The FDW callback functions
must fit into the workings of the PostgreSQL planner.
Here are some notes about what they must do.
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). baserel->baserestrictinfo is particularly interesting, as it contains restriction quals (WHERE clauses) that should be used to filter the rows to be fetched. (The FDW itself is not required to enforce these quals, as the core executor can check them instead.) baserel->reltargetlist can be used to determine which columns need to be fetched; but note that it only lists columns that have to be emitted by the ForeignScan plan node, not columns that are used in qual evaluation but not output by the query.
Various private fields are available for the FDW planning functions to keep information in. Generally, whatever you store in FDW private fields should be palloc'd, so that it will be reclaimed at the end of planning.
baserel->fdw_private is a void pointer that is
available for FDW planning functions to store information relevant to
the particular foreign table. The core planner does not touch it except
to initialize it to NULL when the RelOptInfo node is created.
It is useful for passing information forward from
GetForeignPaths can identify the meaning of different
access paths by storing private information in the
fdw_private field of ForeignPath nodes.
fdw_private is declared as a List pointer, but
could actually contain anything since the core planner does not touch
it. However, best practice is to use a representation that's dumpable
nodeToString, for use with debugging support available
in the backend.
GetForeignPlan can examine the fdw_private
field of the selected ForeignPath node, and can generate
fdw_exprs and fdw_private lists to be
placed in the ForeignScan plan node, where they will be
available at execution time. Both of these lists must be
represented in a form that
copyObject knows how to copy.
The fdw_private list has no other restrictions and is
not interpreted by the core backend in any way. The
fdw_exprs list, if not NIL, is expected to contain
expression trees that are intended to be executed at run time. These
trees will undergo post-processing by the planner to make them fully
GetForeignPlan, generally the passed-in target list can
be copied into the plan node as-is. The passed scan_clauses list
contains the same clauses as baserel->baserestrictinfo,
but may be re-ordered for better execution efficiency. In simple cases
the FDW can just strip RestrictInfo nodes from the
scan_clauses list (using
extract_actual_clauses) and put
all the clauses into the plan node's qual list, which means that all the
clauses will be checked by the executor at run time. More complex FDWs
may be able to check some of the clauses internally, in which case those
clauses can be removed from the plan node's qual list so that the
executor doesn't waste time rechecking them.
As an example, the FDW might identify some restriction clauses of the
form foreign_variable =
sub_expression, which it determines can be executed on
the remote server given the locally-evaluated value of the
sub_expression. The actual identification of such a
clause should happen during
GetForeignPaths, since it would
affect the cost estimate for the path. The path's
fdw_private field would probably include a pointer to
the identified clause's RestrictInfo node. Then
GetForeignPlan would remove that clause from scan_clauses,
but add the sub_expression to fdw_exprs
to ensure that it gets massaged into executable form. It would probably
also put control information into the plan node's
fdw_private field to tell the execution functions what
to do at run time. The query transmitted to the remote server would
involve something like WHERE foreign_variable =
$1, with the parameter value obtained at run time from
evaluation of the fdw_exprs expression tree.
Any clauses removed from the plan node's qual list must instead be added to fdw_recheck_quals or rechecked by RecheckForeignScan in order to ensure correct behavior at the READ COMMITTED isolation level. When a concurrent update occurs for some other table involved in the query, the executor may need to verify that all of the original quals are still satisfied for the tuple, possibly against a different set of parameter values. Using fdw_recheck_quals is typically easier than implementing checks inside RecheckForeignScan, but this method will be insufficient when outer joins have been pushed down, since the join tuples in that case might have some fields go to NULL without rejecting the tuple entirely.
Another ForeignScan field that can be filled by FDWs
is fdw_scan_tlist, which describes the tuples returned by
the FDW for this plan node. For simple foreign table scans this can be
set to NIL, implying that the returned tuples have the
row type declared for the foreign table. A non-NIL value must be a
target list (list of TargetEntrys) containing Vars and/or
expressions representing the returned columns. This might be used, for
example, to show that the FDW has omitted some columns that it noticed
won't be needed for the query. Also, if the FDW can compute expressions
used by the query more cheaply than can be done locally, it could add
those expressions to fdw_scan_tlist. Note that join
plans (created from paths made by
always supply fdw_scan_tlist to describe the set of
columns they will return.
The FDW should always construct at least one path that depends only on
the table's restriction clauses. In join queries, it might also choose
to construct path(s) that depend on join clauses, for example
local_variable. Such clauses will not be found in
baserel->baserestrictinfo but must be sought in the
relation's join lists. A path using such a clause is called a
"parameterized path". It must identify the other relations
used in the selected join clause(s) with a suitable value of
to compute that value. In
local_variable portion of the join clause would be added
to fdw_exprs, and then at run time the case works the
same as for an ordinary restriction clause.
If an FDW supports remote joins,
produce ForeignPaths for potential remote joins in much
the same way as
GetForeignPaths works for base tables.
Information about the intended join can be passed forward
GetForeignPlan in the same ways described above.
However, baserestrictinfo is not relevant for join
relations; instead, the relevant join clauses for a particular join are
GetForeignJoinPaths as a separate parameter
When planning an UPDATE or DELETE,
PlanForeignModify can look up the RelOptInfo
struct for the foreign table and make use of the
baserel->fdw_private data previously created by the
scan-planning functions. However, in INSERT the target
table is not scanned so there is no RelOptInfo for it.
The List returned by
the same restrictions as the fdw_private list of a
ForeignScan plan node, that is it must contain only
copyObject knows how to copy.
INSERT with an ON CONFLICT clause does not support specifying the conflict target, as unique constraints or exclusion constraints on remote tables are not locally known. This in turn implies that ON CONFLICT DO UPDATE is not supported, since the specification is mandatory there.