Fix unnecessary use of moving-aggregate mode with non-moving frame.

When a plain aggregate is used as a window function, and the window
frame start is specified as UNBOUNDED PRECEDING, the frame's head
cannot move so we do not need to use moving-aggregate mode.  The check
for that was put into initialize_peragg(), failing to notice that
ExecInitWindowAgg() calls that function before it's filled in
winstate->frameOptions.  Since makeNode() would have zeroed the field,
this didn't provoke uninitialized-value complaints, nor would the
erroneous decision have resulted in more than a little inefficiency.
Still, it's wrong, so move the initialization of
winstate->frameOptions earlier to make it work properly.

While here, also fix a thinko in a comment.  Both errors crept in in
commit a9d9acb which introduced the moving-aggregate mode.

Spotted by Vallimaharajan G.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/18e7f2a5167.fe36253866818.977923893562469143@zohocorp.com

Author: tglsfdc

src/backend/executor/nodeWindowAgg.c

@@ -2399,6 +2399,9 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
 	winstate->ss.ps.state = estate;
 	winstate->ss.ps.ExecProcNode = ExecWindowAgg;
 
+	/* copy frame options to state node for easy access */
+	winstate->frameOptions = frameOptions;
+
 	/*
 	 * Create expression contexts.  We need two, one for per-input-tuple
 	 * processing and one for per-output-tuple processing.  We cheat a little
@@ -2649,9 +2652,6 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
 	/* Set the status to running */
 	winstate->status = WINDOWAGG_RUN;
 
-	/* copy frame options to state node for easy access */
-	winstate->frameOptions = frameOptions;
-
 	/* initialize frame bound offset expressions */
 	winstate->startOffset = ExecInitExpr((Expr *) node->startOffset,
 										 (PlanState *) winstate);
@@ -2785,7 +2785,7 @@ initialize_peragg(WindowAggState *winstate, WindowFunc *wfunc,
 
 	/*
 	 * Figure out whether we want to use the moving-aggregate implementation,
-	 * and collect the right set of fields from the pg_attribute entry.
+	 * and collect the right set of fields from the pg_aggregate entry.
 	 *
 	 * It's possible that an aggregate would supply a safe moving-aggregate
 	 * implementation and an unsafe normal one, in which case our hand is