Minor performance improvement for SQL-language functions.

Late in the development of commit 0dca5d6, I added a step to copy
the result tlist we extract from the cached final query, because
I was afraid that that might not last as long as the JunkFilter that
we're passing it off to.  However, that turns out to cost a noticeable
number of cycles, and it's really quite unnecessary because the
JunkFilter will not examine that tlist after it's been created.
(ExecFindJunkAttribute would use it, but we don't use that function
on this JunkFilter.)  Hence, remove the copy step.  For safety,
reset the might-become-dangling jf_targetList pointer to NIL.

In passing, remove DR_sqlfunction.cxt, which we don't use anymore;
it's confusing because it's not entirely clear which context it
ought to point at.

Author: tglsfdc

src/backend/executor/functions.c

@@ -45,7 +45,6 @@ typedef struct
 {
 	DestReceiver pub;			/* publicly-known function pointers */
 	Tuplestorestate *tstore;	/* where to put result tuples */
-	MemoryContext cxt;			/* context containing tstore */
 	JunkFilter *filter;			/* filter to convert tuple type */
 } DR_sqlfunction;
 
@@ -787,12 +786,6 @@ init_execution_state(SQLFunctionCachePtr fcache)
 		 */
 		resulttlist = get_sql_fn_result_tlist(plansource->query_list);
 
-		/*
-		 * We need to make a copy to ensure that it doesn't disappear
-		 * underneath us due to plancache invalidation.
-		 */
-		resulttlist = copyObject(resulttlist);
-
 		/*
 		 * If the result is composite, *and* we are returning the whole tuple
 		 * result, we need to insert nulls for any dropped columns.  In the
@@ -807,6 +800,17 @@ init_execution_state(SQLFunctionCachePtr fcache)
 															  slot);
 		else
 			fcache->junkFilter = ExecInitJunkFilter(resulttlist, slot);
+
+		/*
+		 * The resulttlist tree belongs to the plancache and might disappear
+		 * underneath us due to plancache invalidation.  While we could
+		 * forestall that by copying it, that'd just be a waste of cycles,
+		 * because the junkfilter doesn't need it anymore.  (It'd only be used
+		 * by ExecFindJunkAttribute(), which we don't use here.)  To ensure
+		 * there's not a dangling pointer laying about, clear the junkFilter's
+		 * pointer.
+		 */
+		fcache->junkFilter->jf_targetList = NIL;
 	}
 
 	if (fcache->func->returnsTuple)
@@ -1245,7 +1249,6 @@ postquel_start(execution_state *es, SQLFunctionCachePtr fcache)
 		myState = (DR_sqlfunction *) dest;
 		Assert(myState->pub.mydest == DestSQLFunction);
 		myState->tstore = fcache->tstore;
-		myState->cxt = CurrentMemoryContext;
 		myState->filter = fcache->junkFilter;
 	}
 	else