When adding a column to a MyISAM table, MySQL will Create>Copy>Drop>Rename. I have a table which is sufficiently large that it cannot be duplicated in the server's remaining free space. Attempting to add a column then results in the disk filling. Time is not an issue here, neither is table availability, just disk space.
Short of copying the data to another server, truncating, altering, and copying back, is there a way to add a column to a MyISAM table without it creating a temporary table and duplicating all the data?
You could create a new, empty table, manually move the data over in chunks (INSERT into new, DELETE from old a certain number of rows), drop the old table and rename the new table. Essentially what MySQL does, but moving the data over instead of copying it, which should allow you to use less space.
In general I try to keep enough free space to be able to rebuild my largest table. This allows me to run OPTIMIZE TABLE or ALTER TABLE when necessary.
Do you have another disk mounted on your server?
When space is too tight to rebuild a given table, my preferred workaround is to temporarily move some other table(s) to a separate disk volume. I do this by stopping MySQL, moving the relevant data files (MYD and MYI, or ibd), and then creating a symlink at the original location that points at the new location, and starting MySQL. Once the table rebuild is done I reverse that process to move the other table(s) back to their original location.
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I was wondering what happens to the binlog when run an alter using pt-online-schema-change or gh-ost?
for the pt-online-schema-change I have read that it copies the table and use some triggers to apply the changes. I don't know if it create a table from the beginning with the new schema or it just apply the alter after copying the table?
if it alters the table from the beginning, then what happens to binglog?
is the positions different than the previous binglog?
pt-online-schema change copies the table structure and applies the desired ALTER TABLE to the zero-row table. This is virtually instantaneous. Then it creates triggers to mirror changes against the original table. Then it starts copying old data from the original table to the new table.
What happens to the binlog? It gets quite huge. The CREATE TABLE and ALTER TABLE and CREATE TRIGGER are pretty small. DDL is always statement-based in the binlog. The DML changes created by the triggers and the process of copying old data become transactions in the binlog. We prefer row-based binlogs, so these end up being pretty bulky.
gh-ost is similar, but without the triggers. gh-ost reads the binlog to find events that applied to the old table, and it applies those to the new table. Meanwhile, it also copies old data. Together these actions result in a similar volume of extra events in the binlog as occur when using pt-online-schema-change.
So you should check the amount of free disk space before you begin either of these online schema change operations. It will expand the binlogs approximately in proportion to the amount of data to be copied. And of course you need to store two copies of the whole table — the original and the altered version — temporarily, until the original table can be dropped at the end of the process.
I have had to run pt-online-schema change on large tables (500GB+) when I had a disk that was close to being full. It causes some tense moments. I had to PURGE BINARY LOGS periodically to get some more free space, because the schema change would fill the disk to 100% if I didn't! This is not a situation I recommend.
I have very large mysql DB(more than 100 G) and I want to migrate some changes in table. It need to posing that I have no space to make backup for this size, as results, it rolls back all changes. In clear way, when I want to alter table in mysql, it backup for it self in local disk and because of no space on my disk, it 's rolling back. Is there any one to help for this issue?
It depends on the type of ALTER change. Since MySQL 5.6, some alterations can be done "inplace" which does not use extra storage. Read https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl-operations.html for details.
But many types of alterations (basically any change that affects the storage size of a row), still require a "table rebuild" which means it makes a copy of the table in the process of doing the alter. This requires extra storage, usually about the same as the size of the original table. If your server does not have enough storage space to do this, then you cannot alter the table, FULL STOP. You should have addressed this long before the table grew to this size. It is your responsibility to monitor the size of the database and make sure you have enough storage space.
You said in the comments above that it is not possible to add more storage space.
Sometimes InnoDB tablespaces can become "fragmented." That means they could store the same rows using slightly less storage space. You can accomplish defragmentation with OPTIMIZE TABLE <name>; or ALTER TABLE <name> FORCE;
A possible strategy is to optimize each of your tables, starting with the smallest table and doing one by one in order of size. You hope that you have enough space for the copy table needed to do this, and any space freed up by optimizing the small tables will help you to optimize the medium-size tables, and so on. This is a long shot, because it's also possible you don't have enough "wasted" space to recover to make a difference. You may still not be able to alter your largest tables once you're all done with this process. Unfortunately, MySQL has no way of reporting fragmentation of the tables, so you can only try it and see how much space, if any, you recover.
After that, the only other option is to delete data from the table until you can do the alteration. The alter still requires extra storage, but only as much as needed to copy the remaining rows of data. That is, if you delete 50% of the rows, then the tablespace file that stores the copy only needs roughly 50% of the storage space.
You can also try dropping some of the indexes of the table as you do your alteration. Indexes take space in the same tablespace, and it's not uncommon if you have multiple indexes for them to take more space than the data itself. If you include some DROP INDEX operations in your ALTER TABLE statement, the resulting copy will take less space. On the other hand, those indexes might be necessary for proper query optimization, and without the indexes, your application won't be usable.
Is there a way to perform ALTER TABLE in MySQL, telling the server to skip creating a backup of the table first? I have a backup of the table already and I'm doing some tests on it (adding indexes), so I don't care if the table gets corrupted in the process. I'll just restore it from the backup. But what I do care about is for the ALTER TABLE to finish quickly, so I can see the test results.
Given that I have a big MyISAM table (700 GB) it really isn't an option to wait for couple of hours so that MySQL can first finish creating a backup of the original table, before actually adding an index to it.
It's not doing a backup; it is building the new version. (The existing table serves as a backup in case of a crash.)
With InnoDB, there are many flavors of ALTER TABLE -- some of which take essentially zero time, regardless of the size of the table. MyISAM (mostly) does the brute force way: Create an empty table with the new schema; copy all the data and build all the indexes; swap tables. For some alters, InnoDB must also do the brute force way: Example changing the PRIMARY KEY.
I have a 12 GB table full of pictures, I'm trying to rename the blob column that holds the data, and it is taking forever. Can someone give me a blow by blow account of why it is taking so long to rename the column? I would have thought that this operation would be pretty quick, no matter the size of the table?
EDIT: The query I ran is as follows
alter table `rails_production`.`pictures` change `data` `image_file_data` mediumblob NULL
It appears that most of the time is spent waiting for mysql to make a temporary copy of the pictures table, which since it is very large is taking a while to do.
It is on the list of things to do, to change the picture storage from the database to the filesystem.
EDIT2: Mysql Server version: 5.0.51a-24+lenny2 (Debian)
I can't give you the blow-by-blow (feature request #34354 would help, except that it probably wouldn't be back-ported to MySQL 5.0), but the extra time is due to the fact that an ALTER ... CHANGE may change the type of the column (and column attributes, if any), which necessitates converting the values stored in the column and other checks. MySQL 5.0 doesn't include optimizations for when the new type and attributes are the same as the old. From the documentation for ALTER under MySQL 5.0:
In most cases, ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, and then the original table is deleted and the new one is renamed. While ALTER TABLE is executing, the original table is readable by other sessions. Updates and writes to the table are stalled until the new table is ready, and then are automatically redirected to the new table without any failed updates.
[...]
If you use any option to ALTER TABLE other than RENAME, MySQL always creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column).
Under 5.1, ALTER has some additional optimizations:
In some cases, no temporary table is necessary:
Alterations that modify only table metadata and not table data can be made immediately by altering the table's .frm file and not touching table contents. The following changes are fast alterations that can be made this way:
Renaming a column, except for the InnoDB storage engine.
[...]
Because MySQL will rebuild the entire table when you make schema changes.
This is done because it's the only way of doing it in some cases, and it makes it much easier for the server to rebuild it anyway.
Yes mysql does a temporary copy of the table. I don't think there's an easy way around that. You should really think about to store the pictures on the filesystem and only store paths in mysql. That's the only way to fasten it up, I guess.
I have a 12 GB table full of pictures, I'm trying to rename the blob column that holds the data, and it is taking forever. Can someone give me a blow by blow account of why it is taking so long to rename the column? I would have thought that this operation would be pretty quick, no matter the size of the table?
EDIT: The query I ran is as follows
alter table `rails_production`.`pictures` change `data` `image_file_data` mediumblob NULL
It appears that most of the time is spent waiting for mysql to make a temporary copy of the pictures table, which since it is very large is taking a while to do.
It is on the list of things to do, to change the picture storage from the database to the filesystem.
EDIT2: Mysql Server version: 5.0.51a-24+lenny2 (Debian)
I can't give you the blow-by-blow (feature request #34354 would help, except that it probably wouldn't be back-ported to MySQL 5.0), but the extra time is due to the fact that an ALTER ... CHANGE may change the type of the column (and column attributes, if any), which necessitates converting the values stored in the column and other checks. MySQL 5.0 doesn't include optimizations for when the new type and attributes are the same as the old. From the documentation for ALTER under MySQL 5.0:
In most cases, ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, and then the original table is deleted and the new one is renamed. While ALTER TABLE is executing, the original table is readable by other sessions. Updates and writes to the table are stalled until the new table is ready, and then are automatically redirected to the new table without any failed updates.
[...]
If you use any option to ALTER TABLE other than RENAME, MySQL always creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column).
Under 5.1, ALTER has some additional optimizations:
In some cases, no temporary table is necessary:
Alterations that modify only table metadata and not table data can be made immediately by altering the table's .frm file and not touching table contents. The following changes are fast alterations that can be made this way:
Renaming a column, except for the InnoDB storage engine.
[...]
Because MySQL will rebuild the entire table when you make schema changes.
This is done because it's the only way of doing it in some cases, and it makes it much easier for the server to rebuild it anyway.
Yes mysql does a temporary copy of the table. I don't think there's an easy way around that. You should really think about to store the pictures on the filesystem and only store paths in mysql. That's the only way to fasten it up, I guess.