#MYSQL ALTER TABLE ADD COLUMN UPDATE#
UPDATE t SET customercode_new = 'X'||customercode::text ĪLTER TABLE t RENAME COLUMN customercode_new TO customercode It can be done in a couple of steps to avoid the long lock, but applications must be able to cope with the temporary duplication: ALTER TABLE t ADD COLUMN customercode_new text but that would lock the whole table for the re-write. For example, if you have table t and you want to change column customercode integer NOT NULL to text because the customer has decided all customer codes must now begin with an X, you could write: ALTER TABLE t ALTER COLUMN customercode TYPE text USING ( 'X'||customercode::text ) Some normally-slow operations can be sped up to be safe to perform without downtime. The lock levels required by various commands are documented in the locking page. If in doubt, you can generally just try it! All DDL in PostgreSQL is transactional, so it's quite fine to cancel an ALTER TABLE if it takes too long and starts holding up other queries. However, this lock can be quite brief if the table doesn't require re-writing, no new UNIQUE, CHECK or FOREIGN KEY constraints need expensive full-table scans to verify, etc. When you issue an ALTER TABLE in PostgreSQL it will take an ACCESS EXCLUSIVE lock that blocks everything including SELECT. has anyone ever used it in something resemblig production? seems like a nice way to automate a the set of "hacky" ways to do it. Someone just suggested Online Schema Change for MySQL from Facebook script (with a tutorial here and source here). (And, yes, anytime I had to do this before I did it "the right way", backing things up, scheduling downtine etc.but I just want to know if it's possible to do this sort and things "quick and dirty" or if there is any DB system that actually has support for "quick, live and dirty" schema changes) on a live running database?Īgain, I'm mostly referring to Postgres or MySQL as these are what I encounter. does any current database system support doing these things "on-line" without stopping anything? (maybe just delaying the queries that reference a column that is just being changed/deleted)Īnd what does it happen when I just do an ALTER TABLE. I know the correct way is to backup everything schedule downtime and do then do the changes.īut. How about adding columns with NOT NULL constraintĪs we have seen above, when we add a column, the column will be emptyĪnd we need to issue UPDATE statement to fill the values in that column.How do most "popular" (MySQL, Postgres.) database system handle altering tables on live production databases (like adding, deleting or changing the type of colums)? If you want to add multiple columns to employee table you canįor example, to add addr, pin, ph, fax to employees table
#MYSQL ALTER TABLE ADD COLUMN HOW TO#
Like update emp set city='New York' where empno=7782 update emp set city='Dallas' where empno=7934 How to alter table add multiple columns to an existing table To fill the values in CITY column we need to use UPDATE command to The following command alter table emp add city varchar2(20) Īfter issuing the above command the table structure will look like Now we want to add a column "city" to this table. Let's say suppose we have a table by name "emp" with the following EXAMPLES How to add a single column to an existing table. Users table only if the table's owner has granted you the permission to You can add columns to an table using ALTER TABLE command only
WeĬan even specify NOT NULL clause as well as DEFAULT clause. Using this command we can add a single column or multiple columns at once. We use ALTER TABLE ADD COLUMN command to add columns to an existing table.
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