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.