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GO Classes DPP | GATE CS, DA | DBMS | Variable-Length Record

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Consider:

CREATE TABLE products (
    id INTEGER PRIMARY KEY,
    stock INTEGER NOT NULL,
    price INTEGER NOT NULL,
    name VARCHAR(10) NOT NULL,
    category CHAR(6) NOT NULL,
    serial_number CHAR(20)
);

Assume:

  • an $\mathrm{INTEGER}$ occupies $4$ bytes,
     
  • $\mathrm{CHAR(n)}$ occupies exactly $n$ bytes,
     
  • $\mathrm{VARCHAR(n)}$ occupies at most $n$ bytes,
     
  • the record header occupies $8$ bytes,
     
  • $\mathrm{serial\_number}$ may be $\mathrm{NULL}$.

Which of the following can be the size, in bytes, of some record using a variable-length representation?

  1. $18$
     
  2. $27$
     
  3. $48$
     
  4. $55$

1 Answer

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First, calculate the fixed part:

$\mathrm{Header}=8$

$\mathrm{id}=4$

$\mathrm{stock}=4$

$\mathrm{price}=4$

$\mathrm{category}=6$

Therefore:

$8+4+4+4+6=26$

$\mathrm{name}$ is $\mathrm{VARCHAR}(10)$, so it can contribute from $0$ through $10$ bytes.


Case $\mathbf{1}: \mathrm{serial\_number}$ is NULL

Possible record sizes are:

$26\mathrm{\ through\ }36$

Among the options $27$ is possible.

So B is correct.
 

Case $\mathbf{2}: \mathrm{serial\_number}$ is not NULL

$\mathrm{CHAR}(20)$ adds exactly $20$ bytes:

$46\mathrm{\ through\ }56$

Therefore, both $48,\ 55$ are possible.

So C and D are also correct.

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