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>

src/include/utils/plancache.h

@@ -93,8 +93,10 @@ typedef struct CachedPlanSource
 	List	   *invalItems;		/* other dependencies, as PlanInvalItems */
 	struct OverrideSearchPath *search_path;		/* search_path used for
 												 * parsing and planning */
-	Oid			planUserId;		/* User-id that the plan depends on */
 	MemoryContext query_context;	/* context holding the above, or NULL */
+	Oid			rewriteRoleId;	/* Role ID we did rewriting for */
+	bool		rewriteRowSecurity;		/* row_security used during rewrite */
+	bool		dependsOnRLS;	/* is rewritten query specific to the above? */
 	/* If we have a generic plan, this is a reference-counted link to it: */
 	struct CachedPlan *gplan;	/* generic plan, or NULL if not valid */
 	/* Some state flags: */
@@ -109,8 +111,6 @@ typedef struct CachedPlanSource
 	double		generic_cost;	/* cost of generic plan, or -1 if not known */
 	double		total_custom_cost;		/* total cost of custom plans so far */
 	int			num_custom_plans;		/* number of plans included in total */
-	bool		hasRowSecurity; /* planned with row security? */
-	bool		row_security_env;		/* row security setting when planned */
 } CachedPlanSource;
 
 /*
@@ -131,11 +131,12 @@ typedef struct CachedPlan
 	bool		is_oneshot;		/* is it a "oneshot" plan? */
 	bool		is_saved;		/* is CachedPlan in a long-lived context? */
 	bool		is_valid;		/* is the stmt_list currently valid? */
+	Oid			planRoleId;		/* Role ID the plan was created for */
+	bool		dependsOnRole;	/* is plan specific to that role? */
 	TransactionId saved_xmin;	/* if valid, replan when TransactionXmin
 								 * changes from this value */
 	int			generation;		/* parent's generation number for this plan */
 	int			refcount;		/* count of live references to this struct */
-	bool		has_foreign_join;		/* plan has pushed down a foreign join */
 	MemoryContext context;		/* context containing this CachedPlan */
 } CachedPlan;