Projection-Restriction Nodes
A projection-restriction (proj-rest) node describes the result of a projection and/or WHERE clause restriction applied to a subsidiary node. It defines how a subsidiary node is to key into the table and what qualification to use on the table. This type of node displays the following:
• A label identifying it as a proj-rest node
• Storage structure (which is usually heap)
• Total number of pages and tuples
• Query cost information
• Optionally, sort information (whether the data produced by this node is already sorted or requires sorting)
For a description of each of these parts of the display, see
Information on a QEP (see page
Information on a QEP).
Proj-rest nodes are used to remove data irrelevant to the query from a table, so that the minimum amount of data is carried around during the execution of the query. Therefore, only those columns referenced in the target list and WHERE clause for a table are projected, and only those rows that satisfy the WHERE clause restrictions are retained for use higher in the plan.
All you see is the amount of disk I/O required to read the appropriate rows from the node below, and that amount depends on what storage structures were used and the number of pages accessed.