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The ''PRE'' value of a node represents the order in which the XML nodes are visited by a SAX parser. It is actually not stored in the database; instead, it is implicitly given by the table position. As a result, it will change whenever a node with a smaller ''pre'' values is added to or deleted from a database.
Each database node has a persistent ''ID'' vaule, which remains valid after update operations, and which is referenced by the index structures[[Indexes#Value Indexes|value indexes]]. If no updates have been performed on a database, the ''PRE'' and ''ID'' values are identical. This also applies if new nodes are attached to the end of the database. If nodes are deleted or inserted somewhere else, the values diverge, as shown in the next example:
<pre>
</pre>
The [[Database Module#db:node-pre|db:node-pre]] and [[Database Module#db:node-id|db:node-id]] functions can be called to retrieve the ''PRE'' and ''ID'' values of a node, and [[Database Module#db:open-pre|db:open-pre]] and [[Database Module#db:open-id|db:open-id]] can be used to retrieve the original node. As By default, ''ID'' lookups are expensive; . If the [[Options#UPDINDEX|UPDINDEX]] option can be is turned on to build , an additional index and will be maintained to speed up the process.
==Block Storage==
