PostgreSQL 9.5.23 Documentation
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# 56.1. 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 what it believes to be the complete and correct set of access paths for the relation.

    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`. 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_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 should include `CUSTOMPATH_SUPPORT_BACKWARD_SCAN` if the custom path can support a backward scan and `CUSTOMPATH_SUPPORT_MARK_RESTORE` if it can support mark and restore. Both capabilities are optional. 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. `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, of which there are currently only two, as 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.

## 56.1.1. 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 56.2 for more details.

    void (*TextOutCustomPath) (StringInfo str,
                               const CustomPath *node);

Generate additional output when `nodeToString` is invoked on this custom path. This callback is optional. Since `nodeToString` will automatically dump all fields in the structure that it can see, including `custom_private`, this is only useful if the `CustomPath` is actually embedded in a larger struct containing additional fields.

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