branches/innodb+:

Fix Bug#25640:

Introduce an user visible parameter innodb_stats_sample (default 8,
min 1, max 1000) and use that parameter instead of the
BTR_KEY_VAL_ESTIMATE_N_PAGES macro. Remove this macro.

Approved by:	Heikki

btr/btr0cur.c

@@ -55,10 +55,6 @@ can be released by page reorganize, then it is reorganized */
 
 #define BTR_CUR_PAGE_REORGANIZE_LIMIT	(UNIV_PAGE_SIZE / 32)
 
-/* When estimating number of different key values in an index, sample
-this many index pages */
-#define BTR_KEY_VAL_ESTIMATE_N_PAGES	8
-
 /* The structure of a BLOB part header */
 /*--------------------------------------*/
 #define BTR_BLOB_HDR_PART_LEN		0	/* BLOB part len on this
@@ -3174,7 +3170,7 @@ btr_estimate_number_of_different_key_vals(
 
 	/* We sample some pages in the index to get an estimate */
 
-	for (i = 0; i < BTR_KEY_VAL_ESTIMATE_N_PAGES; i++) {
+	for (i = 0; i < srv_stats_sample; i++) {
 		rec_t*	supremum;
 		mtr_start(&mtr);
 
@@ -3263,7 +3259,7 @@ btr_estimate_number_of_different_key_vals(
 	}
 
 	/* If we saw k borders between different key values on
-	BTR_KEY_VAL_ESTIMATE_N_PAGES leaf pages, we can estimate how many
+	srv_stats_sample leaf pages, we can estimate how many
 	there will be in index->stat_n_leaf_pages */
 
 	/* We must take into account that our sample actually represents
@@ -3274,26 +3270,26 @@ btr_estimate_number_of_different_key_vals(
 		index->stat_n_diff_key_vals[j]
 			= ((n_diff[j]
 			    * (ib_longlong)index->stat_n_leaf_pages
-			    + BTR_KEY_VAL_ESTIMATE_N_PAGES - 1
+			    + srv_stats_sample - 1
 			    + total_external_size
 			    + not_empty_flag)
-			   / (BTR_KEY_VAL_ESTIMATE_N_PAGES
+			   / (srv_stats_sample
 			      + total_external_size));
 
 		/* If the tree is small, smaller than
-		10 * BTR_KEY_VAL_ESTIMATE_N_PAGES + total_external_size, then
+		10 * srv_stats_sample + total_external_size, then
 		the above estimate is ok. For bigger trees it is common that we
 		do not see any borders between key values in the few pages
-		we pick. But still there may be BTR_KEY_VAL_ESTIMATE_N_PAGES
+		we pick. But still there may be srv_stats_sample
 		different key values, or even more. Let us try to approximate
 		that: */
 
 		add_on = index->stat_n_leaf_pages
-			/ (10 * (BTR_KEY_VAL_ESTIMATE_N_PAGES
+			/ (10 * (srv_stats_sample
 				 + total_external_size));
 
-		if (add_on > BTR_KEY_VAL_ESTIMATE_N_PAGES) {
-			add_on = BTR_KEY_VAL_ESTIMATE_N_PAGES;
+		if (add_on > srv_stats_sample) {
+			add_on = srv_stats_sample;
 		}
 
 		index->stat_n_diff_key_vals[j] += add_on;

handler/ha_innodb.cc

@@ -8420,6 +8420,12 @@ static MYSQL_SYSVAR_LONG(open_files, innobase_open_files,
   "How many files at the maximum InnoDB keeps open at the same time.",
   NULL, NULL, 300L, 10L, ~0L, 0);
 
+static MYSQL_SYSVAR_ULONG(stats_sample, srv_stats_sample,
+  PLUGIN_VAR_OPCMDARG,
+  "When estimating number of different key values in an index, sample "
+  "this many index pages",
+  NULL, NULL, SRV_STATS_SAMPLE_DEFAULT, 1, 1000, 0);
+
 static MYSQL_SYSVAR_ULONG(sync_spin_loops, srv_n_spin_wait_rounds,
   PLUGIN_VAR_RQCMDARG,
   "Count of spin-loop rounds in InnoDB mutexes",
@@ -8486,6 +8492,7 @@ static struct st_mysql_sys_var* innobase_system_variables[]= {
   MYSQL_SYSVAR(stats_on_metadata),
   MYSQL_SYSVAR(adaptive_hash_index),
   MYSQL_SYSVAR(replication_delay),
+  MYSQL_SYSVAR(stats_sample),
   MYSQL_SYSVAR(status_file),
   MYSQL_SYSVAR(support_xa),
   MYSQL_SYSVAR(sync_spin_loops),

include/srv0srv.h

@@ -128,6 +128,11 @@ extern ibool	srv_innodb_status;
 
 extern ibool	srv_stats_on_metadata;
 
+/* When estimating number of different key values in an index, sample
+this many index pages */
+#define SRV_STATS_SAMPLE_DEFAULT	8
+extern ulong	srv_stats_sample;
+
 extern ibool	srv_use_doublewrite_buf;
 extern ibool	srv_use_checksums;
 

srv/srv0srv.c

@@ -291,6 +291,10 @@ UNIV_INTERN ibool	srv_innodb_status	= FALSE;
 
 UNIV_INTERN ibool	srv_stats_on_metadata	= TRUE;
 
+/* When estimating number of different key values in an index, sample
+this many index pages */
+UNIV_INTERN ulong	srv_stats_sample = SRV_STATS_SAMPLE_DEFAULT;
+
 UNIV_INTERN ibool	srv_use_doublewrite_buf	= TRUE;
 UNIV_INTERN ibool	srv_use_checksums = TRUE;