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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.

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.