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 a9d9acbf2 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

diff --git a/src/backend/executor/nodeWindowAgg.c b/src/backend/executor/nodeWindowAgg.c
index d7c365d31217394c692f5f1d01a95ddb34fdb9e3..418d6d9c426e1406044ef6c8ecf0aff65cd32dad 100644 (file)
--- a/src/backend/executor/nodeWindowAgg.c
+++ b/src/backend/executor/nodeWindowAgg.c
@@ -2397,6 +2397,9 @@ 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
@@ -2647,9 +2650,6 @@ 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);
@@ -2800,7 +2800,7 @@ 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