From: David Rowley <drowley@postgresql.org>
Date: Thu, 15 Feb 2024 00:13:31 +0000 (+1300)
Subject: Clarify the 'rows' parameter in create_append_path
X-Git-Tag: REL_17_BETA1~881
X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=87027cb55bf7b33416880f7f5f5e67f4202d5881;p=postgresql.git

Clarify the 'rows' parameter in create_append_path

This is extracted from a larger patch to improve the UNION planner.
While working on that, I found myself having to check what the 'rows'
parameter is for.  It's not obvious that passing a negative number is the
way to have the rows estimate calculated and to find that out you need
to read code in create_append_path() and in cost_append().

Discussion: https://postgr.es/m/CAApHDvpb_63XQodmxKUF8vb9M7CxyUyT4sWvEgqeQU-GB7QFoQ@mail.gmail.com
---

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 2e1ec41a541..b3902b00329 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1237,6 +1237,10 @@ create_tidrangescan_path(PlannerInfo *root, RelOptInfo *rel,
  *
  * Note that we must handle subpaths = NIL, representing a dummy access path.
  * Also, there are callers that pass root = NULL.
+ *
+ * 'rows', when passed as a non-negative number, will be used to overwrite the
+ * returned path's row estimate.  Otherwise, the row estimate is calculated
+ * by totalling the row estimates from the 'subpaths' list.
  */
 AppendPath *
 create_append_path(PlannerInfo *root,