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Commit35746bc

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Add new FDW API to test for parallel-safety.
This is basically a bug fix; the old code assumes that a ForeignScanis always parallel-safe, but for postgres_fdw, for example, this isdefinitely false. It should be true for file_fdw, though, since aworker can read a file from the filesystem just as well as any otherbackend process.Original patch by Thomas Munro. Documentation, and changes to thecomments, by me.
1 parent9117985 commit35746bc

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4 files changed

+60
-0
lines changed

4 files changed

+60
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lines changed

‎contrib/file_fdw/file_fdw.c‎

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Original file line numberDiff line numberDiff line change
@@ -131,6 +131,8 @@ static void fileEndForeignScan(ForeignScanState *node);
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staticboolfileAnalyzeForeignTable(Relationrelation,
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AcquireSampleRowsFunc*func,
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BlockNumber*totalpages);
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staticboolfileIsForeignScanParallelSafe(PlannerInfo*root,RelOptInfo*rel,
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RangeTblEntry*rte);
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/*
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* Helper functions
@@ -170,6 +172,7 @@ file_fdw_handler(PG_FUNCTION_ARGS)
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fdwroutine->ReScanForeignScan=fileReScanForeignScan;
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fdwroutine->EndForeignScan=fileEndForeignScan;
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fdwroutine->AnalyzeForeignTable=fileAnalyzeForeignTable;
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fdwroutine->IsForeignScanParallelSafe=fileIsForeignScanParallelSafe;
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PG_RETURN_POINTER(fdwroutine);
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}
@@ -761,6 +764,18 @@ fileAnalyzeForeignTable(Relation relation,
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return true;
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}
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/*
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* fileIsForeignScanParallelSafe
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* Reading a file in a parallel worker should work just the same as
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* reading it in the leader, so mark scans safe.
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*/
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staticbool
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fileIsForeignScanParallelSafe(PlannerInfo*root,RelOptInfo*rel,
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RangeTblEntry*rte)
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{
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return true;
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}
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/*
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* check_selective_binary_conversion
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*

‎doc/src/sgml/fdwhandler.sgml‎

Lines changed: 23 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -988,6 +988,29 @@ ImportForeignSchema (ImportForeignSchemaStmt *stmt, Oid serverOid);
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<para>
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<programlisting>
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Size
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IsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
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RangeTblEntry *rte);
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</programlisting>
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Test whether a scan can be performed within a parallel worker. This
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function will only be called when the planner believes that a parallel
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plan might be possible, and should return true if it is safe for that scan
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to run within a parallel worker. This will generally not be the case if
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the remote data source has transaction semantics, unless the worker's
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connection to the data can somehow be made to share the same transaction
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context as the leader.
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</para>
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<para>
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If this callback is not defined, it is assumed that the scan must take
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place within the parallel leader. Note that returning true does not mean
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that the scan itself can be done in parallel, only that the scan can be
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performed within a parallel worker. Therefore, it can be useful to define
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this method even when parallel execution is not supported.
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</para>
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<para>
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<programlisting>
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Size
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EstimateDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt);
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</programlisting>
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Estimate the amount of dynamic shared memory that will be required

‎src/backend/optimizer/path/allpaths.c‎

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Original file line numberDiff line numberDiff line change
@@ -527,6 +527,23 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
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return;
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return;
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}
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/*
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* Ask FDWs whether they can support performing a ForeignScan
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* within a worker. Most often, the answer will be no. For
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* example, if the nature of the FDW is such that it opens a TCP
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* connection with a remote server, each parallel worker would end
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* up with a separate connection, and these connections might not
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* be appropriately coordinated between workers and the leader.
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*/
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if (rte->relkind==RELKIND_FOREIGN_TABLE)
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{
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Assert(rel->fdwroutine);
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if (!rel->fdwroutine->IsForeignScanParallelSafe)
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return;
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if (!rel->fdwroutine->IsForeignScanParallelSafe(root,rel,rte))
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return;
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}
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break;
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caseRTE_SUBQUERY:

‎src/include/foreign/fdwapi.h‎

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Original file line numberDiff line numberDiff line change
@@ -131,6 +131,10 @@ typedef void (*InitializeDSMForeignScan_function) (ForeignScanState *node,
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typedefvoid (*InitializeWorkerForeignScan_function) (ForeignScanState*node,
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shm_toc*toc,
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void*coordinate);
134+
typedefbool (*IsForeignScanParallelSafe_function) (PlannerInfo*root,
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RelOptInfo*rel,
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RangeTblEntry*rte);
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134138
/*
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* FdwRoutine is the struct returned by a foreign-data wrapper's handler
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* function. It provides pointers to the callback functions needed by the
@@ -188,6 +192,7 @@ typedef struct FdwRoutine
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ImportForeignSchema_functionImportForeignSchema;
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/* Support functions for parallelism under Gather node */
195+
IsForeignScanParallelSafe_functionIsForeignScanParallelSafe;
191196
EstimateDSMForeignScan_functionEstimateDSMForeignScan;
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InitializeDSMForeignScan_functionInitializeDSMForeignScan;
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InitializeWorkerForeignScan_functionInitializeWorkerForeignScan;

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