diff options
Diffstat (limited to 'src/include/nodes/relation.h')
| -rw-r--r-- | src/include/nodes/relation.h | 15 |
1 files changed, 11 insertions, 4 deletions
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h index 57747fc4e55..9470df626cc 100644 --- a/src/include/nodes/relation.h +++ b/src/include/nodes/relation.h @@ -1274,15 +1274,22 @@ typedef struct HashPath /* * ProjectionPath represents a projection (that is, targetlist computation) * - * This path node represents using a Result plan node to do a projection. - * It's only needed atop a node that doesn't support projection (such as - * Sort); otherwise we just jam the new desired PathTarget into the lower - * path node, and adjust that node's estimated cost accordingly. + * Nominally, this path node represents using a Result plan node to do a + * projection step. However, if the input plan node supports projection, + * we can just modify its output targetlist to do the required calculations + * directly, and not need a Result. In some places in the planner we can just + * jam the desired PathTarget into the input path node (and adjust its cost + * accordingly), so we don't need a ProjectionPath. But in other places + * it's necessary to not modify the input path node, so we need a separate + * ProjectionPath node, which is marked dummy to indicate that we intend to + * assign the work to the input plan node. The estimated cost for the + * ProjectionPath node will account for whether a Result will be used or not. */ typedef struct ProjectionPath { Path path; Path *subpath; /* path representing input source */ + bool dummypp; /* true if no separate Result is needed */ } ProjectionPath; /* |