The FDW callback functions
PlanForeignModify 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
GetForeignPlan, thereby avoiding
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 by
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
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 executable.
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 foreign_variable = 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 param_info; use
get_baserel_parampathinfo 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
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
PlanForeignModify has the same restrictions as
the fdw_private list of a ForeignScan plan node, that is it must contain
only structures that
how to copy.
For an UPDATE or DELETE against an external data source that supports concurrent updates, it is recommended that the ForeignScan operation lock the rows that it fetches, perhaps via the equivalent of SELECT FOR UPDATE. The FDW may also choose to lock rows at fetch time when the foreign table is referenced in a SELECT FOR UPDATE/SHARE; if it does not, the FOR UPDATE or FOR SHARE option is essentially a no-op so far as the foreign table is concerned. This behavior may yield semantics slightly different from operations on local tables, where row locking is customarily delayed as long as possible: remote rows may get locked even though they subsequently fail locally-applied restriction or join conditions. However, matching the local semantics exactly would require an additional remote access for every row, and might be impossible anyway depending on what locking semantics the external data source provides.
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