Postgres-XC 1.1 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 following callback functions:

    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 involving 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
    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 run-time 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 any FDW-private information provided by `GetForeignPlan`).

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.

    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.)

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

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