Make better use of the new List implementation in a couple of places

In nodeAppend.c and nodeMergeAppend.c there were some foreach loops which
looped over the list of subplans and only performed any work if the
subplan index was found in a Bitmapset.  With the old linked list
implementation of List, this form made sense as accessing the Nth list
element was O(N).  However, thanks to 1cff1b95a we now have array-based
lists, so accessing the Nth element has become O(1).

Here we make the most of the O(1) lookups and just loop over the set
members of the Bitmapset with bms_next_member().  This performs slightly
better when a small number of the list items are in the Bitmapset.  Micro
benchmarks show that when the Bitmapset contains all or most of the list
items then the new code is ever so slightly slower.  In practice, the cost
is so small that it's drowned out by various other things such as locking
the relations belonging to each subplan, etc.

The primary goal here is to leave better code examples around which benefit
better from the new list implementation.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAKJS1f8ZcsLVgkF4wOfRyMYTcPgLFiUAOedFC+U2vK_aFZk-BA@mail.gmail.com

diff --git a/src/backend/executor/nodeAppend.c b/src/backend/executor/nodeAppend.c
index f3be2429dbeb122d186e941d26b2094982441bdc..5ff986ac7d352803ba7671927e62b90f9b1a5458 100644 (file)
--- a/src/backend/executor/nodeAppend.c
+++ b/src/backend/executor/nodeAppend.c
@@ -107,7 +107,6 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
        int                     firstvalid;
        int                     i,
                                j;
-       ListCell   *lc;
 
        /* check for unsupported flags */
        Assert(!(eflags & EXEC_FLAG_MARK));
@@ -211,24 +210,20 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
         *
         * While at it, find out the first valid partial plan.
         */
-       j = i = 0;
+       j = 0;
        firstvalid = nplans;
-       foreach(lc, node->appendplans)
+       i = -1;
+       while ((i = bms_next_member(validsubplans, i)) >= 0)
        {
-               if (bms_is_member(i, validsubplans))
-               {
-                       Plan       *initNode = (Plan *) lfirst(lc);
+               Plan       *initNode = (Plan *) list_nth(node->appendplans, i);
 
-                       /*
-                        * Record the lowest appendplans index which is a valid partial
-                        * plan.
-                        */
-                       if (i >= node->first_partial_plan && j < firstvalid)
-                               firstvalid = j;
+               /*
+                * Record the lowest appendplans index which is a valid partial plan.
+                */
+               if (i >= node->first_partial_plan && j < firstvalid)
+                       firstvalid = j;
 
-                       appendplanstates[j++] = ExecInitNode(initNode, estate, eflags);
-               }
-               i++;
+               appendplanstates[j++] = ExecInitNode(initNode, estate, eflags);
        }
 
        appendstate->as_first_partial_plan = firstvalid;

diff --git a/src/backend/executor/nodeMergeAppend.c b/src/backend/executor/nodeMergeAppend.c
index 7ba53ba185966c01b2fde0e9a880c6e3770ed8f0..18d13377dc3d13e7d9cceb66c64ce94c9fbdfd34 100644 (file)
--- a/src/backend/executor/nodeMergeAppend.c
+++ b/src/backend/executor/nodeMergeAppend.c
@@ -70,7 +70,6 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
        int                     nplans;
        int                     i,
                                j;
-       ListCell   *lc;
 
        /* check for unsupported flags */
        Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -177,16 +176,13 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
         * call ExecInitNode on each of the valid plans to be executed and save
         * the results into the mergeplanstates array.
         */
-       j = i = 0;
-       foreach(lc, node->mergeplans)
+       j = 0;
+       i = -1;
+       while ((i = bms_next_member(validsubplans, i)) >= 0)
        {
-               if (bms_is_member(i, validsubplans))
-               {
-                       Plan       *initNode = (Plan *) lfirst(lc);
+               Plan       *initNode = (Plan *) list_nth(node->mergeplans, i);
 
-                       mergeplanstates[j++] = ExecInitNode(initNode, estate, eflags);
-               }
-               i++;
+               mergeplanstates[j++] = ExecInitNode(initNode, estate, eflags);
        }
 
        mergestate->ps.ps_ProjInfo = NULL;