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50.4. Foreign Data Wrapper Query Planning

The FDW callback functions GetForeignRelSize, GetForeignPaths, and 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 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.

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.

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