custom scan provider handler for PostgreSQL supports a set of experimental facilities which are intended to allow extension modules to add new scan types to the system. Unlike a foreign data wrapper, which is only responsible for knowing how to scan its own foreign tables, a custom scan provider can provide an alternative method of scanning any relation in the system. Typically, the motivation for writing a custom scan provider will be to allow the use of some optimization not supported by the core system, such as caching or some form of hardware acceleration. This chapter outlines how to write a new custom scan provider. Implementing a new type of custom scan is a three-step process. First, during planning, it is necessary to generate access paths representing a scan using the proposed strategy. Second, if one of those access paths is selected by the planner as the optimal strategy for scanning a particular relation, the access path must be converted to a plan. Finally, it must be possible to execute the plan and generate the same results that would have been generated for any other access path targeting the same relation. A custom scan provider will typically add paths 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_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 bitmask, 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. 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 which 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. Plan *(*PlanCustomPath) (PlannerInfo *root, RelOptInfo *rel, CustomPath *best_path, List *tlist, List *clauses); 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 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. A custom scan is represented in a finished plan tree using the following structure: typedef struct CustomScan { Scan scan; uint32 flags; List *custom_exprs; List *custom_private; const CustomScanMethods *methods; } CustomScan; scan must be initialized as for any other scan, including estimated costs, target lists, qualifications, and so on. flags is a bitmask with the same meaning as in CustomPath. custom_exprs should be used to store expression trees that will need to be fixed up by setrefs.c and subselect.c, while custom_private should be used to store other private data that is only used by the custom scan provider itself. Plan trees must be able to be duplicated using copyObject, so all the data stored within these two fields must consist of nodes that function can handle. methods must point to a (usually statically allocated) object implementing the required custom scan methods, which are further detailed below. Node *(*CreateCustomScanState) (CustomScan *cscan); Allocate a CustomScanState for this CustomScan. The actual allocation will often be larger than required for an ordinary CustomScanState, because many scan types will wish to embed that as the first field of a large structure. The value returned must have the node tag and methods set appropriately, but the other fields need not be initialized at this stage; after ExecInitCustomScan performs basic initialization, the BeginCustomScan callback will be invoked to give the custom scan state a chance to do whatever else is needed. void (*TextOutCustomScan) (StringInfo str, const CustomScan *node); Generate additional output when nodeToString is invoked on this custom plan. This callback is optional. Since a CustomScan must be copyable by copyObject, custom scan providers cannot substitute a larger structure that embeds a CustomScan for the structure itself, as would be possible for a CustomPath or CustomScanState. Therefore, providing this callback is unlikely to be useful. When a CustomScan is executed, its execution state is represented by a CustomScanState, which is declared as follows. typedef struct CustomScanState { ScanState ss; uint32 flags; const CustomExecMethods *methods; } CustomScanState; ss must be initialized as for any other scanstate; flags is a bitmask with the same meaning as in CustomPath and CustomScan. methods must point to a (usually statically allocated) object implementing the required custom scan state methods, which are further detailed below. Typically, a CustomScanState, which need not support copyObject, will actually be a larger structure embedding the above as its first member. void (*BeginCustomScan) (CustomScanState *node, EState *estate, int eflags); Complete initialization of the supplied CustomScanState. Some initialization is performed by ExecInitCustomScan, but any private fields should be initialized here. TupleTableSlot *(*ExecCustomScan) (CustomScanState *node); Fetch the next scan tuple. If any tuples remain, it should fill ps_ResultTupleSlot with the next tuple in the current scan direction, and then return the tuple slot. If not, NULL or an empty slot should be returned. void (*EndCustomScan) (CustomScanState *node); Clean up any private data associated with the CustomScanState. This method is required, but may not need to do anything if the associated data does not exist or will be cleaned up automatically. void (*ReScanCustomScan) (CustomScanState *node); Rewind the current scan to the beginning and prepare to rescan the relation. void (*MarkPosCustomScan) (CustomScanState *node); Save the current scan position so that it can subsequently be restored by the RestrPosCustomScan callback. This calback is optional, and need only be supplied if CUSTOMPATH_SUPPORT_MARK_RESTORE flag is set. void (*RestrPosCustomScan) (CustomScanState *node); Restore the previous scan position as saved by the MarkPosCustomScan callback. This callback is optional, and need only be supplied if CUSTOMPATH_SUPPORT_MARK_RESTORE flag is set. void (*ExplainCustomScan) (CustomScanState *node, List *ancestors, ExplainState *es); Output additional information on EXPLAIN that involves custom-scan node. This callback is optional. Common data stored in the ScanState, such as the target list and scan relation, will be shown even without this callback, but the callback allows the display of additional, private state.