MERGE: Its limitation is obvious, the merged tables must be identical MyISAM tables, and MyISAM is even not the default engine for MariaDB. However, TBL accesses a collection of CONNECT tables, but because these tables can be user specified or internally created MYSQL tables, there is no limitation to the type of the tables that can be merged.
TBL is also much more flexible. The merged tables must not be "identical", they just should have the columns defined in the TBL table. If the type of one column in a merged table is not the one of the corresponding column of the TBL table, the column value will be converted. As we have seen, if one column of the TBL table of the TBL column does not exist in one of the merged table, the corresponding value will be set to null. If columns in a sub-table have a different name, they can be accessed by position using the FLAG column option of CONNECT.
However, one limitation of the TBL type regarding MERGE is that TBL tables are currently read-only; INSERT is not supported by TBL. Also, keep using MERGE to access a list of identical MyISAM tables because it will be faster, not passing by the MySQL API.
FEDERATED(X): The main limitation of FEDERATED is to access only MySQL/MariaDB tables. The MYSQL table type of CONNECT has the same limitation but CONNECT provides the ODBC table type and JDBC table type that can access tables of any RDBS providing an ODBC or JDBC driver (including MySQL even it is not really useful!)
Another major limitation of FEDERATED is to access only one table. By combining TBL and MYSQL tables, CONNECT enables to access a collection of local or remote tables as one table. Of course the sub-tables can be on different servers. With one SELECT statement, a company manager will be able to interrogate results coming from all of his subsidiary computers. This is great for distribution, banking, and many other industries.
Remotely executing complex queries
Many companies or administrations must deal with distributed information. CONNECT enables to deal with it efficiently without having to copy it to a centralized database. Let us suppose we have on some remote network machines m1, m2, … mn some information contained in two tables t1 and t2.
Suppose we want to execute on all servers a query such as:
select c1, sum(c2) from t1 a, t2 b where a.id = b.id group by c1;
This raises many problems. Returning the column values of the t1 and t2 tables from all servers can be a lot of network traffic. The group by on the possibly huge resulting tables can be a long process. In addition, the join on the t1 and t2 tables may be relevant only if the joined tuples belong to the same machine, obliging to add a condition on an additional tabid or servid special column.
All this can be avoided and optimized by forcing the query to be locally executed on each server and retrieving only the small results of the group by queries. Here is how to do it. For each remote machine, create a table that will retrieve the locally executed query. For instance for m1:
create table rt1 engine=connect option_list='host=m1' srcdef='select c1, sum(c2) as sc2 from t1 a, t2 b where a.id = b.id group by c1';
Note the alias for the functional column. An alias would be required for the c1
column if its name was different on some machines. The t1 and t2 table names
can also be eventually different on the remote machines. The true names must be
used in the
SRCDEF parameter. This will create a set of tables with two columns
named c1 and sc2.
Then create the table that will retrieve the result of all these tables:
create table rtall engine=connect table_type=tbl table_list='rt1,rt2,…,rtn' option_list='thread=yes';
Now you can retrieve the desired result by:
select c1, sum(sc2) from rtall;
Almost all the work will be done on the remote machines, simultaneously thanks to the thread option, making this query super-fast even on big tables placed on many remote machines.
Providing a list of servers
An interesting case is when the query to run on remote machines is the same for
all of them. It is then possible to avoid declaring all sub-tables. In this
case, the table list option will be used to specify the list of servers the
SRCDEF query must be sent. This will be a list of URL’s and/or Federated
For instance, supposing that federated servers srv1, srv2, … srvn were created for all remote servers, it will be possible to create a tbl table allowing getting the result of a query executed on all of them by:
create table qall [column definition] engine=connect table_type=TBL srcdef='a query' table_list='srv1,srv2,…,srvn' [option_list='thread=yes'];
create table verall engine=connect table_type=TBL srcdef='select @@version' table_list=',server_one'; select * from verall;
Here the server list specifies a void server corresponding to the local running MariaDB and a federated server named server_one.
↑ To generate the columns from the
SRCDEFquery, CONNECT must execute it. This will make sure it is ok. However, if the remote server is not connected yet, or the remote table not existing yet, you can alternatively specify the columns in the create table statement.