maria:bb-10.2-sujatha

Last commit made on 2019-07-17
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Name:
bb-10.2-sujatha
Repository:
lp:maria

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6aa04bc... by Sujatha <email address hidden> on 2019-07-17

MDEV-19925: Column ... cannot be converted from type 'varchar(20)' to type 'varchar(20)'

Description:
============
In row based replication, when replicating from a table with a field with
character set set to UTF8mb3 to the same table with the same field set to
character set UTF8mb4 I get a confusing error message:

For VARCHAR: VARCHAR(1) 'utf8mb3' to VARCHAR(1) 'utf8mb4'
"Column 0 of table 'test.t1' cannot be converted from type 'varchar(3)' to
type 'varchar(1)'"

Similar issue with CHAR type as well.

Issue with respect to BLOB types:

For BLOB: LONGBLOB to TINYBLOB - Error message displays incorrect blob type.
"Column 0 of table 'test.t1' cannot be converted from type 'tinyblob' to type
'tinyblob'"

Analysis:
=========
In Row based replication charset information is not sent as part of metadata
from master to slave.

For VARCHAR field its character length is converted into equivalent
octets/bytes and stored internally. At the time of displaying the data to user
it is converted back to original character length.

For example:
VARCHAR(2)- utf8mb3 is stored as:2*3 = VARCHAR(6)
At the time of displaying it to user
VARCHAR(6)- charset utf8mb3:6/3= VARCHAR(2).

At present the internally converted octect length is sent from master to slave
with out providing the charset information. On slave side if the type
conversion fails 'show_sql_type' function is used to get the type specific
information from metadata. Since there is no charset information is available
the filed type is displayed as VARCHAR(6).

This results in confused error message.

For CHAR fields
CHAR(1)- utf8mb3 - CHAR(3)
CHAR(1)- utf8mb4 - CHAR(4)

'show_sql_type' function which retrieves type information from metadata uses
(bytes/local charset length) to get actual character length. If slave's chaset
is 'utf8mb4' then

CHAR(3/4)-->CHAR(0)
CHAR(4/4)-->CHAR(1).

This results in confused error message.

Analysis for BLOB type issue:

BLOB's length is represented in two forms.
1. Actual length
i.e
  (length < 256) type= MYSQL_TYPE_TINY_BLOB;
  (length < 65536) type= MYSQL_TYPE_BLOB; ...

2. packlength - The number of bytes used to represent the length of the blob
  1- tinyblob
  2- blob ...

In row based replication only the packlength is written in the binary log. On
the slave side this packlength is interpreted as actual length of the blob.
Hence the length is always < 256 and the type is displayed as tiny blob.

Fix:
===
For CHAR and VARCHAR fields display their length in bytes for both source and
target fields. For target field display the charset information if it is
relevant.

For blob type changed the code to use the packlength and display appropriate
blob type in error message.

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