Error in the documentation MariaDB -> mysqlbinlog: `--rewrite-db` and `--database`

Question

In the mysqlbinlog utility documentation, the --rewrite-db=name option has the following explanation

Filtering (e.g. with --database=name ) happens AFTER the database rewrites have been performed.

However, my tests show that the filtering with the --database option happens BEFORE the --rewrite-db option is applied.

I reproduced the issue in the following way: I created a database db1 with a table t1 that contains one column c1. Binlog format = ROW.

CREATE DATABASE db1;
use db1;
CREATE TABLE t1 (c1 varchar(255));

Then I executed the following command:

FLUSH LOGS;

Made an insert into the table

INSERT INTO t1 (c1) VALUES (`row in db1.t1`);

Executed the command again

FLUSH LOGS;

I use the command below (the --vv parameter decrypts the commands) to check what is extracted from the binlogs:

mysqlbinlog mysql-bin.000033 --rewrite-db='db1->new_db_name' --database='new_db_name' -vv

Only comments are output, no INSERT commands. However, if I specify the name db1 in the --database parameter, then INSERT commands will be displayed:

mysqlbinlog mysql-bin.000033 --rewrite-db='db1->new_db_name' --database='db1' -vv

output:

...
...
BINLOG '
eI98YxMBAAAANgAAABwCAAAAADMAAAAAAAEAC25ld19kYl9uYW1lAAJ0MQABDwL8AwFyNVlH
eI98YxcBAAAAMQAAAE0CAAAAADMAAAAAAAEAAf/+DQByb3cgaW4gZGIxLnQxmq6ESw==
'/*!*/;
### INSERT INTO `new_db_name`.`t1`
### SET
###   @1='row in db1.t1' /* VARSTRING(1020) meta=1020 nullable=1 is_null=0 */
...
...

Thus, it turns out that mysqlbinlog first filters the binlog file with the --database parameter and only then makes replacements with the --rewrite-db parameter, which contradicts the documentation.

I tested similar scenarios for MySQL(no MariaDB), and it works as it should, the filtering is run AFTER the replacements.

Do I understand correctly that this is an error in the documentation? Or am I misunderstanding something?

Answer 1

Looking at the code it seems --database option is applied to QUERY_EVENT/QUERY_COMPRESSED_EVENT just before print_use_statement uses the --rewrite-db.

With the TABLE_MAP_EVENT, --database is applied before --rewrite-db is applied to TABLE_MAP_EVENT.

So you are absolutely right, this is a documentation error. MariaDB's KB is editable if you'd like to change this.