For our system we are using multiple databases with the same structure. For example when we have 1000 customers, there will be 1000 databases. We've chosen to give each customers his own database, so we can delete all his data at once without any hassle.
Now I have to update the database structure several times a year. So I began to write a stored procedure which loops through all schemas. But I got stuck with executing a dynamic USE statement.
My code is as follows:
DECLARE V_SCHEMA VARCHAR(100);
SET V_SCHEMA = 'SomeSchemaName';
SET #QUERYSTRING = CONCAT('USE ', V_SCHEMA);
PREPARE S FROM #QUERYSTRING;
EXECUTE S;
DEALLOCATE PREPARE S;
When I execute this code I get an error which says Error Code: 1295. This command is not supported in the prepared statement protocol yet. So I assume that I cannot change the active database in a procedure.
I have searched the internet, but the only thing I found was creating a string of each alter query and prepare/execute/deallocate it. I hope there is a better solution for this. I could write a shell script that loops through the schemas and executes a SQL file on them, but I prefer a stored procedure that takes care of this.
Does anyone know how to make this work?
Thank you for your help!
EDIT: I use the latest stable version of MySQL 5.6
If there are some known databases, then try to write a CASE.
Otherwise, do not execute USE statement using prepared statements; instead, build other statements (SELECT, INSERT, UPDATE, ...) with full name - <database name> + '.' + <object name>, and execute them using prepared statements.
If you put your structure changes into a stored procedure in a temporary schema, you can do this within a Workbench SQL window.
You can build your iteration script using a query against the information_schema, e.g.
SELECT GROUP_CONCAT(CONCAT('USE ',schema_name,'; CALL tmp.Upgrade')
SEPARATOR ';\n') AS BldCode
FROM information_schema.schemata
WHERE schema_name NOT IN
('information_schema', 'performance_schema', 'mysql', 'sakila', 'world', 'tmp')
Since you cannot execute this as a prepared statement, you can copy the SQL result into a new SQL window, and run that.
Please note that the structure changes stored procedure would need to operate on the current schema, rather than specifying schemas.
Related
The purpose of the trigger is exporting an specific data from a table with bcp after inserting data on that table, so I thought doing it in this way, I know that the trigger waits for bcp, that is waiting for a lock on the table to be released, but that lock is held until after the trigger, and for this reason it doesn't work. How can I do it? or do I need to add some function or something for that works?
I'm using SQL Server 2008.
ALTER TRIGGER [TRIGGER] on [TABLE] after INSERT AS BEGIN
DECLARE #CMD NVARCHAR(1000)
SET #CMD = 'cd.. && "C:\Program Files\Microsoft SQL Server\100\Tools\Binn\bcp.exe" "SELECT TOP 1 CODE FROM[TABLE] WITH (NOLOCK) ORDER BY ID DESC" queryout "\\FOLDER\FOLDER\FILE.txt" -T -c -S "[SERVERNAME]"'
EXEC master..XP_CMDSHELL #CMD
END
Don't use bcp in a trigger. Even if you could get it to work it will slow down your database, probably to the point where it will not be usable. bcp is a command line utility and should be treated as one
I recommend that you use an SQL Server Agent to execute extra actions on a scheduled or triggered basis
You can also read this tutorial that will help you get started with Agents
If you do not have SQL Server Agent (Express does not include it), then you have a few other options:
Write your own Agent. Here is an example
Call a stored procedure after insert. This answer used that method to solve similar problem to the one you posted
Use a stored procedure to write data and process the export code
Use a scheduled task processes data on a schedule (this is where bcp can be used without killing server performance)
Not sure if this would work but you could try changing to an INSTEAD OF trigger and do the INSERT and bcp within the trigger
I suspect this may circumvent the lock on the table.
further reading - http://www.c-sharpcorner.com/UploadFile/37db1d/creating-and-managing-triggers-in-sql-server-20052008/
Using the JDBC Java performs a stored procedure, but slowly, so want to check the database stored procedure execution plan, what do you think?Have hundreds of lines of SQL stored procedures, database is Mysql, I know to check the select statement execution plan is to use the explain, but how can a stored procedure?
You could use the mysql optimizer tracer if you have at least mysql v5.6.3.
Basic usage:
SET optimizer_trace="enabled=on";
SELECT ...; # your query here
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
# possibly more queries...
# When done with tracing, disable it:
SET optimizer_trace="enabled=off";
It can trace the following queries:
SELECT; INSERT or REPLACE (with VALUES or SELECT); UPDATE/DELETE and their multi-table variants; all the previous ones prefixed by EXPLAIN; SET (unless it manipulates the optimizer_trace system variable); DO; DECLARE/CASE/IF/RETURN (stored routines language elements); CALL. If one of those statements is prepared and executed in separate steps, preparation and execution are separately traced.
Try to use explain on the select etc. statements separately that are within the storeed procedure.
I have multiple databases with the same structure but different unrelated names, and I want to run this query on all of them:
SELECT FROM <dbname>.`cms_users` WHERE `email` LIKE '%admin.something%'
I searched a bit in the information_schema and found a table called SCHEMATA that contains the databases' names which is ultimately what I need to run the query above and I can do so manually by just replacing the database name myself and creating the query. However I want to do it automatically using a mysql loop but I'm not very certain how can I do this and how can I concatenate the database name to the query and run it.
My pseudo code for this would be as follows:
array dbnames= select `SCHEMA_NAME` from `information_schema`.`SCHEMATA`;
loop start on dbnames
SELECT FROM dbnames[index].`cms_users` WHERE `email` LIKE '%admin.bilsi%';
loop end
Any help to put this or a better logic into mysql syntax? Thanks.
SQL Server and other RDBMS products allow you do scripting in the console. You can use anything you can use in stored procedures. MySQL is unfortunately much more limited and does not allow flow control constructs outside of stored procedures and functions. That means no loops, no if-statements, no cursors. You can use variables, but only the ones that start with #.
Furthermore, if you do a loop, you'll be sending multiple result-sets back to the client. If you're just running queries from a console, this is fine. If the results are something you intend for a program to use, this may not be desirable (it may not be desirable in either case).
If you are doing this in a one-off sort of way, get a list of databases manually, and then use copy and paste to build a query using UNION ALL, like so:
SELECT FROM `first_db`.`cms_users` WHERE `email` LIKE '%admin.bilsi%'
UNION ALL
SELECT FROM `second_db`.`cms_users` WHERE `email` LIKE '%admin.bilsi%'
UNION ALL
SELECT FROM `third_db`.`cms_users` WHERE `email` LIKE '%admin.bilsi%';
If you expect the number of databases to be changing and you don't want to have to update your query, or you are sending it from a program, you can use dynamic SQL. This means building a query in a string variable and then submitting it using MySQL's prepared statement functionality.
On the console, you can use something like this (see: http://dev.mysql.com/doc/refman/5.0/en/sql-syntax-prepared-statements.html):
SELECT GROUP_CONCAT(CONCAT("SELECT FROM `", SCHEMA_NAME, "`.`cms_users` WHERE `email` LIKE '%admin.bilsi%'") SEPARATOR ' UNION ALL ')
INTO #stmt_sql
FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME NOT IN('mysql', 'test', 'tmp', 'information_schema', 'sys', 'performance_schema');
PREPARE stmt FROM #stmt_sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
This generates the query I showed above using information from the INFORMATION_SCHEMA pseudo-database, namely, the list of databases by name (which MySQL incorrectly calls schemas). The rest is just the boilerplate code needed to prepare and execute a prepared statement, as per the linked documentation.
There are other ways, but they are even more tedious and won't buy you much.
my query such as having mysql variable declaration
SET #var1=0, #var2=0;
these variables are used in the select query
which works fantastic in phpmyadmin
but then if i write it as query in yii doesnt work
throws exception doesnt not execute but then if i remove
SET #var1=0, #var2=0;
then query executes but with no data fetched from db because it requires the set variables to fetch the result
how do i declare the set values of mysql in yii?is there any way out
As long as you reuse the same CDbCommand, you can issue multiple queries to the DB using the same connection. That will do what you need (and is what phpMyAdmin does).
Your problem is that you're doing two queries on different connections to the DB and your #vars aren't lasting between connections.
If you have set statements, you're probably writing something that is a little more procedural than a single sql statement is designed to deliver.
I would look at write in a stored procedure to the the job (http://forums.mysql.com/read.php?98,358569). Although they are a bit old school - they will probably do what you want quite effectively.
I'm not a DB expert, but I've inherited some of the responsibility for a fairly large production MySQL DB from a guy who seems to have been somewhat to severely incompetent but with the occasional flares of brilliance. Most issues I've been able to sort out myself, but this one has me stumped and I haven't seen anything addressing it anywhere:
Is there any sane reason to have a stored procedure that is nothing more than a wrapper for a prepared statement?
Something along these lines:
CREATE PROCEDURE foo_search (IN searchParam1 int, IN searchParam2 varchar(255))
BEGIN
SET #local1 = searchParam1;
SET #local2 = searchParam2;
PREPARE stmt FROM
'...'; --Fairly complex nested select statement
EXECUTE stmt USING #local1, #local2;
END
And that's it. My understanding of prepared statements is that their benefit lies in sanitizing input (already handled by the PHP framework we use), and reducing communication back-and-forth (compromised by being within a stored proc).
Is this pure and simple pointless insanity as it appears, or am I missing something?
Mike listed two good reasons. Here are a couple more:
Reduces the amount of network traffic as CALL foo_search(searchParam1, searchParam2) will be less data than sending the entire SQL statement each time.
Preparing the statement on the server may improve performance. After benchmarking several different methods, we found that for complex SQL statements, preparing the statement only once, on the server, performed the best. Here's an example:
CREATE PROCEDURE foo_search (IN searchParam1 int, IN searchParam2 varchar(255))
BEGIN
SET #local1 = searchParam1;
SET #local2 = searchParam2;
IF ISNULL(#foo_search_prepared) THEN
SET #foo_search_prepared = TRUE;
PREPARE stmt FROM
'...'; --Fairly complex nested select statement
END IF;
EXECUTE stmt USING #local1, #local2;
END
There could be.
Perhaps the guy was using a DB connection library that doesn't support prepared statements, so he just wanted get an equivalent effect.
Perhaps he wanted to call that stored procedure from a trigger.