Dynamic SQL alternatives (MySql) - mysql

Question

On current MySql DB server, i have two schemas: "Friends", "Places". Entire DB is organized around stored procedures which are being called from outside. Maybe it's good approach, maybe it's bad but it's not related to this problem i'm having. In this case DB has to be separated from any outside software using it (as i'm only in charge of DB).
Some stored procedures from "Friends" schema refer to tables from "Places" and vice versa. Now, if for example i wanna setup new set of schemas, on the same server, but for another "client" like this:
Friends_clientOne
Places_clientOne
Friends_clientTwo
Places_clientTwo
I'm having a problem - stored procedures referring tables from another schema won't know which schema name to use. Checking and modifying each and every procedure to suite appropriate schema name every time new set is created is not an option. Dynamic SQL is totally new for me - what are other options? How can i, for example, do this:
(stored procedure inside schema Friends_clientOne):
Select * from Places_<getCurrentSchemaSuffix>.someTable;
Please tell me MySql is flexible enough for this :( What about Percona?

Answer 1

The closest thing to what you're describing is a builtin function DATABASE() (http://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_database), which returns the current default database.
The default database isn't necessarily the one that a given table belongs to. It's the database most recently named in a USE <databasename> statement. If you can rely on your application always using the database you mean for that table to belong to, then you can use that function.
However, no SQL implementation allows you to change the table name dynamically during query execution. You can name tables only before doing the prepare of a statement, and hard-coding it into the query. There is no syntax for making the table name variable.
So you'll have to use dynamic SQL even if you use the DATABASE() function.
Percona Server is no different from stock Oracle MySQL for this problem.
Your options for this problem are:
Stop using multiple schemas for each client. Put all of the data for each client into a single schema. This seems simplest.
Design the stored procedures to be unique to each client. You said you didn't want to do this. But for what it's worth, we do this in the stored procedures and triggers in the customer databases I manage at my current job. It's not that bad. We have a "template" version of the CREATE statements for each trigger or procedure, with a placeholder token for the customer ID. When we create a new customer's database, we copy that template code and make a substitution on the customer ID placeholder, then run it.
Put each of your clients' data into their own exclusive instance of MySQL Server. This way you can have multiple schemas per client, but the schema names don't need to be distinct for each client. You can run multiple instances on one server host, they just have to be configured with distinct datadir, port, sock_file, and other log files. Though I have seen this solution used, I don't recommend it, because it has a lot of resource overhead, and it's hard to manage.
Learn to use dynamic SQL.

Answer 2

You can use PREPARED Statement in the Procs like this:
DELIMITER //
CREATE PROCEDURE getPlace (OUT param1 char)
BEGIN
SELECT CONCAT("Select * from Places_", SUBSTRING_INDEX(DATABASE(), '_', -1),".someTable;") INTO #sql;
PREPARE getPlaces from #sql;
EXECUTE getPlaces;
DEALLOCATE PREPARE getPlaces;
END;
//
DELIMITER ;
sample
MariaDB [mysql]> CREATE DATABASE Friends_clientOne;
Query OK, 1 row affected (0.00 sec)
MariaDB [mysql]> CREATE DATABASE Friends_clientTwo;
Query OK, 1 row affected (0.00 sec)
MariaDB [mysql]> CREATE DATABASE Places_clientOne;
Query OK, 1 row affected (0.00 sec)
MariaDB [mysql]> CREATE DATABASE Places_clientTWO;
Query OK, 1 row affected (0.00 sec)
MariaDB [mysql]> CREATE TABLE Places_clientOne.someTable (name varchar(32));
Query OK, 0 rows affected (0.02 sec)
MariaDB [mysql]> CREATE TABLE Places_clientTwo.someTable (name varchar(32));
Query OK, 0 rows affected (0.02 sec)
MariaDB [mysql]> INSERT INTO Places_clientOne.someTable VALUES('text in Places_clientOne.someTable');
Query OK, 1 row affected, 1 warning (0.00 sec)
MariaDB [mysql]> INSERT INTO Places_clientTwo.someTable VALUES('text in Places_clientTwo.someTable');
Query OK, 1 row affected, 1 warning (0.01 sec)
MariaDB [mysql]> use Friends_clientOne;
Database changed
MariaDB [Friends_clientOne]> DELIMITER //
MariaDB [Friends_clientOne]> CREATE PROCEDURE getPlace (OUT param1 char)
-> BEGIN
-> SELECT CONCAT("Select * from Places_", SUBSTRING_INDEX(DATABASE(), '_', -1),".someTable;") INTO #sql;
-> PREPARE getPlaces from #sql;
-> EXECUTE getPlaces;
-> DEALLOCATE PREPARE getPlaces;
-> END;
-> //
Query OK, 0 rows affected (0.03 sec)
MariaDB [Friends_clientOne]> DELIMITER ;
MariaDB [(none)]> use Friends_clientTwo;
Database changed
MariaDB [Friends_clientTwo]> DELIMITER //
MariaDB [Friends_clientTwo]>
MariaDB [Friends_clientTwo]> CREATE PROCEDURE getPlace (OUT param1 char)
-> BEGIN
-> SELECT CONCAT("Select * from Places_", SUBSTRING_INDEX(DATABASE(), '_', -1),".someTable;") INTO #sql;
-> PREPARE getPlaces from #sql;
-> EXECUTE getPlaces;
-> DEALLOCATE PREPARE getPlaces;
-> END;
-> //
Query OK, 0 rows affected (0.02 sec)
MariaDB [Friends_clientTwo]> DELIMITER ;
MariaDB [Friends_clientTwo]> call getPlace(#r);
+----------------------------------+
| name |
+----------------------------------+
| text in Places_clientTwo.someTab |
+----------------------------------+
1 row in set (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
MariaDB [Friends_clientTwo]> use Friends_clientOne;
Database changed
MariaDB [Friends_clientOne]> call getPlace(#r);
+----------------------------------+
| name |
+----------------------------------+
| text in Places_clientOne.someTab |
+----------------------------------+
1 row in set (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
MariaDB [Friends_clientOne]>