Foreign Data Wrapper Query Planning
PostgreSQL 9.6.9 Documentation | |||
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The FDW callback functions
GetForeignRelSize
,
GetForeignPaths
,
GetForeignPlan
,
PlanForeignModify
,
GetForeignJoinPaths
,
GetForeignUpperPaths
, and
PlanDirectModify
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->reltarget->exprs 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
GetForeignRelSize
to
GetForeignPaths
and/or
GetForeignPaths
to
GetForeignPlan
, thereby
avoiding recalculation.
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
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
executable.
In
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
TargetEntry
s) 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
GetForeignJoinPaths
) must
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
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
GetForeignPlan
, the
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,
GetForeignJoinPaths
should
produce
ForeignPath
s for potential remote joins in much
the same way as
GetForeignPaths
works for base tables.
Information about the intended join can be passed forward
to
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
passed to
GetForeignJoinPaths
as a separate parameter
(
extra->restrictlist
).
An FDW might additionally support direct execution of some plan actions
that are above the level of scans and joins, such as grouping or
aggregation. To offer such options, the FDW should generate paths and
insert them into the appropriate
upper relation
. For
example, a path representing remote aggregation should be inserted into
the
UPPERREL_GROUP_AGG
relation, using
add_path
.
This path will be compared on a cost basis with local aggregation
performed by reading a simple scan path for the foreign relation (note
that such a path must also be supplied, else there will be an error at
plan time). If the remote-aggregation path wins, which it usually would,
it will be converted into a plan in the usual way, by
calling
GetForeignPlan
. The recommended place to generate
such paths is in the
GetForeignUpperPaths
callback function, which is called for each upper relation (i.e., each
post-scan/join processing step), if all the base relations of the query
come from the same FDW.
PlanForeignModify
and the other callbacks described in
Section 55.2.4
are designed around the assumption
that the foreign relation will be scanned in the usual way and then
individual row updates will be driven by a local
ModifyTable
plan node. This approach is necessary for the general case where an
update requires reading local tables as well as foreign tables.
However, if the operation could be executed entirely by the foreign
server, the FDW could generate a path representing that and insert it
into the
UPPERREL_FINAL
upper relation, where it would
compete against the
ModifyTable
approach. This approach
could also be used to implement remote
SELECT FOR UPDATE
,
rather than using the row locking callbacks described in
Section 55.2.5
. Keep in mind that a path
inserted into
UPPERREL_FINAL
is responsible for
implementing
all
behavior of the query.
When planning an
UPDATE
or
DELETE
,
PlanForeignModify
and
PlanDirectModify
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
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.