From: Andres Freund <andres@anarazel.de>
Date: Tue, 11 Dec 2018 00:45:59 +0000 (-0800)
Subject: zheap: Properly adjust for metapage in estimate_rel_size().
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=f2515e45f27e26d4693dfaa8c69f3bda62d8300d;p=users%2Fandresfreund%2Fpostgres.git

zheap: Properly adjust for metapage in estimate_rel_size().

Previously *pages wasn't adapted for the metapage, leading to
superflous plan changes. Also, avoid multiple checks.

Author:
Reviewed-By:
Discussion: https://postgr.es/m/
Backpatch:
---

diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 6b33814f39..846866ce37 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -969,6 +969,23 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 			/* it has storage, ok to call the smgr */
 			curpages = RelationGetNumberOfBlocks(rel);
 
+			/* coerce values in pg_class to more desirable types */
+			relpages = (BlockNumber) rel->rd_rel->relpages;
+			reltuples = (double) rel->rd_rel->reltuples;
+			relallvisible = (BlockNumber) rel->rd_rel->relallvisible;
+
+			/*
+			 * If it's a zheap relation, then subtract one page to account for
+			 * the metapage.
+			 */
+			if (RelationStorageIsZHeap(rel))
+			{
+				if (curpages > 0)
+					curpages--;
+				if (relpages > 0)
+					relpages--;
+			}
+
 			/*
 			 * HACK: if the relation has never yet been vacuumed, use a
 			 * minimum size estimate of 10 pages.  The idea here is to avoid
@@ -1000,9 +1017,7 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 			 * minimum to indexes.
 			 */
 			if (curpages < 10 &&
-				(rel->rd_rel->relpages == 0 ||
-				(RelationStorageIsZHeap(rel) &&
-				 rel->rd_rel->relpages == ZHEAP_METAPAGE + 1)) &&
+				relpages == 0 &&
 				!rel->rd_rel->relhassubclass &&
 				rel->rd_rel->relkind != RELKIND_INDEX)
 				curpages = 10;
@@ -1010,27 +1025,13 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 			/* report estimated # pages */
 			*pages = curpages;
 			/* quick exit if rel is clearly empty */
-			if (curpages == 0 || (RelationStorageIsZHeap(rel) &&
-				curpages == ZHEAP_METAPAGE + 1))
+			if (curpages == 0)
 			{
 				*tuples = 0;
 				*allvisfrac = 0;
 				break;
 			}
-			/* coerce values in pg_class to more desirable types */
-			relpages = (BlockNumber) rel->rd_rel->relpages;
-			reltuples = (double) rel->rd_rel->reltuples;
-			relallvisible = (BlockNumber) rel->rd_rel->relallvisible;
 
-			/*
-			 * If it's a zheap relation, then subtract the pages
-			 * to account for the metapage.
-			 */
-			if (relpages > 0 && RelationStorageIsZHeap(rel))
-			{
-				curpages--;
-				relpages--;
-			}
 			/*
 			 * If it's an index, discount the metapage while estimating the
 			 * number of tuples.  This is a kluge because it assumes more than