diff --git a/mysql-test/main/column_compression.result b/mysql-test/main/column_compression.result
index 5ea981cfdc3..e3bccd5f0de 100644
--- a/mysql-test/main/column_compression.result
+++ b/mysql-test/main/column_compression.result
@@ -2672,3 +2672,241 @@ SET column_compression_threshold=0;
 INSERT INTO t1 VALUES('aa');
 SET column_compression_threshold=DEFAULT;
 DROP TABLE t1;
+#
+# MDEV-31724 Compressed varchar values lost on joins when sorting on columns from joined table(s)
+#
+CREATE TABLE t1 (
+id  int(10) unsigned not null,
+txt varchar(5000) COMPRESSED NOT NULL DEFAULT '',
+PRIMARY KEY (id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_bin;
+CREATE TABLE t2 (
+id int(10) unsigned not null,
+n1 bigint(20) NOT NULL,
+n2 bigint(20) NOT NULL,
+n3 bigint(20) NOT NULL,
+PRIMARY KEY (id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_bin;
+INSERT INTO t1 VALUES
+(1, 'short string < 100 chars'),
+(2, 'long string =  99 chars                                                                            '),
+(3, 'long string = 100 chars                                                                            !'),
+(4, 'long string = 101 chars                                                                             !');
+INSERT INTO t2 VALUES
+(1, 24, 1, 1),
+(2, 99, 2, 2),
+(3, 100, 3, 3),
+(4, 101, 4, 4);
+SELECT txt, v.* FROM t1 LEFT JOIN t2 v ON t1.id = v.id;
+txt	id	n1	n2	n3
+short string < 100 chars	1	24	1	1
+long string =  99 chars                                                                            	2	99	2	2
+long string = 100 chars                                                                            !	3	100	3	3
+long string = 101 chars                                                                             !	4	101	4	4
+SELECT txt, v.* FROM t1 LEFT JOIN t2 v ON t1.id = v.id ORDER BY v.n1;
+txt	id	n1	n2	n3
+short string < 100 chars	1	24	1	1
+long string =  99 chars                                                                            	2	99	2	2
+long string = 100 chars                                                                            !	3	100	3	3
+long string = 101 chars                                                                             !	4	101	4	4
+SELECT txt, v.* FROM t1 JOIN t2 v ON t1.id = v.id;
+txt	id	n1	n2	n3
+short string < 100 chars	1	24	1	1
+long string =  99 chars                                                                            	2	99	2	2
+long string = 100 chars                                                                            !	3	100	3	3
+long string = 101 chars                                                                             !	4	101	4	4
+SELECT txt, v.* FROM t1 JOIN t2 v ON t1.id = v.id ORDER BY v.n1;
+txt	id	n1	n2	n3
+short string < 100 chars	1	24	1	1
+long string =  99 chars                                                                            	2	99	2	2
+long string = 100 chars                                                                            !	3	100	3	3
+long string = 101 chars                                                                             !	4	101	4	4
+DROP TABLE t1, t2;
+CREATE OR REPLACE TABLE t1 (
+id INT NOT NULL PRIMARY KEY,
+txt varchar(5000) COMPRESSED NOT NULL DEFAULT ''
+) CHARSET=utf8mb3;
+INSERT INTO t1 VALUES
+(1, REPEAT('a', 10)),
+(2, REPEAT('b', 99)),
+(3, REPEAT('c', 100)),
+(4, REPEAT('d', 121));
+SELECT txt, sysdate(6) FROM t1 ORDER BY 2;
+txt	sysdate(6)
+aaaaaaaaaa	<sysdate>
+bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb	<sysdate>
+cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc	<sysdate>
+ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd	<sysdate>
+DROP TABLE t1;
+CREATE FUNCTION f1(imax INT, jmax INT) RETURNS TEXT
+BEGIN
+DECLARE res TEXT DEFAULT 'x';
+FOR i IN 0..imax
+DO
+FOR j IN 0..jmax
+DO
+SET res=CONCAT(res, ' ', i, ' ', j);
+END FOR;
+END FOR;
+RETURN res;
+END;
+$$
+SET @@column_compression_threshold=32;
+# VARCHAR1, 8bit, truncation
+CREATE TABLE t1 (a VARCHAR(254) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(6,6));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+197	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 6 6 0 6 1 6 2 6 3 6 4 6 5 6 6
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(4) COMPRESSED CHARACTER SET latin1;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	1
+SELECT LENGTH(a), a FROM t1;
+LENGTH(a)	a
+4	x 0 
+DROP TABLE t1;
+CREATE TABLE t1 (a VARCHAR(254) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (REPEAT('a',254));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+254	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(4) COMPRESSED CHARACTER SET latin1;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	1
+SELECT LENGTH(a), a FROM t1;
+LENGTH(a)	a
+4	aaaa
+DROP TABLE t1;
+# VARCHAR1, 8bit, no truncation
+CREATE TABLE t1 (a VARCHAR(250) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(6,6));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+197	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 6 6 0 6 1 6 2 6 3 6 4 6 5 6 6
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(254) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	0
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+197	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 6 6 0 6 1 6 2 6 3 6 4 6 5 6 6
+DROP TABLE t1;
+# VARCHAR2, 8bit, truncation
+CREATE TABLE t1 (a VARCHAR(32000) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+5505	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 31 27 31 28 31 29 31 30 31 31
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(256) COMPRESSED CHARACTER SET latin1;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	1
+Column_decompressions	1
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+256	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 1 17 1 18 1 19 1 20 1 21 1 22
+DROP TABLE t1;
+# VARCHAR2, 8bit, no truncation
+CREATE TABLE t1 (a VARCHAR(32000) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a, 30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a, 30)
+5505	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 31 27 31 28 31 29 31 30 31 31
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(32001) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	0
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a, 30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a, 30)
+5505	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 31 27 31 28 31 29 31 30 31 31
+DROP TABLE t1;
+# VARCHAR1, multi-byte, truncation
+CREATE TABLE t1 (a VARCHAR(80) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(3,3));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+65	x 0 0 0 1 0 2 0 3 1 0 1 1 1 2 	 0 2 1 2 2 2 3 3 0 3 1 3 2 3 3
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(1) COMPRESSED CHARACTER SET utf8mb3;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	1
+SELECT LENGTH(a), a FROM t1;
+LENGTH(a)	a
+1	x
+DROP TABLE t1;
+# VARCHAR1, multi-byte, no truncation
+CREATE TABLE t1 (a VARCHAR(80) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(3,3));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+65	x 0 0 0 1 0 2 0 3 1 0 1 1 1 2 	 0 2 1 2 2 2 3 3 0 3 1 3 2 3 3
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(81) COMPRESSED CHARACTER SET utf8mb3;
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	0
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+65	x 0 0 0 1 0 2 0 3 1 0 1 1 1 2 	 0 2 1 2 2 2 3 3 0 3 1 3 2 3 3
+DROP TABLE t1;
+# VARCHAR2, multi-byte, truncation
+CREATE TABLE t1 (a VARCHAR(10000) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+5505	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 31 27 31 28 31 29 31 30 31 31
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(256) COMPRESSED CHARACTER SET utf8mb3;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	1
+Column_decompressions	1
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+256	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 1 17 1 18 1 19 1 20 1 21 1 22
+DROP TABLE t1;
+# VARCHAR2, multi-byte, no truncation
+CREATE TABLE t1 (a VARCHAR(10000) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+5505	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 31 27 31 28 31 29 31 30 31 31
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(10001) COMPRESSED CHARACTER SET utf8mb3;
+SHOW STATUS LIKE 'Column%compressions';
+Variable_name	Value
+Column_compressions	0
+Column_decompressions	0
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+LENGTH(a)	LEFT(a,30)	RIGHT(a,30)
+5505	x 0 0 0 1 0 2 0 3 0 4 0 5 0 6 	 31 27 31 28 31 29 31 30 31 31
+DROP TABLE t1;
+SET @@column_compression_threshold=DEFAULT;
+DROP FUNCTION f1;
+#
+# End of 10.4 tests
+#
diff --git a/mysql-test/main/column_compression.test b/mysql-test/main/column_compression.test
index 1560b23ad3e..29e0218749c 100644
--- a/mysql-test/main/column_compression.test
+++ b/mysql-test/main/column_compression.test
@@ -266,3 +266,178 @@ SET column_compression_threshold=0;
 INSERT INTO t1 VALUES('aa');
 SET column_compression_threshold=DEFAULT;
 DROP TABLE t1;
+
+
+--echo #
+--echo # MDEV-31724 Compressed varchar values lost on joins when sorting on columns from joined table(s)
+--echo #
+
+CREATE TABLE t1 (
+  id  int(10) unsigned not null,
+  txt varchar(5000) COMPRESSED NOT NULL DEFAULT '',
+  PRIMARY KEY (id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_bin;
+
+CREATE TABLE t2 (
+  id int(10) unsigned not null,
+  n1 bigint(20) NOT NULL,
+  n2 bigint(20) NOT NULL,
+  n3 bigint(20) NOT NULL,
+  PRIMARY KEY (id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_bin;
+
+INSERT INTO t1 VALUES
+(1, 'short string < 100 chars'),
+(2, 'long string =  99 chars                                                                            '),
+(3, 'long string = 100 chars                                                                            !'),
+(4, 'long string = 101 chars                                                                             !');
+
+INSERT INTO t2 VALUES
+(1, 24, 1, 1),
+(2, 99, 2, 2),
+(3, 100, 3, 3),
+(4, 101, 4, 4);
+
+SELECT txt, v.* FROM t1 LEFT JOIN t2 v ON t1.id = v.id;
+SELECT txt, v.* FROM t1 LEFT JOIN t2 v ON t1.id = v.id ORDER BY v.n1;
+SELECT txt, v.* FROM t1 JOIN t2 v ON t1.id = v.id;
+SELECT txt, v.* FROM t1 JOIN t2 v ON t1.id = v.id ORDER BY v.n1;
+
+DROP TABLE t1, t2;
+
+CREATE OR REPLACE TABLE t1 (
+  id INT NOT NULL PRIMARY KEY,
+  txt varchar(5000) COMPRESSED NOT NULL DEFAULT ''
+) CHARSET=utf8mb3;
+
+INSERT INTO t1 VALUES
+(1, REPEAT('a', 10)),
+(2, REPEAT('b', 99)),
+(3, REPEAT('c', 100)),
+(4, REPEAT('d', 121));
+
+--replace_column 2 <sysdate>
+--sorted_result
+SELECT txt, sysdate(6) FROM t1 ORDER BY 2;
+DROP TABLE t1;
+
+
+DELIMITER $$;
+CREATE FUNCTION f1(imax INT, jmax INT) RETURNS TEXT
+BEGIN
+  DECLARE res TEXT DEFAULT 'x';
+  FOR i IN 0..imax
+  DO
+    FOR j IN 0..jmax
+    DO
+      SET res=CONCAT(res, ' ', i, ' ', j);
+    END FOR;
+  END FOR;
+  RETURN res;
+END;
+$$
+DELIMITER ;$$
+
+
+# Let's override the default threshold (100) to force
+# comression for VARCHAR1+MB, for example, for:
+# VARCHAR(80) CHARACTER SET utf8mb3
+
+SET @@column_compression_threshold=32;
+
+--echo # VARCHAR1, 8bit, truncation
+CREATE TABLE t1 (a VARCHAR(254) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(6,6));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(4) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), a FROM t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a VARCHAR(254) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (REPEAT('a',254));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(4) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), a FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR1, 8bit, no truncation
+CREATE TABLE t1 (a VARCHAR(250) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(6,6));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(254) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR2, 8bit, truncation
+CREATE TABLE t1 (a VARCHAR(32000) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(256) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR2, 8bit, no truncation
+CREATE TABLE t1 (a VARCHAR(32000) COMPRESSED CHARACTER SET latin1);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a, 30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(32001) COMPRESSED CHARACTER SET latin1;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a, 30) FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR1, multi-byte, truncation
+CREATE TABLE t1 (a VARCHAR(80) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(3,3));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(1) COMPRESSED CHARACTER SET utf8mb3;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), a FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR1, multi-byte, no truncation
+CREATE TABLE t1 (a VARCHAR(80) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(3,3));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(81) COMPRESSED CHARACTER SET utf8mb3;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR2, multi-byte, truncation
+CREATE TABLE t1 (a VARCHAR(10000) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(256) COMPRESSED CHARACTER SET utf8mb3;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+DROP TABLE t1;
+
+--echo # VARCHAR2, multi-byte, no truncation
+CREATE TABLE t1 (a VARCHAR(10000) COMPRESSED CHARACTER SET utf8mb3);
+INSERT INTO t1 VALUES (f1(31,31));
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+FLUSH STATUS;
+ALTER IGNORE TABLE t1 MODIFY a VARCHAR(10001) COMPRESSED CHARACTER SET utf8mb3;
+SHOW STATUS LIKE 'Column%compressions';
+SELECT LENGTH(a), LEFT(a,30), RIGHT(a,30) FROM t1;
+DROP TABLE t1;
+
+SET @@column_compression_threshold=DEFAULT;
+
+DROP FUNCTION f1;
+
+--echo #
+--echo # End of 10.4 tests
+--echo #
diff --git a/sql/field_conv.cc b/sql/field_conv.cc
index 7ec93e032e6..7e60e37a36e 100644
--- a/sql/field_conv.cc
+++ b/sql/field_conv.cc
@@ -532,6 +532,40 @@ static void do_expand_string(Copy_field *copy)
 }
 
 
+/*
+  Copy from a Field_varstring with length_bytes==1
+  into another Field_varstring with length_bytes==1
+  when the target column is not shorter than the source column.
+  We don't need to calculate the prefix in this case. It works for
+  - non-compressed and compressed columns
+  - single byte and multi-byte character sets
+*/
+static void do_varstring1_no_truncation(Copy_field *copy)
+{
+  uint length= (uint) *(uchar*) copy->from_ptr;
+  DBUG_ASSERT(length <= copy->to_length - 1);
+  *(uchar*) copy->to_ptr= (uchar) length;
+  memcpy(copy->to_ptr+1, copy->from_ptr + 1, length);
+}
+
+/*
+  Copy from a Field_varstring with length_bytes==2
+  into another Field_varstring with length_bytes==2
+  when the target column is not shorter than the source column.
+  We don't need to calculate the prefix in this case. It works for
+  - non-compressed and compressed columns
+  - single byte and multi-byte character sets
+*/
+static void do_varstring2_no_truncation(Copy_field *copy)
+{
+  uint length= uint2korr(copy->from_ptr);
+  DBUG_ASSERT(length <= copy->to_length - HA_KEY_BLOB_LENGTH);
+  int2store(copy->to_ptr, length);
+  memcpy(copy->to_ptr + HA_KEY_BLOB_LENGTH,
+         copy->from_ptr + HA_KEY_BLOB_LENGTH, length);
+}
+
+
 static void do_varstring1(Copy_field *copy)
 {
   uint length= (uint) *(uchar*) copy->from_ptr;
@@ -776,6 +810,21 @@ Field::Copy_func *Field_varstring::get_copy_func(const Field *from) const
       length_bytes != ((const Field_varstring*) from)->length_bytes ||
       !compression_method() != !from->compression_method())
     return do_field_string;
+
+  if (field_length >= from->field_length)
+    return length_bytes == 1 ? do_varstring1_no_truncation :
+                               do_varstring2_no_truncation;
+
+  if (compression_method())
+  {
+    /*
+      Truncation is going to happen, so we need to calculate prefixes.
+      Can't calculate prefixes directly on compressed data,
+      need to go through val_str() to uncompress.
+    */
+    return do_field_string;
+  }
+
   return length_bytes == 1 ?
          (from->charset()->mbmaxlen == 1 ? do_varstring1 : do_varstring1_mb) :
          (from->charset()->mbmaxlen == 1 ? do_varstring2 : do_varstring2_mb);