Microsoft® SQL Server® 2012 Bible

1101

Chapter 48: Data Compression

48

written to an anchor row at the beginning of each page. This method is repeated for all

subsequent columns in the table.

FIGURE 48-1

The sample data before page compression is enabled.

ggghh

ggghii

gggiii

ggggh

hhhh

ggggii hhhh

ghik

ghik

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The values are then updated with the prefi x (see Figure 48-2). The fi rst value, ggghh,

begins with four letters of the prefi x, so 4h is written, which saves space on the page. The

compression ratio is much better for values that include more of the prefi x — for example,

ggghii is compressed into just the number 6 because it contains six characters of the pre-

fi x with nothing remaining.

FIGURE 48-2

Prefi x compression identifi es the best prefi x for each column and then stores the prefi x char-

acter count in each row instead of the prefi x characters.

4h

6

ggghii

3iii

4h

0hhhh

6 0hhhh

ggggii

4

ghik

4

Page Header

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