The FDW callback functions
GetForeignPlan 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 baserel 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.
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.