Command Representation of Queries

OLE DB uses a tree format to represent commands associated with the Command object. For Indexing Service, the command represents a query. A typical query involves the following steps, where the terms in brackets suggest actions familiar to database users:

• Step 1 [PROJECT]: Choose the columns you want to display in the search results table and copy the appropriate values from the files that had hits.
• Step 2 [SELECT]: Define a search on the content and property indexes for some values you specify. The query languages supported allow you to express property-value queries as relational queries or pattern-matching queries in addition to expressing content queries. For each part of the search expression, indicate the values that should be searched for in specific properties (including the contents property) for a file. Execute the query.
• Step 3 [SORT]: Sort the results by one or more columns in the specified order.

When creating a machine-readable command structure to represent these steps, Indexing Service uses a DBCOMMANDTREE structure, which is an array of zero or more nodes, each describing a part of one of the steps. Nodes include an operation field, a data type identifier field, a data field, and links to other nodes that are children or siblings of this node.

DBCOMMANDTREE structures that represent queries as described in Step 2 are known as select trees. DBCOMMANDTREE structures that represent all three steps are known as full trees.

A full tree contains nodes that represent all three steps but in reverse order. Trees are processed upward from the base with final operations occurring at the top. A typical query tree contains a SELECT node that creates a results table, a PROJECT node that copies values from the results table to the results display table, and a SORT node that specifies how the results-display table gets sorted before being available for display.

An example of a full tree is shown in Example of Command Tree.