Databases Reference
In-Depth Information
Physical operators, such as the Index Scan and the Hash Aggregate physical operators,
seen in Figure 1-2, are represented as icons in a graphical plan. The first icon is called the
result operator; it represents the SELECT statement, and is usually the root element in
the plan.
Operators implement a basic function or operation of the execution engine; for
example, a logical join operation could be implemented by any of three different
physical join operators: Nested Loops Join, Merge Join or Hash Join. Obviously, there
are many more operators implemented in the execution engine, and all of them are
documented in Books Online, if you're curious about them. The Query Optimizer
builds an execution plan, choosing from these operators, which may read records from
the database, like the Index Scan operator shown in the previous plan, or may read
records from another operator, like the Hash Aggregate, which is reading records from
the Index Scan operator.
After the operator performs some function on the records it has read, the results are
output to its parent. This data flow is represented by arrows between the operators; the
thickness of the arrows corresponds to the relative number of rows. You can hover the
mouse pointer over an arrow to get more information about that data flow, displayed
as a tooltip. For example, if you hover the mouse pointer over the arrow between the
Index Scan and the Hash Aggregate operators (shown in Figure 1-2), you will get the data
flow information between these operators, as shown in Figure 1-3.
Figure 1-3:
Data flow between Index Scan and Hash Aggregate operators.
