Avoid invalidating all foreign-join cached plans when user mappings change.

We must not push down a foreign join when the foreign tables involved
should be accessed under different user mappings.  Previously we tried
to enforce that rule literally during planning, but that meant that the
resulting plans were dependent on the current contents of the
pg_user_mapping catalog, and we had to blow away all cached plans
containing any remote join when anything at all changed in pg_user_mapping.
This could have been improved somewhat, but the fact that a syscache inval
callback has very limited info about what changed made it hard to do better
within that design.  Instead, let's change the planner to not consider user
mappings per se, but to allow a foreign join if both RTEs have the same
checkAsUser value.  If they do, then they necessarily will use the same
user mapping at runtime, and we don't need to know specifically which one
that is.  Post-plan-time changes in pg_user_mapping no longer require any
plan invalidation.

This rule does give up some optimization ability, to wit where two foreign
table references come from views with different owners or one's from a view
and one's directly in the query, but nonetheless the same user mapping
would have applied.  We'll sacrifice the first case, but to not regress
more than we have to in the second case, allow a foreign join involving
both zero and nonzero checkAsUser values if the nonzero one is the same as
the prevailing effective userID.  In that case, mark the plan as only
runnable by that userID.

The plancache code already had a notion of plans being userID-specific,
in order to support RLS.  It was a little confused though, in particular
lacking clarity of thought as to whether it was the rewritten query or just
the finished plan that's dependent on the userID.  Rearrange that code so
that it's clearer what depends on which, and so that the same logic applies
to both RLS-injected role dependency and foreign-join-injected role
dependency.

Note that this patch doesn't remove the other issue mentioned in the
original complaint, which is that while we'll reliably stop using a foreign
join if it's disallowed in a new context, we might fail to start using a
foreign join if it's now allowed, but we previously created a generic
cached plan that didn't use one.  It was agreed that the chance of winning
that way was not high enough to justify the much larger number of plan
invalidations that would have to occur if we tried to cause it to happen.

In passing, clean up randomly-varying spelling of EXPLAIN commands in
postgres_fdw.sql, and fix a COSTS ON example that had been allowed to
leak into the committed tests.

This reverts most of commits fbe5a3fb7 and 5d4171d1c, which were the
previous attempt at ensuring we wouldn't push down foreign joins that
span permissions contexts.

Etsuro Fujita and Tom Lane

Discussion: <d49c1e5b-f059-20f4-c132-e9752ee0113e@lab.ntt.co.jp>

1 files changed, 39 insertions, 43 deletions

diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index a0a284b901b..806600ed107 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -15,8 +15,6 @@
 #include "postgres.h"
 
 #include "miscadmin.h"
-#include "catalog/pg_class.h"
-#include "foreign/foreign.h"
 #include "optimizer/clauses.h"
 #include "optimizer/cost.h"
 #include "optimizer/pathnode.h"
@@ -107,7 +105,6 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	rel->consider_startup = (root->tuple_fraction > 0);
 	rel->consider_param_startup = false;		/* might get changed later */
 	rel->consider_parallel = false;		/* might get changed later */
-	rel->rel_parallel_workers = -1;		/* set up in GetRelationInfo */
 	rel->reltarget = create_empty_pathtarget();
 	rel->pathlist = NIL;
 	rel->ppilist = NIL;
@@ -129,8 +126,10 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	rel->allvisfrac = 0;
 	rel->subroot = NULL;
 	rel->subplan_params = NIL;
+	rel->rel_parallel_workers = -1;		/* set up in GetRelationInfo */
 	rel->serverid = InvalidOid;
-	rel->umid = InvalidOid;
+	rel->userid = rte->checkAsUser;
+	rel->useridiscurrent = false;
 	rel->fdwroutine = NULL;
 	rel->fdw_private = NULL;
 	rel->baserestrictinfo = NIL;
@@ -170,30 +169,6 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 			break;
 	}
 
-	/* For foreign tables get the user mapping */
-	if (rte->relkind == RELKIND_FOREIGN_TABLE)
-	{
-		/*
-		 * This should match what ExecCheckRTEPerms() does.
-		 *
-		 * Note that if the plan ends up depending on the user OID in any way
-		 * - e.g. if it depends on the computed user mapping OID - we must
-		 * ensure that it gets invalidated in the case of a user OID change.
-		 * See RevalidateCachedQuery and more generally the hasForeignJoin
-		 * flags in PlannerGlobal and PlannedStmt.
-		 *
-		 * It's possible, and not necessarily an error, for rel->umid to be
-		 * InvalidOid even though rel->serverid is set.  That just means there
-		 * is a server with no user mapping.
-		 */
-		Oid			userid;
-
-		userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
-		rel->umid = GetUserMappingId(userid, rel->serverid, true);
-	}
-	else
-		rel->umid = InvalidOid;
-
 	/* Save the finished struct in the query's simple_rel_array */
 	root->simple_rel_array[relid] = rel;
 
@@ -423,8 +398,10 @@ build_join_rel(PlannerInfo *root,
 	joinrel->allvisfrac = 0;
 	joinrel->subroot = NULL;
 	joinrel->subplan_params = NIL;
+	joinrel->rel_parallel_workers = -1;
 	joinrel->serverid = InvalidOid;
-	joinrel->umid = InvalidOid;
+	joinrel->userid = InvalidOid;
+	joinrel->useridiscurrent = false;
 	joinrel->fdwroutine = NULL;
 	joinrel->fdw_private = NULL;
 	joinrel->baserestrictinfo = NIL;
@@ -435,24 +412,43 @@ build_join_rel(PlannerInfo *root,
 
 	/*
 	 * Set up foreign-join fields if outer and inner relation are foreign
-	 * tables (or joins) belonging to the same server and using the same user
-	 * mapping.
-	 *
-	 * Otherwise those fields are left invalid, so FDW API will not be called
-	 * for the join relation.
+	 * tables (or joins) belonging to the same server and assigned to the same
+	 * user to check access permissions as.  In addition to an exact match of
+	 * userid, we allow the case where one side has zero userid (implying
+	 * current user) and the other side has explicit userid that happens to
+	 * equal the current user; but in that case, pushdown of the join is only
+	 * valid for the current user.  The useridiscurrent field records whether
+	 * we had to make such an assumption for this join or any sub-join.
 	 *
-	 * For FDWs like file_fdw, which ignore user mapping, the user mapping id
-	 * associated with the joining relation may be invalid. A valid serverid
-	 * distinguishes between a pushed down join with no user mapping and a
-	 * join which can not be pushed down because of user mapping mismatch.
+	 * Otherwise these fields are left invalid, so GetForeignJoinPaths will
+	 * not be called for the join relation.
 	 */
 	if (OidIsValid(outer_rel->serverid) &&
-		inner_rel->serverid == outer_rel->serverid &&
-		inner_rel->umid == outer_rel->umid)
+		inner_rel->serverid == outer_rel->serverid)
 	{
-		joinrel->serverid = outer_rel->serverid;
-		joinrel->umid = outer_rel->umid;
-		joinrel->fdwroutine = outer_rel->fdwroutine;
+		if (inner_rel->userid == outer_rel->userid)
+		{
+			joinrel->serverid = outer_rel->serverid;
+			joinrel->userid = outer_rel->userid;
+			joinrel->useridiscurrent = outer_rel->useridiscurrent || inner_rel->useridiscurrent;
+			joinrel->fdwroutine = outer_rel->fdwroutine;
+		}
+		else if (!OidIsValid(inner_rel->userid) &&
+				 outer_rel->userid == GetUserId())
+		{
+			joinrel->serverid = outer_rel->serverid;
+			joinrel->userid = outer_rel->userid;
+			joinrel->useridiscurrent = true;
+			joinrel->fdwroutine = outer_rel->fdwroutine;
+		}
+		else if (!OidIsValid(outer_rel->userid) &&
+				 inner_rel->userid == GetUserId())
+		{
+			joinrel->serverid = outer_rel->serverid;
+			joinrel->userid = inner_rel->userid;
+			joinrel->useridiscurrent = true;
+			joinrel->fdwroutine = outer_rel->fdwroutine;
+		}
 	}
 
 	/*