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BITMAP INDEXES

BITMAP INDEXES. Sai Priya Rama Gopal SJSU ID : 007506408 Class ID: 125. Introduction. A bitmap index is a special kind of index that stores the bulk of its data as bit arrays (commonly called "bitmaps"). It answers most queries by performing bitwise logical operations on these bitmaps.

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BITMAP INDEXES

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  1. BITMAP INDEXES SaiPriya Rama Gopal SJSU ID : 007506408 Class ID: 125

  2. Introduction • A bitmap index is a special kind of index that stores the bulk of its data as bit arrays (commonly called "bitmaps"). • It answers most queries by performing bitwise logical operations on these bitmaps. • The bitmap index is designed for cases where number of distinct values is low, in other words, the values repeat very frequently.

  3. Example • Suppose a file consists of records with two fields, F and G, of type integer and string, respectively. The current file has six records, numbered 1 through 6, with the following values in order:

  4. Example (contd…) • A bitmap index for the first field, F, would have three bit-vectors, each of length 6 as shown in the table. • In each case, the 1's indicate in which records the corresponding string appears. • Table 2

  5. Example (contd…) • A bitmap index for the first field, G, would have three bit-vectors, each of length 6 as shown in the table. • In each case, the 1's indicate in which records the corresponding string appears. • Table 3

  6. Motivation for Bitmap Indexes • Bitmap indexes can help answer range queries. • Example: • Given is the data of a jewelry stores. The attributes considered are age and salary. • Table 4

  7. Motivation (contd…) • A bitmap index for the first field Age, would have seven bit-vectors, each of length 12 as shown in the table. • In each case, the 1's indicate in which records the corresponding string appears. • Table 5

  8. Motivation (contd…) • A bitmap index for the second field Salary, would have ten bit-vectors, each of length 12 as shown in the table. • In each case, the 1's indicate in which records the corresponding string appears. • Table 5

  9. Motivation (contd…) • Suppose we want to find the jewelry buyers with an age in the range 45-55 and a salary in the range 100-200. • We first find the bit-vectors for the age values in this range; in this example there are only two: 010000000100 and 001110000010, for 45 and 50, respectively. If we take their bitwise OR, we have a new bit-vector with 1 in position i if and only if the ith record has an age in the desired range. • This bit-vector is 011110000110.

  10. Motivation (contd…) • Next, we find the bit-vectors for the salaries between 100 and 200 thousand. • There are four, corresponding to salaries 100, 110, 120, and 140; their bitwise OR is 000111100000.

  11. Motivation (contd…) • The last step is to take the bitwise AND of the two bit-vectors we calculated by OR. • That is: 011110000110 AND 000111100000 ----------------------------------- 000110000000

  12. Motivation (contd…) • We thus find that only the fourth and fifth records, which are (50,100) and (50,120), are in the desired range.

  13. Issues for Bitmap Indexes • Managing Bitmap indexes • Memory requirement.

  14. Managing Bitmap indexes • Finding Bit-Vectors • Finding Records • Handling modifications to the data file. • Record numbers must remain fixed once assigned. • Changes to the data file require the bitmap index to change as well.

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