Path的定义：
/* * Type "Path" is used as-is for sequential-scan paths, as well as some other * simple plan types that we don't need any extra information in the path for. * For other path types it is the first component of a larger struct. * * "pathtype" is the NodeTag of the Plan node we could build from this Path. * It is partially redundant with the Path's NodeTag, but allows us to use * the same Path type for multiple Plan types when there is no need to * distinguish the Plan type during path processing. * * "param_info", if not NULL, links to a ParamPathInfo that identifies outer * relation(s) that provide parameter values to each scan of this path. * That means this path can only be joined to those rels by means of nestloop * joins with this path on the inside. Also note that a parameterized path * is responsible for testing all "movable" joinclauses involving this rel * and the specified outer rel(s). * * "rows" is the same as parent->rows in simple paths, but in parameterized * paths and UniquePaths it can be less than parent->rows, reflecting the * fact that we've filtered by extra join conditions or removed duplicates. * * "pathkeys" is a List of PathKey nodes (see above), describing the sort * ordering of the path's output rows. * Type "Path" is used as-is
