Question

We have MYSQL table of MERGE Storage Engine which merges data from 40 tables (“table01” through “table40“). Once we run a SELECT query we get all the entries from all the merged MySQL tables. The structure and behaviour of MERGE table is as expected.

However it happens that we need to identify which merged table row entry came from or corresponds to which merged table, i.e.

• rowA => “table01“
• rowB => “table12“
• rowC => “table12“
• rowD => “table35“
• rowE => “table39“
• row… => “table…“

Is there a way to do this with MySQL MERGE storage engine?

Answer 1

Not that elegant but you could add an identifying integer column on each of your tables, specify that column with a default value of 1 ,2 (depending on which table it is – you could even chuck in a lookup table (tableId –> tableName) and then you should be able to tell from which table a given row came.

You’d have to make the same changes to the MERGE table too of course…

Lovingly ripped off the MERGE MySQL docs:

CREATE TABLE t1 (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,message CHAR(20)) ENGINE=MyISAM;

CREATE TABLE t2 (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,message CHAR(20)) ENGINE=MyISAM;

INSERT INTO t1 (message) VALUES ('Testing'),('table'),('t1');
INSERT INTO t2 (message) VALUES ('Testing'),('table'),('t2');

CREATE TABLE total (a INT NOT NULL AUTO_INCREMENT,message CHAR(20),INDEX(a)) ENGINE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;

alter table t1 add column identifier int unsigned not null default 1;
alter table t2 add column identifier int unsigned not null default 2;

drop table total;

CREATE TABLE total (a INT NOT NULL AUTO_INCREMENT,message CHAR(20),identifier int unsigned,INDEX(a)) ENGINE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;

You will have the rather dull task of altering all 40 tables though…