Fix Bug#30423 "InnoDBs treatment of NULL in index stats causes bad

"rows examined" estimates". This change implements "innodb_stats_method"
with options of "nulls_equal", "nulls_unequal" and "null_ignored".
      
rb://553 approved by Marko

storage/innodb_plugin/handler/ha_innodb.cc

@@ -174,6 +174,25 @@ static char*	internal_innobase_data_file_path	= NULL;
 
 static char*	innodb_version_str = (char*) INNODB_VERSION_STR;
 
+/** Possible values for system variable "innodb_stats_method". The values
+are defined the same as its corresponding MyISAM system variable
+"myisam_stats_method"(see "myisam_stats_method_names"), for better usability */
+static const char* innodb_stats_method_names[] = {
+	"nulls_equal",
+	"nulls_unequal",
+	"nulls_ignored",
+	NullS
+};
+
+/** Used to define an enumerate type of the system variable innodb_stats_method.
+This is the same as "myisam_stats_method_typelib" */
+static TYPELIB innodb_stats_method_typelib = {
+	array_elements(innodb_stats_method_names) - 1,
+	"innodb_stats_method_typelib",
+	innodb_stats_method_names,
+	NULL
+};
+
 /* The following counter is used to convey information to InnoDB
 about server activity: in selects it is not sensible to call
 srv_active_wake_master_thread after each fetch or search, we only do
@@ -7507,6 +7526,65 @@ innobase_get_mysql_key_number_for_index(
 
         return(0);
 }
+
+/*********************************************************************//**
+Calculate Record Per Key value. Need to exclude the NULL value if
+innodb_stats_method is set to "nulls_ignored"
+@return estimated record per key value */
+static
+ha_rows
+innodb_rec_per_key(
+/*===============*/
+	dict_index_t*	index,		/*!< in: dict_index_t structure */
+	ulint		i,		/*!< in: the column we are
+					calculating rec per key */
+	ha_rows		records)	/*!< in: estimated total records */
+{
+	ha_rows		rec_per_key;
+
+	ut_ad(i < dict_index_get_n_unique(index));
+
+	/* Note the stat_n_diff_key_vals[] stores the diff value with
+	n-prefix indexing, so it is always stat_n_diff_key_vals[i + 1] */
+	if (index->stat_n_diff_key_vals[i + 1] == 0) {
+
+		rec_per_key = records;
+	} else if (srv_innodb_stats_method == SRV_STATS_NULLS_IGNORED) {
+		ib_int64_t	num_null;
+
+		/* Number of rows with NULL value in this
+		field */
+		num_null = records - index->stat_n_non_null_key_vals[i];
+
+		/* In theory, index->stat_n_non_null_key_vals[i]
+		should always be less than the number of records.
+		Since this is statistics value, the value could
+		have slight discrepancy. But we will make sure
+		the number of null values is not a negative number. */
+		num_null = (num_null < 0) ? 0 : num_null;
+
+		/* If the number of NULL values is the same as or
+		large than that of the distinct values, we could
+		consider that the table consists mostly of NULL value. 
+		Set rec_per_key to 1. */
+		if (index->stat_n_diff_key_vals[i + 1] <= num_null) {
+			rec_per_key = 1;
+		} else {
+			/* Need to exclude rows with NULL values from
+			rec_per_key calculation */
+			rec_per_key = (ha_rows)(
+				(records - num_null)
+				/ (index->stat_n_diff_key_vals[i + 1]
+				   - num_null));
+		}
+	} else {
+		rec_per_key = (ha_rows)
+			 (records / index->stat_n_diff_key_vals[i + 1]);
+	}
+
+	return(rec_per_key);
+}
+
 /*********************************************************************//**
 Returns statistics information of the table to the MySQL interpreter,
 in various fields of the handle object. */
@@ -7737,13 +7815,8 @@ ha_innobase::info_low(
 					break;
 				}
 
-				if (index->stat_n_diff_key_vals[j + 1] == 0) {
-
-					rec_per_key = stats.records;
-				} else {
-					rec_per_key = (ha_rows)(stats.records /
-					 index->stat_n_diff_key_vals[j + 1]);
-				}
+				rec_per_key = innodb_rec_per_key(
+					index, j, stats.records);
 
 				/* Since MySQL seems to favor table scans
 				too much over index searches, we pretend
@@ -10934,6 +11007,13 @@ static MYSQL_SYSVAR_STR(change_buffering, innobase_change_buffering,
   innodb_change_buffering_validate,
   innodb_change_buffering_update, "inserts"); 
 
+static MYSQL_SYSVAR_ENUM(stats_method, srv_innodb_stats_method,
+   PLUGIN_VAR_RQCMDARG,
+  "Specifies how InnoDB index statistics collection code should "
+  "treat NULLs. Possible values are NULLS_EQUAL (default), "
+  "NULLS_UNEQUAL and NULLS_IGNORED",
+   NULL, NULL, SRV_STATS_NULLS_EQUAL, &innodb_stats_method_typelib);
+
 #if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
 static MYSQL_SYSVAR_UINT(change_buffering_debug, ibuf_debug,
   PLUGIN_VAR_RQCMDARG,
@@ -10988,6 +11068,7 @@ static struct st_mysql_sys_var* innobase_system_variables[]= {
   MYSQL_SYSVAR(stats_on_metadata),
   MYSQL_SYSVAR(stats_sample_pages),
   MYSQL_SYSVAR(adaptive_hash_index),
+  MYSQL_SYSVAR(stats_method),
   MYSQL_SYSVAR(replication_delay),
   MYSQL_SYSVAR(status_file),
   MYSQL_SYSVAR(strict_mode),