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 6b33814f39ca6954a48abebeeeaa727645acb3f3..846866ce3743ce437e47258e0ccf63552b02914d 100644 (file)
--- 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