MDEV-23600 Division by 0 in row_search_with_covering_prefix

The InnoDB index fields store bytes, not characters.
Remove some unnecessary conversions from characters to bytes.

This also fixes MDEV-20422 and the wrong-result bug MDEV-12486.

mysql-test/include/innodb_prefix_index_cluster_optimization.combinations

+[covering]
+innodb_prefix_index_cluster_optimization=on
+
+[unoptimized]
+innodb_prefix_index_cluster_optimization=off

mysql-test/include/innodb_prefix_index_cluster_optimization.inc

+--source include/have_innodb.inc

mysql-test/r/fast_prefix_index_fetch_innodb.result

-drop table if exists prefixinno;
+SET @save_opt= @@GLOBAL.innodb_prefix_index_cluster_optimization;
 set global innodb_prefix_index_cluster_optimization = ON;
 show variables like 'innodb_prefix_index_cluster_optimization';
 Variable_name	Value
@@ -346,10 +346,10 @@ f1
 🐱🌑
 select @cluster_lookups;
 @cluster_lookups
-2
+1
 select @cluster_lookups_avoided;
 @cluster_lookups_avoided
-0
+1
 # Eligible - record length is shorter than prefix length
 SELECT f1 FROM t1 FORCE INDEX (`f1`) WHERE f1 like '🌑%';
 f1
@@ -366,10 +366,10 @@ f1
 🌒
 select @cluster_lookups;
 @cluster_lookups
-1
+0
 select @cluster_lookups_avoided;
 @cluster_lookups_avoided
-1
+2
 DROP TABLE t1;
 CREATE TABLE t1(
 col1 INT,
@@ -398,4 +398,49 @@ select @cluster_lookups_avoided;
 @cluster_lookups_avoided
 0
 DROP TABLE t1;
-set global innodb_prefix_index_cluster_optimization = OFF;
+#
+# MDEV-23600 Division by 0 in row_search_with_covering_prefix()
+#
+CREATE TABLE t(c POINT UNIQUE) ENGINE=InnoDB;
+INSERT t SET c=POINT(1,1);
+SELECT * FROM t WHERE c > (SELECT MAX(c) FROM t);
+c
+DROP TABLE t;
+#
+# MDEV-12486 Wrong results with innodb_prefix_index_cluster_optimization
+#
+CREATE TABLE wp_blogs (
+blog_id bigint(20) NOT NULL auto_increment,
+site_id bigint(20) NOT NULL default '0',
+domain varchar(200) NOT NULL default '',
+path varchar(100) NOT NULL default '',
+registered datetime NOT NULL default '0000-00-00 00:00:00',
+last_updated datetime NOT NULL default '0000-00-00 00:00:00',
+public tinyint(2) NOT NULL default '1',
+archived tinyint(2) NOT NULL default '0',
+mature tinyint(2) NOT NULL default '0',
+spam tinyint(2) NOT NULL default '0',
+deleted tinyint(2) NOT NULL default '0',
+lang_id int(11) NOT NULL default '0',
+PRIMARY KEY  (blog_id),
+KEY domain (domain(50),path(5)),
+KEY lang_id (lang_id)
+) ENGINE=InnoDB DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
+INSERT INTO wp_blogs (domain, path) VALUES
+('domain.no', '/fondsinvesteringer/'), ('domain.no', '/'),
+('foo', 'bar'), ('bar', 'foo'), ('foo', 'foo'), ('bar', 'bar'),
+('foo', 'foobar'), ('bar', 'foobar'), ('foobar', 'foobar');
+SET GLOBAL innodb_prefix_index_cluster_optimization=off;
+SELECT blog_id FROM wp_blogs WHERE domain IN ('domain.no')
+AND path IN ( '/fondsinvesteringer/', '/' );
+blog_id
+2
+1
+SET GLOBAL innodb_prefix_index_cluster_optimization=on;
+SELECT blog_id FROM wp_blogs WHERE domain IN ('domain.no')
+AND path IN ( '/fondsinvesteringer/', '/' );
+blog_id
+2
+1
+DROP TABLE wp_blogs;
+SET GLOBAL innodb_prefix_index_cluster_optimization = @save_opt;

mysql-test/t/fast_prefix_index_fetch_innodb.test

 -- source include/have_innodb.inc
 
---disable_warnings
-drop table if exists prefixinno;
---enable_warnings
-
+SET @save_opt= @@GLOBAL.innodb_prefix_index_cluster_optimization;
 set global innodb_prefix_index_cluster_optimization = ON;
 show variables like 'innodb_prefix_index_cluster_optimization';
 
@@ -665,4 +662,49 @@ select @cluster_lookups;
 select @cluster_lookups_avoided;
 
 DROP TABLE t1;
-set global innodb_prefix_index_cluster_optimization = OFF;
+
+--echo #
+--echo # MDEV-23600 Division by 0 in row_search_with_covering_prefix()
+--echo #
+CREATE TABLE t(c POINT UNIQUE) ENGINE=InnoDB;
+INSERT t SET c=POINT(1,1);
+SELECT * FROM t WHERE c > (SELECT MAX(c) FROM t);
+DROP TABLE t;
+
+--echo #
+--echo # MDEV-12486 Wrong results with innodb_prefix_index_cluster_optimization
+--echo #
+CREATE TABLE wp_blogs (
+  blog_id bigint(20) NOT NULL auto_increment,
+  site_id bigint(20) NOT NULL default '0',
+  domain varchar(200) NOT NULL default '',
+  path varchar(100) NOT NULL default '',
+  registered datetime NOT NULL default '0000-00-00 00:00:00',
+  last_updated datetime NOT NULL default '0000-00-00 00:00:00',
+  public tinyint(2) NOT NULL default '1',
+  archived tinyint(2) NOT NULL default '0',
+  mature tinyint(2) NOT NULL default '0',
+  spam tinyint(2) NOT NULL default '0',
+  deleted tinyint(2) NOT NULL default '0',
+  lang_id int(11) NOT NULL default '0',
+  PRIMARY KEY  (blog_id),
+  KEY domain (domain(50),path(5)),
+  KEY lang_id (lang_id)
+) ENGINE=InnoDB DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
+
+INSERT INTO wp_blogs (domain, path) VALUES
+('domain.no', '/fondsinvesteringer/'), ('domain.no', '/'),
+('foo', 'bar'), ('bar', 'foo'), ('foo', 'foo'), ('bar', 'bar'),
+('foo', 'foobar'), ('bar', 'foobar'), ('foobar', 'foobar');
+
+SET GLOBAL innodb_prefix_index_cluster_optimization=off;
+SELECT blog_id FROM wp_blogs WHERE domain IN ('domain.no')
+AND path IN ( '/fondsinvesteringer/', '/' );
+
+SET GLOBAL innodb_prefix_index_cluster_optimization=on;
+SELECT blog_id FROM wp_blogs WHERE domain IN ('domain.no')
+AND path IN ( '/fondsinvesteringer/', '/' );
+
+DROP TABLE wp_blogs;
+
+SET GLOBAL innodb_prefix_index_cluster_optimization = @save_opt;

mysql-test/t/innodb_ext_key.combinations

+[on]
+optimizer_switch=extended_keys=on
+
+[off]
+optimizer_switch=extended_keys=off

storage/innobase/row/row0sel.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1997, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
-Copyright (c) 2015, 2018, MariaDB Corporation.
+Copyright (c) 2015, 2020, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3723,9 +3723,16 @@ bool row_search_with_covering_prefix(
 		return false;
 	}
 
+	/* We can avoid a clustered index lookup if
+	all of the following hold:
+	(1) all columns are in the secondary index
+	(2) all values for columns that are prefix-only
+	indexes are shorter than the prefix size
+	This optimization can avoid many IOs for certain schemas. */
 	for (ulint i = 0; i < prebuilt->n_template; i++) {
 		mysql_row_templ_t* templ = prebuilt->mysql_template + i;
 		ulint j = templ->rec_prefix_field_no;
+		ut_ad(!templ->mbminlen == !templ->mbmaxlen);
 
 		/** Condition (1) : is the field in the index. */
 		if (j == ULINT_UNDEFINED) {
@@ -3735,33 +3742,29 @@ bool row_search_with_covering_prefix(
 		/** Condition (2): If this is a prefix index then
 		row's value size shorter than prefix length. */
 
-		if (!templ->rec_field_is_prefix) {
+		if (!templ->rec_field_is_prefix
+		    || rec_offs_nth_sql_null(offsets, j)) {
 			continue;
 		}
 
-		ulint rec_size = rec_offs_nth_size(offsets, j);
 		const dict_field_t* field = dict_index_get_nth_field(index, j);
-		ulint max_chars = field->prefix_len / templ->mbmaxlen;
 
-		ut_a(field->prefix_len > 0);
-
-		if (rec_size < max_chars) {
-			/* Record in bytes shorter than the index
-			prefix length in char. */
+		if (!field->prefix_len) {
 			continue;
 		}
 
-		if (rec_size * templ->mbminlen >= field->prefix_len) {
+		const ulint rec_size = rec_offs_nth_size(offsets, j);
+
+		if (rec_size >= field->prefix_len) {
 			/* Shortest representation string by the
 			byte length of the record is longer than the
 			maximum possible index prefix. */
 			return false;
 		}
 
-		size_t num_chars = rec_field_len_in_chars(
-			field->col, j, rec, offsets);
-
-		if (num_chars >= max_chars) {
+		if (templ->mbminlen != templ->mbmaxlen
+		    && rec_field_len_in_chars(field->col, j, rec, offsets)
+		    >= field->prefix_len / templ->mbmaxlen) {
 			/* No of chars to store the record exceeds
 			the index prefix character length. */
 			return false;

storage/xtradb/row/row0sel.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1997, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
-Copyright (c) 2015, 2018, MariaDB Corporation.
+Copyright (c) 2015, 2020, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3740,9 +3740,16 @@ bool row_search_with_covering_prefix(
 		return false;
 	}
 
+	/* We can avoid a clustered index lookup if
+	all of the following hold:
+	(1) all columns are in the secondary index
+	(2) all values for columns that are prefix-only
+	indexes are shorter than the prefix size
+	This optimization can avoid many IOs for certain schemas. */
 	for (ulint i = 0; i < prebuilt->n_template; i++) {
 		mysql_row_templ_t* templ = prebuilt->mysql_template + i;
 		ulint j = templ->rec_prefix_field_no;
+		ut_ad(!templ->mbminlen == !templ->mbmaxlen);
 
 		/** Condition (1) : is the field in the index. */
 		if (j == ULINT_UNDEFINED) {
@@ -3752,34 +3759,29 @@ bool row_search_with_covering_prefix(
 		/** Condition (2): If this is a prefix index then
 		row's value size shorter than prefix length. */
 
-		if (!templ->rec_field_is_prefix) {
+		if (!templ->rec_field_is_prefix
+		    || rec_offs_nth_sql_null(offsets, j)) {
 			continue;
 		}
 
-		ulint rec_size = rec_offs_nth_size(offsets, j);
 		const dict_field_t* field = dict_index_get_nth_field(index, j);
-		ulint max_chars = field->prefix_len / templ->mbmaxlen;
 
-		ut_a(field->prefix_len > 0);
-
-		if (rec_size < max_chars) {
-			/* Record in bytes shorter than the index
-			prefix length in char. */
+		if (!field->prefix_len) {
 			continue;
 		}
 
-		if (rec_size * templ->mbminlen >= field->prefix_len) {
+		const ulint rec_size = rec_offs_nth_size(offsets, j);
+
+		if (rec_size >= field->prefix_len) {
 			/* Shortest representation string by the
 			byte length of the record is longer than the
 			maximum possible index prefix. */
 			return false;
 		}
 
-
-		size_t num_chars = rec_field_len_in_chars(
-			field->col, j, rec, offsets);
-
-		if (num_chars >= max_chars) {
+		if (templ->mbminlen != templ->mbmaxlen
+		    && rec_field_len_in_chars(field->col, j, rec, offsets)
+		    >= field->prefix_len / templ->mbmaxlen) {
 			/* No of chars to store the record exceeds
 			the index prefix character length. */
 			return false;