TAL Distributors Exercises For the following exercises, you will answer problems

Question

TAL Distributors Exercises

For the following exercises, you will answer problems and questions from management at TAL Distributors. You do

not use the TAL Distributors database for any of these exercises.

3. A user queries the Item table in the TAL Distributors database over the company intranet. Assume the Item

table contains 5,000 rows, each row is 1,000 bits long, the access delay is 2.5 seconds, the transmission rate is

50,000 bits per second, and only 20 of the 5,000 rows in the Item table satisfy the query conditions. Calculate

the total communication time required for this query based on retrieving all table rows one row at a time, and

then calculate the total communication time required based on retrieving the 20 rows that satisfy the query conditions

in a single message.

Explanation / Answer

5000 x 1000 = 5 000 000 bits is the total size to be read

each access needs 2.5 seconds

to access all the rows the time required will be 5000000 * 2.5 = 125 00000 secs

for 20 rows to be read seperately we need 20 * 1000 * 2.5 = 5000 secs

transmission time over the net is 50,000 bits per second, so 5000 000 bits will need (500 000 / 50000) 100 secs over the net

totally the communication time will be 100 + 125 00 000 = 125 00 100 secs.

but when a data base is read, the rows are not accessed individually, they will be read in a block - the block size isdetermined by the dbms

depending upon that the access time for the database will be different - it will reduce - for eg. if block size is 512 k, then 5000 000 bits will be read in one instance - totally 2.5 secs

so without knowing the read capacity we cannot give a correct answer.

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