PostgreSQL 9.3.20 Documentation
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# 52.2. Foreign Data Wrapper Callback Routines

The FDW handler function returns a palloc'd `FdwRoutine` struct containing pointers to the callback functions described below. The scan-related functions are required, the rest are optional.

The `FdwRoutine` struct type is declared in `src/include/foreign/fdwapi.h`, which see for additional details.

## 52.2.1. FDW Routines For Scanning Foreign Tables

    void
    GetForeignRelSize (PlannerInfo *root,
                       RelOptInfo *baserel,
                       Oid foreigntableid);

Obtain relation size estimates for a foreign table. This is called at the beginning of planning for a query that scans a foreign table. `root` is the planner's global information about the query; `baserel` is the planner's information about this table; and `foreigntableid` is the `pg_class` OID of the foreign table. (`foreigntableid` could be obtained from the planner data structures, but it's passed explicitly to save effort.)

This function should update `baserel->rows` to be the expected number of rows returned by the table scan, after accounting for the filtering done by the restriction quals. The initial value of `baserel->rows` is just a constant default estimate, which should be replaced if at all possible. The function may also choose to update `baserel->width` if it can compute a better estimate of the average result row width.

See Section 52.4 for additional information.

    void
    GetForeignPaths (PlannerInfo *root,
                     RelOptInfo *baserel,
                     Oid foreigntableid);

Create possible access paths for a scan on a foreign table. This is called during query planning. The parameters are the same as for `GetForeignRelSize`, which has already been called.

This function must generate at least one access path (`ForeignPath` node) for a scan on the foreign table and must call `add_path` to add each such path to `baserel->pathlist`. It's recommended to use `create_foreignscan_path` to build the `ForeignPath` nodes. The function can generate multiple access paths, e.g., a path which has valid `pathkeys` to represent a pre-sorted result. Each access path must contain cost estimates, and can contain any FDW-private information that is needed to identify the specific scan method intended.

See Section 52.4 for additional information.

    ForeignScan *
    GetForeignPlan (PlannerInfo *root,
                    RelOptInfo *baserel,
                    Oid foreigntableid,
                    ForeignPath *best_path,
                    List *tlist,
                    List *scan_clauses);

Create a `ForeignScan` plan node from the selected foreign access path. This is called at the end of query planning. The parameters are as for `GetForeignRelSize`, plus the selected `ForeignPath` (previously produced by `GetForeignPaths`), the target list to be emitted by the plan node, and the restriction clauses to be enforced by the plan node.

This function must create and return a `ForeignScan` plan node; it's recommended to use `make_foreignscan` to build the `ForeignScan` node.

See Section 52.4 for additional information.

    void
    BeginForeignScan (ForeignScanState *node,
                      int eflags);

Begin executing a foreign scan. This is called during executor startup. It should perform any initialization needed before the scan can start, but not start executing the actual scan (that should be done upon the first call to `IterateForeignScan`). The `ForeignScanState` node has already been created, but its `fdw_state` field is still NULL. Information about the table to scan is accessible through the `ForeignScanState` node (in particular, from the underlying `ForeignScan` plan node, which contains any FDW-private information provided by `GetForeignPlan`). `eflags` contains flag bits describing the executor's operating mode for this plan node.

Note that when `(eflags & EXEC_FLAG_EXPLAIN_ONLY)` is true, this function should not perform any externally-visible actions; it should only do the minimum required to make the node state valid for `ExplainForeignScan` and `EndForeignScan`.

    TupleTableSlot *
    IterateForeignScan (ForeignScanState *node);

Fetch one row from the foreign source, returning it in a tuple table slot (the node's `ScanTupleSlot` should be used for this purpose). Return NULL if no more rows are available. The tuple table slot infrastructure allows either a physical or virtual tuple to be returned; in most cases the latter choice is preferable from a performance standpoint. Note that this is called in a short-lived memory context that will be reset between invocations. Create a memory context in `BeginForeignScan` if you need longer-lived storage, or use the `es_query_cxt` of the node's `EState`.

The rows returned must match the column signature of the foreign table being scanned. If you choose to optimize away fetching columns that are not needed, you should insert nulls in those column positions.

Note that PostgreSQL's executor doesn't care whether the rows returned violate any `NOT NULL` constraints that were defined on the foreign table columns — but the planner does care, and may optimize queries incorrectly if `NULL` values are present in a column declared not to contain them. If a `NULL` value is encountered when the user has declared that none should be present, it may be appropriate to raise an error (just as you would need to do in the case of a data type mismatch).

    void
    ReScanForeignScan (ForeignScanState *node);

Restart the scan from the beginning. Note that any parameters the scan depends on may have changed value, so the new scan does not necessarily return exactly the same rows.

    void
    EndForeignScan (ForeignScanState *node);

End the scan and release resources. It is normally not important to release palloc'd memory, but for example open files and connections to remote servers should be cleaned up.

## 52.2.2. FDW Routines For Updating Foreign Tables

If an FDW supports writable foreign tables, it should provide some or all of the following callback functions depending on the needs and capabilities of the FDW:

    void
    AddForeignUpdateTargets (Query *parsetree,
                             RangeTblEntry *target_rte,
                             Relation target_relation);

`UPDATE` and `DELETE` operations are performed against rows previously fetched by the table-scanning functions. The FDW may need extra information, such as a row ID or the values of primary-key columns, to ensure that it can identify the exact row to update or delete. To support that, this function can add extra hidden, or "junk", target columns to the list of columns that are to be retrieved from the foreign table during an `UPDATE` or `DELETE`.

To do that, add `TargetEntry` items to `parsetree->targetList`, containing expressions for the extra values to be fetched. Each such entry must be marked `resjunk` = `true`, and must have a distinct `resname` that will identify it at execution time. Avoid using names matching `ctid`N`` or `wholerow`N``, as the core system can generate junk columns of these names.

This function is called in the rewriter, not the planner, so the information available is a bit different from that available to the planning routines. `parsetree` is the parse tree for the `UPDATE` or `DELETE` command, while `target_rte` and `target_relation` describe the target foreign table.

If the `AddForeignUpdateTargets` pointer is set to `NULL`, no extra target expressions are added. (This will make it impossible to implement `DELETE` operations, though `UPDATE` may still be feasible if the FDW relies on an unchanging primary key to identify rows.)

    List *
    PlanForeignModify (PlannerInfo *root,
                       ModifyTable *plan,
                       Index resultRelation,
                       int subplan_index);

Perform any additional planning actions needed for an insert, update, or delete on a foreign table. This function generates the FDW-private information that will be attached to the `ModifyTable` plan node that performs the update action. This private information must have the form of a `List`, and will be delivered to `BeginForeignModify` during the execution stage.

`root` is the planner's global information about the query. `plan` is the `ModifyTable` plan node, which is complete except for the `fdwPrivLists` field. `resultRelation` identifies the target foreign table by its rangetable index. `subplan_index` identifies which target of the `ModifyTable` plan node this is, counting from zero; use this if you want to index into `plan->plans` or other substructure of the `plan` node.

See Section 52.4 for additional information.

If the `PlanForeignModify` pointer is set to `NULL`, no additional plan-time actions are taken, and the `fdw_private` list delivered to `BeginForeignModify` will be NIL.

    void
    BeginForeignModify (ModifyTableState *mtstate,
                        ResultRelInfo *rinfo,
                        List *fdw_private,
                        int subplan_index,
                        int eflags);

Begin executing a foreign table modification operation. This routine is called during executor startup. It should perform any initialization needed prior to the actual table modifications. Subsequently, `ExecForeignInsert`, `ExecForeignUpdate` or `ExecForeignDelete` will be called for each tuple to be inserted, updated, or deleted.

`mtstate` is the overall state of the `ModifyTable` plan node being executed; global data about the plan and execution state is available via this structure. `rinfo` is the `ResultRelInfo` struct describing the target foreign table. (The `ri_FdwState` field of `ResultRelInfo` is available for the FDW to store any private state it needs for this operation.) `fdw_private` contains the private data generated by `PlanForeignModify`, if any. `subplan_index` identifies which target of the `ModifyTable` plan node this is. `eflags` contains flag bits describing the executor's operating mode for this plan node.

Note that when `(eflags & EXEC_FLAG_EXPLAIN_ONLY)` is true, this function should not perform any externally-visible actions; it should only do the minimum required to make the node state valid for `ExplainForeignModify` and `EndForeignModify`.

If the `BeginForeignModify` pointer is set to `NULL`, no action is taken during executor startup.

    TupleTableSlot *
    ExecForeignInsert (EState *estate,
                       ResultRelInfo *rinfo,
                       TupleTableSlot *slot,
                       TupleTableSlot *planSlot);

Insert one tuple into the foreign table. `estate` is global execution state for the query. `rinfo` is the `ResultRelInfo` struct describing the target foreign table. `slot` contains the tuple to be inserted; it will match the rowtype definition of the foreign table. `planSlot` contains the tuple that was generated by the `ModifyTable` plan node's subplan; it differs from `slot` in possibly containing additional "junk" columns. (The `planSlot` is typically of little interest for `INSERT` cases, but is provided for completeness.)

The return value is either a slot containing the data that was actually inserted (this might differ from the data supplied, for example as a result of trigger actions), or NULL if no row was actually inserted (again, typically as a result of triggers). The passed-in `slot` can be re-used for this purpose.

The data in the returned slot is used only if the `INSERT` query has a `RETURNING` clause. Hence, the FDW could choose to optimize away returning some or all columns depending on the contents of the `RETURNING` clause. However, some slot must be returned to indicate success, or the query's reported row count will be wrong.

If the `ExecForeignInsert` pointer is set to `NULL`, attempts to insert into the foreign table will fail with an error message.

    TupleTableSlot *
    ExecForeignUpdate (EState *estate,
                       ResultRelInfo *rinfo,
                       TupleTableSlot *slot,
                       TupleTableSlot *planSlot);

Update one tuple in the foreign table. `estate` is global execution state for the query. `rinfo` is the `ResultRelInfo` struct describing the target foreign table. `slot` contains the new data for the tuple; it will match the rowtype definition of the foreign table. `planSlot` contains the tuple that was generated by the `ModifyTable` plan node's subplan; it differs from `slot` in possibly containing additional "junk" columns. In particular, any junk columns that were requested by `AddForeignUpdateTargets` will be available from this slot.

The return value is either a slot containing the row as it was actually updated (this might differ from the data supplied, for example as a result of trigger actions), or NULL if no row was actually updated (again, typically as a result of triggers). The passed-in `slot` can be re-used for this purpose.

The data in the returned slot is used only if the `UPDATE` query has a `RETURNING` clause. Hence, the FDW could choose to optimize away returning some or all columns depending on the contents of the `RETURNING` clause. However, some slot must be returned to indicate success, or the query's reported row count will be wrong.

If the `ExecForeignUpdate` pointer is set to `NULL`, attempts to update the foreign table will fail with an error message.

    TupleTableSlot *
    ExecForeignDelete (EState *estate,
                       ResultRelInfo *rinfo,
                       TupleTableSlot *slot,
                       TupleTableSlot *planSlot);

Delete one tuple from the foreign table. `estate` is global execution state for the query. `rinfo` is the `ResultRelInfo` struct describing the target foreign table. `slot` contains nothing useful upon call, but can be used to hold the returned tuple. `planSlot` contains the tuple that was generated by the `ModifyTable` plan node's subplan; in particular, it will carry any junk columns that were requested by `AddForeignUpdateTargets`. The junk column(s) must be used to identify the tuple to be deleted.

The return value is either a slot containing the row that was deleted, or NULL if no row was deleted (typically as a result of triggers). The passed-in `slot` can be used to hold the tuple to be returned.

The data in the returned slot is used only if the `DELETE` query has a `RETURNING` clause. Hence, the FDW could choose to optimize away returning some or all columns depending on the contents of the `RETURNING` clause. However, some slot must be returned to indicate success, or the query's reported row count will be wrong.

If the `ExecForeignDelete` pointer is set to `NULL`, attempts to delete from the foreign table will fail with an error message.

    void
    EndForeignModify (EState *estate,
                      ResultRelInfo *rinfo);

End the table update and release resources. It is normally not important to release palloc'd memory, but for example open files and connections to remote servers should be cleaned up.

If the `EndForeignModify` pointer is set to `NULL`, no action is taken during executor shutdown.

    int
    IsForeignRelUpdatable (Relation rel);

Report which update operations the specified foreign table supports. The return value should be a bitmask of rule event numbers indicating which operations are supported by the foreign table, using the `CmdType` enumeration; that is, `(1 << CMD_UPDATE) = 4` for `UPDATE`, `(1 << CMD_INSERT) = 8` for `INSERT`, and `(1 << CMD_DELETE) = 16` for `DELETE`.

If the `IsForeignRelUpdatable` pointer is set to `NULL`, foreign tables are assumed to be insertable, updatable, or deletable if the FDW provides `ExecForeignInsert`, `ExecForeignUpdate`, or `ExecForeignDelete` respectively. This function is only needed if the FDW supports some tables that are updatable and some that are not. (Even then, it's permissible to throw an error in the execution routine instead of checking in this function. However, this function is used to determine updatability for display in the `information_schema` views.)

## 52.2.3. FDW Routines for `EXPLAIN`

    void
    ExplainForeignScan (ForeignScanState *node,
                        ExplainState *es);

Print additional `EXPLAIN` output for a foreign table scan. This function can call `ExplainPropertyText` and related functions to add fields to the `EXPLAIN` output. The flag fields in `es` can be used to determine what to print, and the state of the `ForeignScanState` node can be inspected to provide run-time statistics in the `EXPLAIN ANALYZE` case.

If the `ExplainForeignScan` pointer is set to `NULL`, no additional information is printed during `EXPLAIN`.

    void
    ExplainForeignModify (ModifyTableState *mtstate,
                          ResultRelInfo *rinfo,
                          List *fdw_private,
                          int subplan_index,
                          struct ExplainState *es);

Print additional `EXPLAIN` output for a foreign table update. This function can call `ExplainPropertyText` and related functions to add fields to the `EXPLAIN` output. The flag fields in `es` can be used to determine what to print, and the state of the `ModifyTableState` node can be inspected to provide run-time statistics in the `EXPLAIN ANALYZE` case. The first four arguments are the same as for `BeginForeignModify`.

If the `ExplainForeignModify` pointer is set to `NULL`, no additional information is printed during `EXPLAIN`.

## 52.2.4. FDW Routines for `ANALYZE`

    bool
    AnalyzeForeignTable (Relation relation,
                         AcquireSampleRowsFunc *func,
                         BlockNumber *totalpages);

This function is called when ANALYZE is executed on a foreign table. If the FDW can collect statistics for this foreign table, it should return `true`, and provide a pointer to a function that will collect sample rows from the table in `func`, plus the estimated size of the table in pages in `totalpages`. Otherwise, return `false`.

If the FDW does not support collecting statistics for any tables, the `AnalyzeForeignTable` pointer can be set to `NULL`.

If provided, the sample collection function must have the signature

    int
    AcquireSampleRowsFunc (Relation relation, int elevel,
                           HeapTuple *rows, int targrows,
                           double *totalrows,
                           double *totaldeadrows);

A random sample of up to `targrows` rows should be collected from the table and stored into the caller-provided `rows` array. The actual number of rows collected must be returned. In addition, store estimates of the total numbers of live and dead rows in the table into the output parameters `totalrows` and `totaldeadrows`. (Set `totaldeadrows` to zero if the FDW does not have any concept of dead rows.)

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