PostgreSQL
58.1. 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 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
. 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 is currently only one.
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.
58.1.1. 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 58.2 for more details.
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Chapter 58. Writing A Custom Scan Provider |
58.2. Creating Custom Scan Plans |
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