. Discuss some factors on the basis of which optimizer can decide to use either Sort Merge Join or Hash based Join. 

 

Note: A concise, coherent and to the point comment is preferred over lengthy comment having irrelevant details. Comments, posted on regular Lesson's MDB or sent through email will NOT be considered in any case. For any queries please email at CS614@vu.edu.pk

 

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Replies to This Discussion

i think Sort merge join is good to use what u think about SORT MERGE JOIN.

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Nope

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meray buss main hota tou awwww oeee kernay walo ka dunya say safaya ker deta.

khair GDB banao tou share ker dena

any one have gdb solution or idea kindly share

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Cs614 gab sol

A "sort merge" join is performed by sorting the two data sets to be joined according to the join keys and then merging them together. The merge is very cheap, but the sort can be prohibitively expensive especially if the sort spills to disk. The cost of the sort can be lowered if one of the data sets can be accessed in sorted order via an index, although accessing a high proportion of blocks of a table via an index scan can also be very expensive in comparison to a full table scan.

 

A hash join is performed by hashing one data set into memory based on join columns and reading the other one and probing the hash table for matches. The hash join is very low cost when the hash table can be held entirely in memory, with the total cost amounting to very little more than the cost of reading the data sets. The cost rises if the hash table has to be spilled to disk in a one-pass sort, and rises considerably for a multipass sort.

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Discuss some factors on the basis of which optimizer can decide to use either Sort Merge Join or Hash based Join. 

Iska kia Meaning hy ???

mean sql optimiser kub or ku mergsort ya phir hash sort use karega

sort merge r hash merge dono ko explain krna h ?? ya just usy jo best h??

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