Drupal 7 Module Development

Chapter 9
Note that the {node_access} table uses the node id (nid) as its primary key. As a
result, it can only be used to check access for content that has already been created.
This means that node access modules cannot assert permissions for the Create
operation. To do so, your module must implement hook_node_access() and
respond to the 'create' operation.
Rebuilding the access permissions will also trigger node_query_node_access_
alter() to begin rewriting node queries for the View, Update, and Delete actions.
However, modules that only implement hook_node_access() do not write records
to the {node_access} table, nor do they impose query altering logic. As a result,
we might consider hook_node_access() to be an easier (and often more efficient)
approach to solving the problem of node access.
When using a node access module, Drupal will add a JOIN statement to
every node query. This can have minor performance implications for your
site if you have large amounts of content.
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