PostgreSQL 9.1.24 Documentation
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# 50.2. Foreign Data Wrapper Callback Routines

The FDW handler function returns a palloc'd `FdwRoutine` struct containing pointers to the following callback functions:

    FdwPlan *
    PlanForeignScan (Oid foreigntableid,
                     PlannerInfo *root,
                     RelOptInfo *baserel);

Plan a scan on a foreign table. This is called when a query is planned. `foreigntableid` is the `pg_class` OID of the foreign table. `root` is the planner's global information about the query, and `baserel` is the planner's information about this table. The function must return a palloc'd struct that contains cost estimates plus any FDW-private information that is needed to execute the foreign scan at a later time. (Note that the private information must be represented in a form that `copyObject` knows how to copy.)

The information in `root` and `baserel` can be used to reduce the amount of information that has to be fetched from the foreign table (and therefore reduce the cost estimate). `baserel->baserestrictinfo` is particularly interesting, as it contains restriction quals (`WHERE` clauses) that can be used to filter the rows to be fetched. (The FDW is not required to enforce these quals, as the finished plan will recheck them anyway.) `baserel->reltargetlist` can be used to determine which columns need to be fetched.

In addition to returning cost estimates, the function should update `baserel->rows` to be the expected number of rows returned by the 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.

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

Print additional `EXPLAIN` output for a foreign table scan. This can just return if there is no need to print anything. Otherwise, it should 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 runtime statistics in the `EXPLAIN ANALYZE` case.

    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 a pointer to the `FdwPlan` structure returned by `PlanForeignScan`).

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 the `NOT NULL` constraints which 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.

The `FdwRoutine` and `FdwPlan` struct types are declared in `src/include/foreign/fdwapi.h`, which see for additional details.

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