External Sort
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M blocks available for an operation

PAGES(R)


PAGES(R) < M,

Cost of sort = Reading R completely once, sort and output, PAGES(R)

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PAGES(R) >> M

External sort:


Step 1: While there are more pages in R: Read in groups of M pages,
sort and write to disk

Read R once fuly and written R once fully -> 2*PAGES(R)

Step 2: Merge from each block and output.


PAGES(R) = 8 M=4

Step 1: Cost = 16 pages
Resulted in 2 sorted groups (4 pages wide total)

Step 2: Cost = Read R once more, 8 pages and output.


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Ex:

PAGES(R) = 1,000

M = 20

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Step 1:   cost = 2,000 pages

1,000/20 = 50 sorted groups

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Step 2:  cost = 2,000 pages (read/write)

ceil(50/20) = 3 sorted  groups

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Step 2: merge and output cost = 1,000 pages

Cost = 5,000 pages


====================


PAGES(R) = 1,000

M = 100


Step 1:   cost = 2,000 pages

1,000/100 = 10 sorted groups

-------

Step 2: read/merge and output, Cost = 1,000 pages