Writing A Foreign Data Wrapper
foreign data wrapper
handler for
All operations on a foreign table are handled through its foreign data
wrapper, which consists of a set of functions that the planner and
executor call. The foreign data wrapper is responsible for fetching
data from the remote data source and returning it to the
PostgreSQL executor. This chapter outlines how
to write a new foreign data wrapper.
The foreign data wrappers included in the standard distribution are good
references when trying to write your own. Look into the
contrib/file_fdw> subdirectory of the source tree.
The reference page also has
some useful details.
The SQL standard specifies an interface for writing foreign data wrappers.
However, PostgreSQL does not implement that API, because the effort to
accommodate it into PostgreSQL would be large, and the standard API hasn't
gained wide adoption anyway.
Foreign Data Wrapper Functions
The FDW author needs to implement a handler function, and optionally
a validator function. Both functions must be written in a compiled
language such as C, using the version-1 interface.
For details on C language calling conventions and dynamic loading,
see .
The handler function simply returns a struct of function pointers to
callback functions that will be called by the planner and executor.
Most of the effort in writing an FDW is in implementing these callback
functions.
The handler function must be registered with
PostgreSQL as taking no arguments and
returning the special pseudo-type fdw_handler. The
callback functions are plain C functions and are not visible or
callable at the SQL level. The callback functions are described in
.
The validator function is responsible for validating options given in
CREATE and ALTER commands for its
foreign data wrapper, as well as foreign servers, user mappings, and
foreign tables using the wrapper.
The validator function must be registered as taking two arguments, a
text array containing the options to be validated, and an OID
representing the type of object the options are associated with (in
the form of the OID of the system catalog the object would be stored
in, either
ForeignDataWrapperRelationId>,
ForeignServerRelationId>,
UserMappingRelationId>,
or ForeignTableRelationId>).
If no validator function is supplied, options are not checked at object
creation time or object alteration time.
Foreign Data Wrapper Callback Routines
The FDW handler function returns a palloc'd FdwRoutine>
struct containing pointers to the following callback functions:
FdwPlan *
PlanForeignScan (Oid foreigntableid,
PlannerInfo *root,
RelOptInfo *baserel);
Plan a scan on a foreign table. This is called when a query is planned.
foreigntableid> is the pg_class> OID of the
foreign table. root> is the planner's global information
about the query, and baserel> is the planner's information
about this table.
The function must return a palloc'd struct that contains cost estimates
plus any FDW-private information that is needed to execute the foreign
scan at a later time. (Note that the private information must be
represented in a form that copyObject> knows how to copy.)
The information in root> and baserel> can be used
to reduce the amount of information that has to be fetched from the
foreign table (and therefore reduce the cost estimate).
baserel->baserestrictinfo> is particularly interesting, as
it contains restriction quals (WHERE> clauses) that can be
used to filter the rows to be fetched. (The FDW is not required to
enforce these quals, as the finished plan will recheck them anyway.)
baserel->reltargetlist> can be used to determine which
columns need to be fetched.
In addition to returning cost estimates, the function should update
baserel->rows> to be the expected number of rows returned
by the 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.
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 runtime 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 a pointer to the
FdwPlan> structure returned by
PlanForeignScan>).
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.
The FdwRoutine> and FdwPlan> struct types
are declared in src/include/foreign/fdwapi.h>, which see
for additional details.