59.3. Executing Custom Scans
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## 59.3. Executing Custom Scans

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` is initialized as for any other scan state, except that if the scan is for a join rather than a base relation, `ss.ss_currentRelation` is left NULL. `flags` is a bit mask 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.

### 59.3.1. Custom Scan Execution Callbacks

    void (*BeginCustomScan) (CustomScanState *node,
                             EState *estate,
                             int eflags);

Complete initialization of the supplied `CustomScanState`. Standard fields have been initialized 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 it does not need to do anything if there is no associated data or it 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 callback is optional, and need only be supplied if the `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 the `CUSTOMPATH_SUPPORT_MARK_RESTORE` flag is set.

    Size (*EstimateDSMCustomScan) (CustomScanState *node,
                                   ParallelContext *pcxt);

Estimate the amount of dynamic shared memory that will be required for parallel operation. This may be higher than the amount that will actually be used, but it must not be lower. The return value is in bytes. This callback is optional, and need only be supplied if this custom scan provider supports parallel execution.

    void (*InitializeDSMCustomScan) (CustomScanState *node,
                                     ParallelContext *pcxt,
                                     void *coordinate);

Initialize the dynamic shared memory that will be required for parallel operation. `coordinate` points to a shared memory area of size equal to the return value of `EstimateDSMCustomScan`. This callback is optional, and need only be supplied if this custom scan provider supports parallel execution.

    void (*ReInitializeDSMCustomScan) (CustomScanState *node,
                                       ParallelContext *pcxt,
                                       void *coordinate);

Re-initialize the dynamic shared memory required for parallel operation when the custom-scan plan node is about to be re-scanned. This callback is optional, and need only be supplied if this custom scan provider supports parallel execution. Recommended practice is that this callback reset only shared state, while the `ReScanCustomScan` callback resets only local state. Currently, this callback will be called before `ReScanCustomScan`, but it's best not to rely on that ordering.

    void (*InitializeWorkerCustomScan) (CustomScanState *node,
                                        shm_toc *toc,
                                        void *coordinate);

Initialize a parallel worker's local state based on the shared state set up by the leader during `InitializeDSMCustomScan`. This callback is optional, and need only be supplied if this custom scan provider supports parallel execution.

    void (*ShutdownCustomScan) (CustomScanState *node);

Release resources when it is anticipated the node will not be executed to completion. This is not called in all cases; sometimes, `EndCustomScan` may be called without this function having been called first. Since the DSM segment used by parallel query is destroyed just after this callback is invoked, custom scan providers that wish to take some action before the DSM segment goes away should implement this method.

    void (*ExplainCustomScan) (CustomScanState *node,
                               List *ancestors,
                               ExplainState *es);

Output additional information for `EXPLAIN` of a custom-scan plan 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.

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