59.1. Creating Custom Scan Paths
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## 59.1. Creating Custom Scan Paths #

A custom scan provider will typically add paths for a base relation by setting the following hook, which is called after the core code has generated all the access paths it can for the relation (except for Gather and Gather Merge paths, which are made after this call so that they can use partial paths added by the hook):

    typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root,
                                                RelOptInfo *rel,
                                                Index rti,
                                                RangeTblEntry *rte);
    extern PGDLLIMPORT set_rel_pathlist_hook_type set_rel_pathlist_hook;

Although this hook function can be used to examine, modify, or remove paths generated by the core system, a custom scan provider will typically confine itself to generating `CustomPath` objects and adding them to `rel` using `add_path`, or `add_partial_path` if they are partial paths. The custom scan provider is responsible for initializing the `CustomPath` object, which is declared like this:

    typedef struct CustomPath
    {
        Path      path;
        uint32    flags;
        List     *custom_paths;
        List     *custom_restrictinfo;
        List     *custom_private;
        const CustomPathMethods *methods;
    } CustomPath;

`path` must be initialized as for any other path, including the row-count estimate, start and total cost, and sort ordering provided by this path. `flags` is a bit mask, which specifies whether the scan provider can support certain optional capabilities. `flags` should include `CUSTOMPATH_SUPPORT_BACKWARD_SCAN` if the custom path can support a backward scan, `CUSTOMPATH_SUPPORT_MARK_RESTORE` if it can support mark and restore, and `CUSTOMPATH_SUPPORT_PROJECTION` if it can perform projections. (If `CUSTOMPATH_SUPPORT_PROJECTION` is not set, the scan node will only be asked to produce Vars of the scanned relation; while if that flag is set, the scan node must be able to evaluate scalar expressions over these Vars.) An optional `custom_paths` is a list of `Path` nodes used by this custom-path node; these will be transformed into `Plan` nodes by planner. As described below, custom paths can be created for join relations as well. In such a case, `custom_restrictinfo` should be used to store the set of join clauses to apply to the join the custom path replaces. Otherwise it should be NIL. `custom_private` can be used to store the custom path's private data. Private data should be stored in a form that can be handled by `nodeToString`, so that debugging routines that attempt to print the custom path will work as designed. `methods` must point to a (usually statically allocated) object implementing the required custom path methods, which are further detailed below.

A custom scan provider can also provide join paths. Just as for base relations, such a path must produce the same output as would normally be produced by the join it replaces. To do this, the join provider should set the following hook, and then within the hook function, create `CustomPath` path(s) for the join relation.

    typedef void (*set_join_pathlist_hook_type) (PlannerInfo *root,
                                                 RelOptInfo *joinrel,
                                                 RelOptInfo *outerrel,
                                                 RelOptInfo *innerrel,
                                                 JoinType jointype,
                                                 JoinPathExtraData *extra);
    extern PGDLLIMPORT set_join_pathlist_hook_type set_join_pathlist_hook;

This hook will be invoked repeatedly for the same join relation, with different combinations of inner and outer relations; it is the responsibility of the hook to minimize duplicated work.

Note also that the set of join clauses to apply to the join, which is passed as `extra->restrictlist`, varies depending on the combination of inner and outer relations. A `CustomPath` path generated for the `joinrel` must contain the set of join clauses it uses, which will be used by the planner to convert the `CustomPath` path into a plan, if it is selected by the planner as the best path for the `joinrel`.

### 59.1.1. Custom Scan Path Callbacks #

    Plan *(*PlanCustomPath) (PlannerInfo *root,
                             RelOptInfo *rel,
                             CustomPath *best_path,
                             List *tlist,
                             List *clauses,
                             List *custom_plans);

Convert a custom path to a finished plan. The return value will generally be a `CustomScan` object, which the callback must allocate and initialize. See Section 59.2 for more details.

    List *(*ReparameterizeCustomPathByChild) (PlannerInfo *root,
                                              List *custom_private,
                                              RelOptInfo *child_rel);

This callback is called while converting a path parameterized by the top-most parent of the given child relation `child_rel` to be parameterized by the child relation. The callback is used to reparameterize any paths or translate any expression nodes saved in the given `custom_private` member of a `CustomPath`. The callback may use `reparameterize_path_by_child`, `adjust_appendrel_attrs` or `adjust_appendrel_attrs_multilevel` as required.

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