[MDEV-9319] ALTER from a bigger to a smaller blob type truncates too much data Created: 2015-12-24  Updated: 2017-10-12  Resolved: 2015-12-29

Status: Closed
Project: MariaDB Server
Component/s: Data Definition - Alter Table
Affects Version/s: 5.5, 10.0, 10.1, 10.2
Fix Version/s: 5.5.48, 10.0.24, 10.1.11

Type: Bug Priority: Major
Reporter: Alexander Barkov Assignee: Alexander Barkov
Resolution: Fixed Votes: 0
Labels: upstream, upstream-5.5

Issue Links:
Blocks
blocks MDEV-4912 Data type plugin API version 1 Closed
Relates
relates to MDEV-13530 VARBINARY doesn't convert to to BLOB ... Closed

 Description    

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a BLOB) ENGINE=MyISAM;
 
INSERT INTO t1 VALUES (REPEAT(0x61,65000));
 
SELECT LENGTH(a) FROM t1;
 
ALTER TABLE t1 MODIFY a TINYBLOB;
 
SELECT LENGTH(a) FROM t1;

returns 65000 before ALTER:

+-----------+
 
| LENGTH(a) |
 
+-----------+
 
|     65000 |
 
+-----------+

and returns 232 after ALTER:

+-----------+
 
| LENGTH(a) |
 
+-----------+
 
|       232 |
 
+-----------+

This looks wrong. The expected length after ALTER is 255.

It also does not produce any warnings on truncation. Probably it should.

 Comments   
Comment by Alexander Barkov [ 2015-12-24 ]

In case of a multi-byte TEXT field, truncation breaks characters apart:

SET NAMES utf8;
 
DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a TEXT CHARACTER SET utf8) ENGINE=MyISAM;
 
INSERT INTO t1 VALUES (REPEAT('Ａ',100));
 
SELECT OCTET_LENGTH(a) FROM t1;
 
ALTER TABLE t1 MODIFY a TINYTEXT CHARACTER SET utf8;
 
SELECT OCTET_LENGTH(a),a FROM t1;
 

+-----------------+----------------------------------------------+
 
| OCTET_LENGTH(a) | a                                            |
 
+-----------------+----------------------------------------------+
 
|              44 | ＡＡＡＡＡＡＡＡＡＡＡＡＡＡ??                 |
 
+-----------------+----------------------------------------------+

Notice the the garbage at the end.

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