postgrespro.git / summary / log / commit / refs

commit    efdcca55a3df27a12efb84a18bce6ea739927b80
Author:   David Rowley <drowley@postgresql.org>
Date:     Mon Jul 22 07:03:12 2019 +0000

    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


 src/backend/executor/nodeAppend.c      | 25 ++++++++++---------------
 src/backend/executor/nodeMergeAppend.c | 14 +++++---------
 2 files changed, 15 insertions(+), 24 deletions(-)

diff --git a/src/backend/executor/nodeAppend.c b/src/backend/executor/nodeAppend.c
index f3be2429db..5ff986ac7d 100644
--- 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 7ba53ba185..18d13377dc 100644
--- 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;


[parent: 23bccc823d43]