Foreign Data Wrapper Callback Routines
PostgreSQL 9.2.20 Documentation | ||||
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The FDW handler function returns a palloc'd FdwRoutine struct containing pointers to the following callback functions:
void GetForeignRelSize (PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid);
Obtain relation size estimates for a foreign table. This is called at the beginning of planning for a query involving a foreign table. root is the planner's global information about the query; baserel is the planner's information about this table; and foreigntableid is the pg_class OID of the foreign table. ( foreigntableid could be obtained from the planner data structures, but it's passed explicitly to save effort.)
This function should update baserel->rows to be the expected number of rows returned by the table scan, after accounting for the filtering done by the restriction quals. The initial value of baserel->rows is just a constant default estimate, which should be replaced if at all possible. The function may also choose to update baserel->width if it can compute a better estimate of the average result row width.
See Section 50.4 for additional information.
void GetForeignPaths (PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid);
Create possible access paths for a scan on a foreign table.
This is called during query planning.
The parameters are the same as for
GetForeignRelSize
,
which has already been called.
This function must generate at least one access path
(
ForeignPath
node) for a scan on the foreign table and
must call
add_path
to add each such path to
baserel->pathlist
. It's recommended to use
create_foreignscan_path
to build the
ForeignPath
nodes. The function can generate multiple
access paths, e.g., a path which has valid
pathkeys
to
represent a pre-sorted result. Each access path must contain cost
estimates, and can contain any FDW-private information that is needed to
identify the specific scan method intended.
See Section 50.4 for additional information.
ForeignScan * GetForeignPlan (PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid, ForeignPath *best_path, List *tlist, List *scan_clauses);
Create a
ForeignScan
plan node from the selected foreign
access path. This is called at the end of query planning.
The parameters are as for
GetForeignRelSize
, plus
the selected
ForeignPath
(previously produced by
GetForeignPaths
), the target list to be emitted by the
plan node, and the restriction clauses to be enforced by the plan node.
This function must create and return a
ForeignScan
plan
node; it's recommended to use
make_foreignscan
to build the
ForeignScan
node.
See Section 50.4 for additional information.
void ExplainForeignScan (ForeignScanState *node, ExplainState *es);
Print additional
EXPLAIN
output for a foreign table scan.
This can just return if there is no need to print anything.
Otherwise, it should call
ExplainPropertyText
and
related functions to add fields to the
EXPLAIN
output.
The flag fields in
es
can be used to determine what to
print, and the state of the
ForeignScanState
node
can be inspected to provide run-time statistics in the
EXPLAIN
ANALYZE
case.
void BeginForeignScan (ForeignScanState *node, int eflags);
Begin executing a foreign scan. This is called during executor startup.
It should perform any initialization needed before the scan can start,
but not start executing the actual scan (that should be done upon the
first call to
IterateForeignScan
).
The
ForeignScanState
node has already been created, but
its
fdw_state
field is still NULL. Information about
the table to scan is accessible through the
ForeignScanState
node (in particular, from the underlying
ForeignScan
plan node, which contains any FDW-private
information provided by
GetForeignPlan
).
Note that when
(eflags & EXEC_FLAG_EXPLAIN_ONLY)
is
true, this function should not perform any externally-visible actions;
it should only do the minimum required to make the node state valid
for
ExplainForeignScan
and
EndForeignScan
.
TupleTableSlot * IterateForeignScan (ForeignScanState *node);
Fetch one row from the foreign source, returning it in a tuple table slot
(the node's
ScanTupleSlot
should be used for this
purpose). Return NULL if no more rows are available. The tuple table
slot infrastructure allows either a physical or virtual tuple to be
returned; in most cases the latter choice is preferable from a
performance standpoint. Note that this is called in a short-lived memory
context that will be reset between invocations. Create a memory context
in
BeginForeignScan
if you need longer-lived storage, or use
the
es_query_cxt
of the node's
EState
.
The rows returned must match the column signature of the foreign table being scanned. If you choose to optimize away fetching columns that are not needed, you should insert nulls in those column positions.
Note that PostgreSQL 's executor doesn't care whether the rows returned violate the NOT NULL constraints which were defined on the foreign table columns - but the planner does care, and may optimize queries incorrectly if NULL values are present in a column declared not to contain them. If a NULL value is encountered when the user has declared that none should be present, it may be appropriate to raise an error (just as you would need to do in the case of a data type mismatch).
void ReScanForeignScan (ForeignScanState *node);
Restart the scan from the beginning. Note that any parameters the scan depends on may have changed value, so the new scan does not necessarily return exactly the same rows.
void EndForeignScan (ForeignScanState *node);
End the scan and release resources. It is normally not important to release palloc'd memory, but for example open files and connections to remote servers should be cleaned up.
bool AnalyzeForeignTable (Relation relation, AcquireSampleRowsFunc *func, BlockNumber *totalpages);
This function is called when
ANALYZE
is executed on
a foreign table. If the FDW can collect statistics for this
foreign table, it should return
true
, and provide a pointer
to a function that will collect sample rows from the table in
func
, plus the estimated size of the table in pages in
totalpages
. Otherwise, return
false
.
If the FDW does not support collecting statistics for any tables, the
AnalyzeForeignTable
pointer can be set to
NULL
.
If provided, the sample collection function must have the signature
int AcquireSampleRowsFunc (Relation relation, int elevel, HeapTuple *rows, int targrows, double *totalrows, double *totaldeadrows);
A random sample of up to targrows rows should be collected from the table and stored into the caller-provided rows array. The actual number of rows collected must be returned. In addition, store estimates of the total numbers of live and dead rows in the table into the output parameters totalrows and totaldeadrows . (Set totaldeadrows to zero if the FDW does not have any concept of dead rows.)
The FdwRoutine struct type is declared in src/include/foreign/fdwapi.h , which see for additional details.