Physical Database Design

3.3 Query Cost Evaluation—An Example 39

Number of block accesses (step 1)

= read S + write 10% of rows to TEMP1

= 1 + 1

= 2.

Step 2. We then project SH over snum, pnum forming TEMP2 (snum, pnum) at 13

bytes per row. No indexes are required since a projection operation requires a full

table scan.

Figure 3.2 Query execution plan for option 1B.

40 CHAPTER 3 Query Optimization and Plan Selection

Number of block accesses (step 2)

= r

ead SH + write TEMP2

= 174 + 87

= 261.

Step 3. Next we project P over pnum, pname forming TEMP3 (pnum, pname) at 18

b

ytes per row. Again, no index is required for a projection operation.

Number of block accesses (step 3)

= read P + ...