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Add new FDW API to test for parallel-safety.

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This is basically a bug fix; the old code assumes that a ForeignScan
is always parallel-safe, but for postgres_fdw, for example, this is
definitely false.  It should be true for file_fdw, though, since a
worker can read a file from the filesystem just as well as any other
backend process.

Original patch by Thomas Munro.  Documentation, and changes to the
comments, by me.
This commit is contained in:
Robert Haas
2016-02-26 16:14:46 +05:30
parent 9117985b6b
commit 35746bc348
4 changed files with 60 additions and 0 deletions
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17
src/backend/optimizer/path/allpaths.c
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@ -527,6 +527,23 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
return;
return;
}
/*
* Ask FDWs whether they can support performing a ForeignScan
* within a worker. Most often, the answer will be no. For
* example, if the nature of the FDW is such that it opens a TCP
* connection with a remote server, each parallel worker would end
* up with a separate connection, and these connections might not
* be appropriately coordinated between workers and the leader.
*/
if (rte->relkind == RELKIND_FOREIGN_TABLE)
{
Assert(rel->fdwroutine);
if (!rel->fdwroutine->IsForeignScanParallelSafe)
return;
if (!rel->fdwroutine->IsForeignScanParallelSafe(root, rel, rte))
return;
}
break;
case RTE_SUBQUERY: