Use list_copy_head() instead of list_truncate(list_copy(...), ...)

Truncating off the end of a freshly copied List is not a very efficient
way of copying the first N elements of a List.

In many of the cases that are updated here, the pattern was only being
used to remove the final element of a List.  That's about the best case
for it, but there were many instances where the truncate trimming the List
down much further.

4cc832f94 added list_copy_head(), so let's use it in cases where it's
useful.

Author: David Rowley
Discussion: https://postgr.es/m/1986787.1657666922%40sss.pgh.pa.us

7 files changed, 25 insertions, 30 deletions

diff --git a/src/backend/catalog/objectaddress.c b/src/backend/catalog/objectaddress.c
index 2d21db46901..6080ff8f5f7 100644
--- a/src/backend/catalog/objectaddress.c
+++ b/src/backend/catalog/objectaddress.c
@@ -1453,7 +1453,7 @@ get_object_address_relobject(ObjectType objtype, List *object,
 				 errmsg("must specify relation and object name")));
 
 	/* Extract relation name and open relation. */
-	relname = list_truncate(list_copy(object), nnames - 1);
+	relname = list_copy_head(object, nnames - 1);
 	relation = table_openrv_extended(makeRangeVarFromNameList(relname),
 									 AccessShareLock,
 									 missing_ok);
@@ -1528,7 +1528,7 @@ get_object_address_attribute(ObjectType objtype, List *object,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("column name must be qualified")));
 	attname = strVal(llast(object));
-	relname = list_truncate(list_copy(object), list_length(object) - 1);
+	relname = list_copy_head(object, list_length(object) - 1);
 	/* XXX no missing_ok support here */
 	relation = relation_openrv(makeRangeVarFromNameList(relname), lockmode);
 	reloid = RelationGetRelid(relation);
@@ -1581,7 +1581,7 @@ get_object_address_attrdef(ObjectType objtype, List *object,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("column name must be qualified")));
 	attname = strVal(llast(object));
-	relname = list_truncate(list_copy(object), list_length(object) - 1);
+	relname = list_copy_head(object, list_length(object) - 1);
 	/* XXX no missing_ok support here */
 	relation = relation_openrv(makeRangeVarFromNameList(relname), lockmode);
 	reloid = RelationGetRelid(relation);
@@ -1715,7 +1715,7 @@ get_object_address_opf_member(ObjectType objtype,
 	 * address.  The rest can be used directly by get_object_address_opcf().
 	 */
 	membernum = atoi(strVal(llast(linitial(object))));
-	copy = list_truncate(list_copy(linitial(object)), list_length(linitial(object)) - 1);
+	copy = list_copy_head(linitial(object), list_length(linitial(object)) - 1);
 
 	/* no missing_ok support here */
 	famaddr = get_object_address_opcf(OBJECT_OPFAMILY, copy, false);
diff --git a/src/backend/commands/dropcmds.c b/src/backend/commands/dropcmds.c
index c9b5732448e..26157eb4e3f 100644
--- a/src/backend/commands/dropcmds.c
+++ b/src/backend/commands/dropcmds.c
@@ -145,8 +145,7 @@ owningrel_does_not_exist_skipping(List *object, const char **msg, char **name)
 	List	   *parent_object;
 	RangeVar   *parent_rel;
 
-	parent_object = list_truncate(list_copy(object),
-								  list_length(object) - 1);
+	parent_object = list_copy_head(object, list_length(object) - 1);
 
 	if (schema_does_not_exist_skipping(parent_object, msg, name))
 		return true;
@@ -419,8 +418,8 @@ does_not_exist_skipping(ObjectType objtype, Node *object)
 			{
 				msg = gettext_noop("trigger \"%s\" for relation \"%s\" does not exist, skipping");
 				name = strVal(llast(castNode(List, object)));
-				args = NameListToString(list_truncate(list_copy(castNode(List, object)),
-													  list_length(castNode(List, object)) - 1));
+				args = NameListToString(list_copy_head(castNode(List, object),
+													   list_length(castNode(List, object)) - 1));
 			}
 			break;
 		case OBJECT_POLICY:
@@ -428,8 +427,8 @@ does_not_exist_skipping(ObjectType objtype, Node *object)
 			{
 				msg = gettext_noop("policy \"%s\" for relation \"%s\" does not exist, skipping");
 				name = strVal(llast(castNode(List, object)));
-				args = NameListToString(list_truncate(list_copy(castNode(List, object)),
-													  list_length(castNode(List, object)) - 1));
+				args = NameListToString(list_copy_head(castNode(List, object),
+													   list_length(castNode(List, object)) - 1));
 			}
 			break;
 		case OBJECT_EVENT_TRIGGER:
@@ -441,8 +440,8 @@ does_not_exist_skipping(ObjectType objtype, Node *object)
 			{
 				msg = gettext_noop("rule \"%s\" for relation \"%s\" does not exist, skipping");
 				name = strVal(llast(castNode(List, object)));
-				args = NameListToString(list_truncate(list_copy(castNode(List, object)),
-													  list_length(castNode(List, object)) - 1));
+				args = NameListToString(list_copy_head(castNode(List, object),
+													   list_length(castNode(List, object)) - 1));
 			}
 			break;
 		case OBJECT_FDW:
diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index 48d9d43dac6..b0b211891c3 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -1620,7 +1620,7 @@ process_owned_by(Relation seqrel, List *owned_by, bool for_identity)
 		RangeVar   *rel;
 
 		/* Separate relname and attr name */
-		relname = list_truncate(list_copy(owned_by), nnames - 1);
+		relname = list_copy_head(owned_by, nnames - 1);
 		attrname = strVal(llast(owned_by));
 
 		/* Open and lock rel to ensure it won't go away meanwhile */
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index e9342097e52..358bb2aed6f 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2053,9 +2053,8 @@ accumulate_append_subpath(Path *path, List **subpaths, List **special_subpaths)
 			*subpaths = list_concat(*subpaths,
 									list_copy_tail(apath->subpaths,
 												   apath->first_partial_path));
-			new_special_subpaths =
-				list_truncate(list_copy(apath->subpaths),
-							  apath->first_partial_path);
+			new_special_subpaths = list_copy_head(apath->subpaths,
+												  apath->first_partial_path);
 			*special_subpaths = list_concat(*special_subpaths,
 											new_special_subpaths);
 			return;
@@ -3086,8 +3085,8 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel,
 										   root->query_pathkeys);
 		else if (npathkeys > 0)
 			useful_pathkeys_list = lappend(useful_pathkeys_list,
-										   list_truncate(list_copy(root->query_pathkeys),
-														 npathkeys));
+										   list_copy_head(root->query_pathkeys,
+														  npathkeys));
 	}
 
 	return useful_pathkeys_list;
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 0ef70ad7f11..7d176e7b00a 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -3026,14 +3026,12 @@ expand_indexqual_rowcompare(PlannerInfo *root,
 
 			rc->rctype = (RowCompareType) op_strategy;
 			rc->opnos = new_ops;
-			rc->opfamilies = list_truncate(list_copy(clause->opfamilies),
-										   matching_cols);
-			rc->inputcollids = list_truncate(list_copy(clause->inputcollids),
-											 matching_cols);
-			rc->largs = list_truncate(copyObject(var_args),
-									  matching_cols);
-			rc->rargs = list_truncate(copyObject(non_var_args),
-									  matching_cols);
+			rc->opfamilies = list_copy_head(clause->opfamilies,
+											matching_cols);
+			rc->inputcollids = list_copy_head(clause->inputcollids,
+											  matching_cols);
+			rc->largs = list_copy_head(var_args, matching_cols);
+			rc->rargs = list_copy_head(non_var_args, matching_cols);
 			iclause->indexquals = list_make1(make_simple_restrictinfo(root,
 																	  (Expr *) rc));
 		}
diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c
index 985b5d8de9b..b5d6c977eef 100644
--- a/src/backend/optimizer/path/pathkeys.c
+++ b/src/backend/optimizer/path/pathkeys.c
@@ -2464,7 +2464,7 @@ truncate_useless_pathkeys(PlannerInfo *root,
 	else if (nuseful == list_length(pathkeys))
 		return pathkeys;
 	else
-		return list_truncate(list_copy(pathkeys), nuseful);
+		return list_copy_head(pathkeys, nuseful);
 }
 
 /*
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 76606faa3e4..e37f2933eb8 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -1397,8 +1397,7 @@ create_append_plan(PlannerInfo *root, AppendPath *best_path, int flags)
 	 */
 	if (tlist_was_changed && (flags & (CP_EXACT_TLIST | CP_SMALL_TLIST)))
 	{
-		tlist = list_truncate(list_copy(plan->plan.targetlist),
-							  orig_tlist_length);
+		tlist = list_copy_head(plan->plan.targetlist, orig_tlist_length);
 		return inject_projection_plan((Plan *) plan, tlist,
 									  plan->plan.parallel_safe);
 	}
@@ -1557,7 +1556,7 @@ create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path,
 	 */
 	if (tlist_was_changed && (flags & (CP_EXACT_TLIST | CP_SMALL_TLIST)))
 	{
-		tlist = list_truncate(list_copy(plan->targetlist), orig_tlist_length);
+		tlist = list_copy_head(plan->targetlist, orig_tlist_length);
 		return inject_projection_plan(plan, tlist, plan->parallel_safe);
 	}
 	else