PostgreSQL 9.6.24 Documentation | |||
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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 in CustomPath. custom_plans can be used to store child Plan nodes. custom_exprs should be used to store expression trees that will need to be fixed up by setrefs.c and subselect.c, while custom_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 a CustomScan 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 using copyObject
, 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 a CustomScan for the structure itself, as would be possible for a CustomPath or CustomScanState.
Node *(*CreateCustomScanState) (CustomScan *cscan);
Allocate a CustomScanState for this CustomScan. The actual allocation will often be larger than required for an ordinary CustomScanState, because many providers will wish to embed that as the first field of a larger structure. The value returned must have the node tag and methods set appropriately, but other fields should be left as zeroes at this stage; after ExecInitCustomScan
performs basic initialization, the BeginCustomScan
callback will be invoked to give the custom scan provider a chance to do whatever else is needed.