56.2. Creating Custom Scan Plans
A custom scan is represented in a finished plan tree using the following structure:
typedef struct CustomScan{ Scan scan; uint32 flags; List *custom_plans; List *custom_exprs; List *custom_private; List *custom_scan_tlist; Bitmapset *custom_relids; const CustomScanMethods *methods;} CustomScan;scan must be initialized as for any other scan, including estimated costs, target lists, qualifications, and so on.flags is a bit mask with the same meaning as inCustomPath.custom_plans can be used to store childPlan nodes.custom_exprs should be used to store expression trees that will need to be fixed up bysetrefs.c andsubselect.c, whilecustom_private should be used to store other private data that is only used by the custom scan provider itself.custom_scan_tlist can be NIL when scanning a base relation, indicating that the custom scan returns scan tuples that match the base relation's row type. Otherwise it is a target list describing the actual scan tuples.custom_scan_tlist must be provided for joins, and could be provided for scans if the custom scan provider can compute some non-Var expressions.custom_relids is set by the core code to the set of relations (range table indexes) that this scan node handles; except when this scan is replacing a join, it will have only one member.methods must point to a (usually statically allocated) object implementing the required custom scan methods, which are further detailed below.
When aCustomScan scans a single relation,scan.scanrelid must be the range table index of the table to be scanned. When it replaces a join,scan.scanrelid should be zero.
Plan trees must be able to be duplicated usingcopyObject, so all the data stored within the“custom” fields must consist of nodes that that function can handle. Furthermore, custom scan providers cannot substitute a larger structure that embeds aCustomScan for the structure itself, as would be possible for aCustomPath orCustomScanState.
56.2.1. Custom Scan Plan Callbacks
Node *(*CreateCustomScanState) (CustomScan *cscan);
Allocate aCustomScanState for thisCustomScan. The actual allocation will often be larger than required for an ordinaryCustomScanState, because many providers will wish to embed that as the first field of a larger structure. The value returned must have the node tag andmethods set appropriately, but other fields should be left as zeroes at this stage; afterExecInitCustomScan performs basic initialization, theBeginCustomScan callback will be invoked to give the custom scan provider a chance to do whatever else is needed.
void (*TextOutCustomScan) (StringInfo str, const CustomScan *node);
Generate additional output whennodeToString is invoked on this custom plan node. This callback is optional. SincenodeToString will automatically dump all fields in the structure, including the substructure of the“custom” fields, there is usually not much need for this callback.