From 0614f8c718a23da7d24a6745d9f2cdc611999e81 Mon Sep 17 00:00:00 2001
From: "heikki@hundin.mysql.fi" <>
Date: Thu, 15 Jul 2004 09:08:51 +0300
Subject: btr0cur.c:   Do not add + 1 to the InnoDB index cardinality estimate
 if the B-tree just contains one page; the fix made in March 2004 caused
 InnoDB systematically to overestimate the cardinality of empty or small
 tables by 1

---
 innobase/btr/btr0cur.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

(limited to 'innobase')

diff --git a/innobase/btr/btr0cur.c b/innobase/btr/btr0cur.c
index 8974200efe8..b07b06765e6 100644
--- a/innobase/btr/btr0cur.c
+++ b/innobase/btr/btr0cur.c
@@ -2707,8 +2707,11 @@ btr_estimate_number_of_different_key_vals(
 			rec = page_rec_get_next(rec);
 		}
 		
+
 		if (n_cols == dict_index_get_n_unique_in_tree(index)) {
-			/* We add one because we know that the first record
+
+			/* If there is more than one leaf page in the tree,
+			we add one because we know that the first record
 			on the page certainly had a different prefix than the
 			last record on the previous index page in the
 			alphabetical order. Before this fix, if there was
@@ -2716,7 +2719,11 @@ btr_estimate_number_of_different_key_vals(
 			algorithm grossly underestimated the number of rows
 			in the table. */
 
-			n_diff[n_cols]++;
+			if (btr_page_get_prev(page, &mtr) != FIL_NULL
+			    || btr_page_get_next(page, &mtr) != FIL_NULL) {
+
+				n_diff[n_cols]++;
+			}
 		}
 
 		total_external_size +=
-- 
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