Executing Custom Scans
| PostgreSQL 9.5.20 Documentation | |||
|---|---|---|---|
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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.
56.3.1. 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.
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.