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><H1
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><A
NAME="FDW-PLANNING"
>52.4. Foreign Data Wrapper Query Planning</A
></H1
><P
> The FDW callback functions <CODE
CLASS="FUNCTION"
>GetForeignRelSize</CODE
>,
<CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
>, <CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
>, and
<CODE
CLASS="FUNCTION"
>PlanForeignModify</CODE
> must fit into the workings of the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> planner. Here are some notes about what
they must do.
</P
><P
> The information in <TT
CLASS="LITERAL"
>root</TT
> and <TT
CLASS="LITERAL"
>baserel</TT
> can be used
to reduce the amount of information that has to be fetched from the
foreign table (and therefore reduce the cost).
<TT
CLASS="LITERAL"
>baserel->baserestrictinfo</TT
> is particularly interesting, as
it contains restriction quals (<TT
CLASS="LITERAL"
>WHERE</TT
> 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.)
<TT
CLASS="LITERAL"
>baserel->reltargetlist</TT
> 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 <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node, not
columns that are used in qual evaluation but not output by the query.
</P
><P
> 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.
</P
><P
> <TT
CLASS="LITERAL"
>baserel->fdw_private</TT
> is a <TT
CLASS="TYPE"
>void</TT
> 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 <TT
CLASS="LITERAL"
>baserel</TT
> node is created.
It is useful for passing information forward from
<CODE
CLASS="FUNCTION"
>GetForeignRelSize</CODE
> to <CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
> and/or
<CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
> to <CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
>, thereby
avoiding recalculation.
</P
><P
> <CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
> can identify the meaning of different
access paths by storing private information in the
<TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> field of <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> nodes.
<TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> is declared as a <TT
CLASS="TYPE"
>List</TT
> 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 <CODE
CLASS="FUNCTION"
>nodeToString</CODE
>, for use with debugging support available
in the backend.
</P
><P
> <CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
> can examine the <TT
CLASS="STRUCTFIELD"
>fdw_private</TT
>
field of the selected <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> node, and can generate
<TT
CLASS="STRUCTFIELD"
>fdw_exprs</TT
> and <TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> lists to be
placed in the <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node, where they will be
available at execution time. Both of these lists must be
represented in a form that <CODE
CLASS="FUNCTION"
>copyObject</CODE
> knows how to copy.
The <TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> list has no other restrictions and is
not interpreted by the core backend in any way. The
<TT
CLASS="STRUCTFIELD"
>fdw_exprs</TT
> 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.
</P
><P
> In <CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
>, 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 <TT
CLASS="LITERAL"
>baserel->baserestrictinfo</TT
>,
but may be re-ordered for better execution efficiency. In simple cases
the FDW can just strip <TT
CLASS="STRUCTNAME"
>RestrictInfo</TT
> nodes from the
scan_clauses list (using <CODE
CLASS="FUNCTION"
>extract_actual_clauses</CODE
>) 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.
</P
><P
> As an example, the FDW might identify some restriction clauses of the
form <TT
CLASS="REPLACEABLE"
><I
>foreign_variable</I
></TT
> <TT
CLASS="LITERAL"
>=</TT
>
<TT
CLASS="REPLACEABLE"
><I
>sub_expression</I
></TT
>, which it determines can be executed on
the remote server given the locally-evaluated value of the
<TT
CLASS="REPLACEABLE"
><I
>sub_expression</I
></TT
>. The actual identification of such a
clause should happen during <CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
>, since it would
affect the cost estimate for the path. The path's
<TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> field would probably include a pointer to
the identified clause's <TT
CLASS="STRUCTNAME"
>RestrictInfo</TT
> node. Then
<CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
> would remove that clause from scan_clauses,
but add the <TT
CLASS="REPLACEABLE"
><I
>sub_expression</I
></TT
> to <TT
CLASS="STRUCTFIELD"
>fdw_exprs</TT
>
to ensure that it gets massaged into executable form. It would probably
also put control information into the plan node's
<TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> field to tell the execution functions what
to do at run time. The query transmitted to the remote server would
involve something like <TT
CLASS="LITERAL"
>WHERE <TT
CLASS="REPLACEABLE"
><I
>foreign_variable</I
></TT
> =
$1</TT
>, with the parameter value obtained at run time from
evaluation of the <TT
CLASS="STRUCTFIELD"
>fdw_exprs</TT
> expression tree.
</P
><P
> 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
<TT
CLASS="REPLACEABLE"
><I
>foreign_variable</I
></TT
> <TT
CLASS="LITERAL"
>=</TT
>
<TT
CLASS="REPLACEABLE"
><I
>local_variable</I
></TT
>. Such clauses will not be found in
<TT
CLASS="LITERAL"
>baserel->baserestrictinfo</TT
> but must be sought in the
relation's join lists. A path using such a clause is called a
<SPAN
CLASS="QUOTE"
>"parameterized path"</SPAN
>. It must identify the other relations
used in the selected join clause(s) with a suitable value of
<TT
CLASS="LITERAL"
>param_info</TT
>; use <CODE
CLASS="FUNCTION"
>get_baserel_parampathinfo</CODE
>
to compute that value. In <CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
>, the
<TT
CLASS="REPLACEABLE"
><I
>local_variable</I
></TT
> portion of the join clause would be added
to <TT
CLASS="STRUCTFIELD"
>fdw_exprs</TT
>, and then at run time the case works the
same as for an ordinary restriction clause.
</P
><P
> When planning an <TT
CLASS="COMMAND"
>UPDATE</TT
> or <TT
CLASS="COMMAND"
>DELETE</TT
>,
<CODE
CLASS="FUNCTION"
>PlanForeignModify</CODE
> can look up the <TT
CLASS="STRUCTNAME"
>RelOptInfo</TT
>
struct for the foreign table and make use of the
<TT
CLASS="LITERAL"
>baserel->fdw_private</TT
> data previously created by the
scan-planning functions. However, in <TT
CLASS="COMMAND"
>INSERT</TT
> the target
table is not scanned so there is no <TT
CLASS="STRUCTNAME"
>RelOptInfo</TT
> for it.
The <TT
CLASS="STRUCTNAME"
>List</TT
> returned by <CODE
CLASS="FUNCTION"
>PlanForeignModify</CODE
> has
the same restrictions as the <TT
CLASS="STRUCTFIELD"
>fdw_private</TT
> list of a
<TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node, that is it must contain only
structures that <CODE
CLASS="FUNCTION"
>copyObject</CODE
> knows how to copy.
</P
><P
> For an <TT
CLASS="COMMAND"
>UPDATE</TT
> or <TT
CLASS="COMMAND"
>DELETE</TT
> against an external data
source that supports concurrent updates, it is recommended that the
<TT
CLASS="LITERAL"
>ForeignScan</TT
> operation lock the rows that it fetches, perhaps
via the equivalent of <TT
CLASS="COMMAND"
>SELECT FOR UPDATE</TT
>. The FDW may also
choose to lock rows at fetch time when the foreign table is referenced
in a <TT
CLASS="COMMAND"
>SELECT FOR UPDATE/SHARE</TT
>; if it does not, the
<TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
> 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.
</P
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