Duplicate Key Values

Duplicate key values occur when the value of an indexed column is identical for multiple rows. For example, suppose that the third and fourth leaf nodes of a B-tree structure contain the key value Smith. Suppose further that this value is duplicated six times, as Figure 1 illustrates.
Figure 1. Leaf Nodes 3 and 4
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The first item on the third leaf page contains the duplicate key value, Smith, and the rowid information for the first physical row in the table that contains the duplicate key value. To conserve space, the second item does not repeat the key value Smith but instead contains just the rowid information. This process continues throughout the page; no other key values are on the leaf, only rowid information.

The first item on the fourth leaf page again contains the duplicated key value and rowid information. Subsequent items contain only rowid information.

Now consider the branch node. The third item in the branch node contains the same key value and rowid as the largest item in the third leaf node and a node pointer to it. The fourth item would contain only a node pointer to the fourth leaf node, thus saving the space of an additional duplicate key value.

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