PostgreSQL 9.3.25 Documentation | ||||
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The FDW handler function returns a palloc'd FdwRoutine struct containing pointers to the callback functions described below. The scan-related functions are required, the rest are optional.
The FdwRoutine struct type is declared in src/include/foreign/fdwapi.h, which see for additional details.
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 that scans 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 52.4 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 52.4 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 52.4 for additional information.
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
). eflags contains flag bits describing the executor's
operating mode for this plan node.
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 any NOT NULL constraints that 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.
If an FDW supports writable foreign tables, it should provide some or all of the following callback functions depending on the needs and capabilities of the FDW:
void AddForeignUpdateTargets (Query *parsetree, RangeTblEntry *target_rte, Relation target_relation);
UPDATE and DELETE operations are performed against rows previously fetched by the table-scanning functions. The FDW may need extra information, such as a row ID or the values of primary-key columns, to ensure that it can identify the exact row to update or delete. To support that, this function can add extra hidden, or "junk", target columns to the list of columns that are to be retrieved from the foreign table during an UPDATE or DELETE.
To do that, add TargetEntry items to parsetree->targetList, containing expressions for the extra values to be fetched. Each such entry must be marked resjunk = true, and must have a distinct resname that will identify it at execution time. Avoid using names matching ctidN or wholerowN, as the core system can generate junk columns of these names.
This function is called in the rewriter, not the planner, so the information available is a bit different from that available to the planning routines. parsetree is the parse tree for the UPDATE or DELETE command, while target_rte and target_relation describe the target foreign table.
If the AddForeignUpdateTargets
pointer is set to NULL, no extra target
expressions are added. (This will make it impossible to implement
DELETE operations, though UPDATE may still be feasible if the FDW relies on an
unchanging primary key to identify rows.)
List * PlanForeignModify (PlannerInfo *root, ModifyTable *plan, Index resultRelation, int subplan_index);
Perform any additional planning actions needed for an insert,
update, or delete on a foreign table. This function generates the
FDW-private information that will be attached to the ModifyTable plan node that performs the update
action. This private information must have the form of a List, and will be delivered to BeginForeignModify
during the execution
stage.
root is the planner's global information about the query. plan is the ModifyTable plan node, which is complete except for the fdwPrivLists field. resultRelation identifies the target foreign table by its rangetable index. subplan_index identifies which target of the ModifyTable plan node this is, counting from zero; use this if you want to index into plan->plans or other substructure of the plan node.
See Section 52.4 for additional information.
If the PlanForeignModify
pointer
is set to NULL, no additional plan-time
actions are taken, and the fdw_private
list delivered to BeginForeignModify
will be NIL.
void BeginForeignModify (ModifyTableState *mtstate, ResultRelInfo *rinfo, List *fdw_private, int subplan_index, int eflags);
Begin executing a foreign table modification operation. This
routine is called during executor startup. It should perform any
initialization needed prior to the actual table modifications.
Subsequently, ExecForeignInsert
,
ExecForeignUpdate
or ExecForeignDelete
will be called for each tuple
to be inserted, updated, or deleted.
mtstate is the overall state of the
ModifyTable plan node being executed;
global data about the plan and execution state is available via
this structure. rinfo is the ResultRelInfo struct describing the target
foreign table. (The ri_FdwState field
of ResultRelInfo is available for the
FDW to store any private state it needs for this operation.)
fdw_private contains the private data
generated by PlanForeignModify
, if
any. subplan_index identifies which target
of the ModifyTable plan node this is.
eflags contains flag bits describing the
executor's operating mode for this plan node.
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 ExplainForeignModify
and EndForeignModify
.
If the BeginForeignModify
pointer
is set to NULL, no action is taken during
executor startup.
TupleTableSlot * ExecForeignInsert (EState *estate, ResultRelInfo *rinfo, TupleTableSlot *slot, TupleTableSlot *planSlot);
Insert one tuple into the foreign table. estate is global execution state for the query. rinfo is the ResultRelInfo struct describing the target foreign table. slot contains the tuple to be inserted; it will match the rowtype definition of the foreign table. planSlot contains the tuple that was generated by the ModifyTable plan node's subplan; it differs from slot in possibly containing additional "junk" columns. (The planSlot is typically of little interest for INSERT cases, but is provided for completeness.)
The return value is either a slot containing the data that was actually inserted (this might differ from the data supplied, for example as a result of trigger actions), or NULL if no row was actually inserted (again, typically as a result of triggers). The passed-in slot can be re-used for this purpose.
The data in the returned slot is used only if the INSERT query has a RETURNING clause. Hence, the FDW could choose to optimize away returning some or all columns depending on the contents of the RETURNING clause. However, some slot must be returned to indicate success, or the query's reported row count will be wrong.
If the ExecForeignInsert
pointer
is set to NULL, attempts to insert into
the foreign table will fail with an error message.
TupleTableSlot * ExecForeignUpdate (EState *estate, ResultRelInfo *rinfo, TupleTableSlot *slot, TupleTableSlot *planSlot);
Update one tuple in the foreign table. estate is global execution state for the query.
rinfo is the ResultRelInfo struct describing the target
foreign table. slot contains the new data
for the tuple; it will match the rowtype definition of the foreign
table. planSlot contains the tuple that
was generated by the ModifyTable plan
node's subplan; it differs from slot in
possibly containing additional "junk"
columns. In particular, any junk columns that were requested by
AddForeignUpdateTargets
will be
available from this slot.
The return value is either a slot containing the row as it was actually updated (this might differ from the data supplied, for example as a result of trigger actions), or NULL if no row was actually updated (again, typically as a result of triggers). The passed-in slot can be re-used for this purpose.
The data in the returned slot is used only if the UPDATE query has a RETURNING clause. Hence, the FDW could choose to optimize away returning some or all columns depending on the contents of the RETURNING clause. However, some slot must be returned to indicate success, or the query's reported row count will be wrong.
If the ExecForeignUpdate
pointer
is set to NULL, attempts to update the
foreign table will fail with an error message.
TupleTableSlot * ExecForeignDelete (EState *estate, ResultRelInfo *rinfo, TupleTableSlot *slot, TupleTableSlot *planSlot);
Delete one tuple from the foreign table. estate is global execution state for the query.
rinfo is the ResultRelInfo struct describing the target
foreign table. slot contains nothing
useful upon call, but can be used to hold the returned tuple.
planSlot contains the tuple that was
generated by the ModifyTable plan
node's subplan; in particular, it will carry any junk columns that
were requested by AddForeignUpdateTargets
. The junk column(s) must
be used to identify the tuple to be deleted.
The return value is either a slot containing the row that was deleted, or NULL if no row was deleted (typically as a result of triggers). The passed-in slot can be used to hold the tuple to be returned.
The data in the returned slot is used only if the DELETE query has a RETURNING clause. Hence, the FDW could choose to optimize away returning some or all columns depending on the contents of the RETURNING clause. However, some slot must be returned to indicate success, or the query's reported row count will be wrong.
If the ExecForeignDelete
pointer
is set to NULL, attempts to delete from
the foreign table will fail with an error message.
void EndForeignModify (EState *estate, ResultRelInfo *rinfo);
End the table update 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.
If the EndForeignModify
pointer is
set to NULL, no action is taken during
executor shutdown.
int IsForeignRelUpdatable (Relation rel);
Report which update operations the specified foreign table supports. The return value should be a bitmask of rule event numbers indicating which operations are supported by the foreign table, using the CmdType enumeration; that is, (1 << CMD_UPDATE) = 4 for UPDATE, (1 << CMD_INSERT) = 8 for INSERT, and (1 << CMD_DELETE) = 16 for DELETE.
If the IsForeignRelUpdatable
pointer is set to NULL, foreign tables are
assumed to be insertable, updatable, or deletable if the FDW
provides ExecForeignInsert
,
ExecForeignUpdate
, or ExecForeignDelete
respectively. This function is
only needed if the FDW supports some tables that are updatable and
some that are not. (Even then, it's permissible to throw an error
in the execution routine instead of checking in this function.
However, this function is used to determine updatability for
display in the information_schema
views.)
void ExplainForeignScan (ForeignScanState *node, ExplainState *es);
Print additional EXPLAIN output for a
foreign table scan. This function can 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.
If the ExplainForeignScan
pointer
is set to NULL, no additional information
is printed during EXPLAIN.
void ExplainForeignModify (ModifyTableState *mtstate, ResultRelInfo *rinfo, List *fdw_private, int subplan_index, struct ExplainState *es);
Print additional EXPLAIN output for a
foreign table update. This function can 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 ModifyTableState node can be inspected to provide
run-time statistics in the EXPLAIN ANALYZE
case. The first four arguments are the same as for BeginForeignModify
.
If the ExplainForeignModify
pointer is set to NULL, no additional
information is printed during EXPLAIN.
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.)