I need to rename two tables in one atomic operation so that user will never be able to see the database in its intermediate state.
I'm using MySQL and noticed that this case is perfectly described in the documentation:
13.3.3 Statements That Cause an Implicit Commit
The statements listed in this section (and any synonyms for them)
implicitly end any transaction active in the current session, as if
you had done a COMMIT before executing the statement
[...]
Data definition language (DDL) statements that define or modify
database objects. ALTER DATABASE ... UPGRADE DATA DIRECTORY NAME,
ALTER EVENT, ALTER PROCEDURE, ALTER SERVER, ALTER TABLE, ALTER VIEW,
CREATE DATABASE, CREATE EVENT, CREATE INDEX, CREATE PROCEDURE, CREATE
SERVER, CREATE TABLE, CREATE TRIGGER, CREATE VIEW, DROP DATABASE, DROP
EVENT, DROP INDEX, DROP PROCEDURE, DROP SERVER, DROP TABLE, DROP
TRIGGER, DROP VIEW, INSTALL PLUGIN (as of MySQL 5.7.6), RENAME TABLE,
TRUNCATE TABLE, UNINSTALL PLUGIN (as of MySQL 5.7.6).
But maybe there's some kind of workaround or something like this?
My situation looks like this:
I have a current data set in the table named current
I gathered a new data set in the table named next
I need to rename the current table to the current_%current_date_time% and the next table to the current in one atomic operation
Well, easy...
RENAME TABLE current TO current_20151221, next TO current;
as is stated in the manual. There it says that it's an atomic operation. Just to clear this up, implicit commits have nothing to do with it. That's a different story. That just says, that those statements end an open transaction.
Related
I'm using a transaction to make alterations to my database, this way if anything fails during the alterations i can roll back without any harm having been done.
However, since i run my queries based on a list of queries that puts my database in it's final state (any time i need to make changes, i simply add a new rule to the list), it fails when i try to drop a column that was added during the same transaction.
Example:
START TRANSACTION;
ALTER TABLE "servers" ADD COLUMN "test" INTEGER NOT NULL;
ALTER TABLE "servers" DROP COLUMN "test";
COMMIT;
When I run this, i get something along the lines of
column "test" in the middle of being added, try again later
I understand why this is happening, since the transaction hasn't been committed yet the column doesn't exist to be dropped. However is there a way around this specifically so that I can drop the column in the same transaction? Or at least queue it to be deleted once the transaction is committed.
I feel it worth noting that the queries being run within the transaction are generated using Eloquent ORM Blueprints.
MySQL doesn't support table changes in transactions. From the documentation:
Some statements cannot be rolled back. In general, these include data definition language (DDL) statements, such as those that create or drop databases, those that create, drop, or alter tables or stored routines.
I want be able to debug easily my scripts in Mysql, like in MSSQL (run a chunk of the script then verify the tables and so on), but the temporary tables are not persisted on the server.
For example :
CREATE temporary table a(i int);
INSERT INTO a VALUE (1);
SELECT * FROM a;
If I run the whole script it returns me the right result, but if I run it statement by statement on the insert I get the following error:
SQL.sql: Error (2,13): Table 'test.a' doesn't exist
I suppose this is a server configuration problem.
Temporary tables are dropped when the transaction is over.
from dev.mysql:
Temporary Tables:
You can use the TEMPORARY keyword when creating a
table. A TEMPORARY table is visible only to the current connection,
and is dropped automatically when the connection is closed. This means
that two different connections can use the same temporary table name
without conflicting with each other or with an existing non-TEMPORARY
table of the same name. (The existing table is hidden until the
temporary table is dropped.) To create temporary tables, you must have
the CREATE TEMPORARY TABLES privilege.
Note CREATE TABLE does not automatically commit the current active
transaction if you use the TEMPORARY keyword.
So if you run all these sql in deferent transactions you temporary table wont exist when you run the insert statement.
If these executions are executed in diferent transactions depend on what interface you use. Thats wy if you "run the whole script it returns me the right result" because its all in the same transaction.
You can try to force it to run on the same transaction with:
START TRANSACTION;
<SQL QUERYS>
COMMIT;
anyway i recomend you MySQL Workbench as interface.
my best regards, i hope this help you.
I'm doing an update to MySQL Database which includes MySQL scripts that make ALTER TABLE sentences, as well as DIU sentences (delete, insert, update).
The idea is to make a transactional update, so if a sentence fails, a rollback is made, but if I put ALTER TABLE sentences or others specified in http://dev.mysql.com/doc/refman/5.0/en/implicit-commit.html an implicit commit is made, so I can't make a complete rollback, because the indicated operations remains commited.
I tried to use mysqldump to make a backup which is used in case of error (mysql returns distinct to zero), but it is too slow and can fail too.
What can I do? I need this to ensure that future updates are safe and not too slow, because databases contains between 30-100 GB of data.
dump and reload might be your best options instead of alter table.
From mysql prompt or from the database script:
select * from mydb.myt INTO OUTFILE '/var/lib/mysql/mydb.myt.out';
drop table mydb.myt;
create tablemyt(your table ddl here)
load data infile '/var/lib/mysql/mydb.myt.out' INTO TABLE mydb.myt;
Check this out:
http://everythingmysql.ning.com/profiles/blogs/whats-faster-than-alter
I think it offers good guidance on "alternatives to alter".
Look at pt-online-schema change.
You can configure it to leave the 'old' table around after the online ALTER is completed. The old table will have an underscore prefix. If bad things happen, drop the tables you altered and renamed the OLD tables to the original tables. If everything is OK, then just drop the OLD tables.
http://www.percona.com/doc/percona-toolkit/2.1/pt-online-schema-change.html
I noticed that the concept of temporary tables in these two systems is different, and I have a musing.. I have the following scenario in MySQL:
Drop temporary table 'a' if exists
Create temporary table 'a'
Populate it with data through a stored procedure
Use the data in another stored procedure
How can I implement the same scenario in Oracle? Can I (in one procedure preferable) create a temporary table, populate it, and insert data in another (non-temporary) table?
I think that I can use a (global) temporary table which truncates on commit, and avoid steps 1&2, but I need someone else's opinion too.
In Oracle, you very rarely need a temporary table in the first place. You commonly need temporary tables in other databases because those databases do not implement multi-version read consistency and there is the potential that someone reading data from the table would be blocked while your procedure runs or that your procedure would do a dirty read if it didn't save off the data to a separate structure. You don't need global temporary tables in Oracle for either of these reasons because readers don't block writers and dirty reads are not possible.
If you just need a temporary place to store data while you perform PL/SQL computations, PL/SQL collections are more commonly used than temporary tables in Oracle. This way, you're not pushing data back and forth from the PL/SQL engine to the SQL engine and back to the PL/SQL engine.
CREATE PROCEDURE do_some_processing
AS
TYPE emp_collection_typ IS TABLE OF emp%rowtype;
l_emps emp_collection_type;
CURSOR emp_cur
IS SELECT *
FROM emp;
BEGIN
OPEN emp_cur;
LOOP
FETCH emp_cur
BULK COLLECT INTO l_emps
LIMIT 100;
EXIT WHEN l_emps.count = 0;
FOR i IN 1 .. l_emps.count
LOOP
<<do some complicated processing>>
END LOOP;
END LOOP;
END;
You can create a global temporary table (outside of the procedure) and use the global temporary table inside your procedure just as you would use any other table. So you can continue to use temporary tables if you so desire. But I can count on one hand the number of times I really needed a temporary table in Oracle.
You are right, temporary tables will work work you.
If you decide stick with regular tables you may want to use the advice #Johan gave, along with
ALTER TABLE <table name> NOLOGGING;
to make this perform a bit faster.
I see no problem in the scheme your are using.
Note that it doesn't have to be a temp-table, you can use a sort of kind of memory table as well.
Do this by creating a table as usual, then do
ALTER TABLE <table_name> CACHE;
This will prioritize the table for storage in memory.
As long as you fill and empty the table in short order you don't need to do step 1 & 2.
Remember the cache modifier is just a hint. The table still ages in the cache and will be pushed out of memory eventually.
Just do:
Populate cache-table with data through a stored procedure
Use the data in another stored procedure, but don't wait to long.
2a. Clear the data in the cache table.
In your MySQL version, I didn't see a step 5 to drop the table a. So, if you want or don't mind having the data in the table persist you could also use a materialized view and simply refresh on demand. With a materialized view you do not need to manage any INSERT statements, just include the SQL:
CREATE MATERIALIZED VIEW my_mv
NOCACHE -- NOCACHE/CACHE: Optional, cache places the table in the most recently used part of the LRU blocks
BUILD IMMEDIATE -- BUILD DEFERRED or BUILD IMMEDIATE
REFRESH ON DEMAND
WITH PRIMARY KEY -- Optional: creates PK column
AS
SELECT *
FROM ....;
Then in your other stored procedure, call:
BEGIN
dbms_mview.refresh ('my_mv', 'c'); -- 'c' = Complete
END;
That said, a global temporary table will work as well, but you manage the insert and exceptions.
using MySQL 5.1.36, I am trying to write trigger which drops scratch tables form "scratch" database.
CREATE DEFINER=`root`#`localhost` TRIGGER
`jobq`.`DropScratch`
BEFORE DELETE ON jobq.jobq FOR EACH ROW
BEGIN
DECLARE tblname VARCHAR(128);
set tblname=concat('scratch.',OLD.jobname);
DROP TABLE IF EXISTS tblname;
END;
I am always getting an error:
Explicit or implicit commit is not allowed in stored function or trigger.
Can I somehow overcome this restriction?
Thank you beforehand
Arman
The primary problem here is that you are not allowed to drop a table within a trigger. That's what the error message is getting at when it says "implicit commit" is not allowed. The drop table does an implicit commit.
So you will need to figure out a different way to do this other than a trigger. One way would be to set up a cron job which compares the data in information_schema.tables to the jobq table to look for tables in the scratch DB that can be dropped, and then drop them.
I should also point out that the way you are trying to dynamically create a drop table statement will not work. That is going to drop a table named literally "tblname", not "scratch.jobname". If you want to drop a table dynamically you will need to build the drop table statement in a separate scripting language, such as python, perl, shell, etc.
Good luck!