I need to Add & Delete merged tables in the UNION=() line. According to the MySQL docs it says:
DROP the MERGE table and re-create it.
Use ALTER TABLE tbl_name UNION=(...) to change the list of
underlying tables.
The only "DROP" I'm aware of is DROP TABLE tablename; Are these instructions suggesting that I drop the MRG_MyISAM table, then recreate it with an empty UNION=() field? To then be followed by an ALTER TABLE tbl_name UNION=(...) with all the tables I need to have connected?
If possible, could you post an example of the commands?
Thanks
Oh boy, am I late here. But this page is in the top google search results for "alter table tbl_name union=(...)". So I guess it needs an answer
So here's the answer.
To change the union list of underlying tables for merge table you only need to execute this statement
alter table tbl_name union=(`t1`,`t2`,`t3`);
where t1,t2,t3 is a list of tables you want to have in a union.
You can drop merge table and recreate it with a new list of underlying tables.
Drop statement execute on merge table will only delete the merge table itself and won't affect underlying tables.
But altering it should be sufficient. And you don't need to recreate it with empty union, if you ever do that, just use list of tables that you want to have in it.
For more, please refer to documentation:
https://dev.mysql.com/doc/refman/5.7/en/merge-storage-engine.html
Related
Some databases, like MySQL [1] and PostgreSQL [2], support bundling of certain compatible ALTER TABLE statements (as non-standard SQL).
For example we can have:
ALTER TABLE `my_table`
DROP COLUMN `column_1`,
DROP COLUMN `column_2`,
...
or
ALTER TABLE
MODIFY `column_1` ... ,
MODIFY `column_2` ... ,
instead of having individual statements:
ALTER TABLE `my_table` DROP COLUMN `column_1`;
ALTER TABLE `my_table` DROP COLUMN `column_2`;
or
ALTER TABLE `my_table` MODIFY `column_1` ... ;
ALTER TABLE `my_table` MODIFY `column_2` ... ;
etc
For comparison of the same feature, PostgreSQL [2], which also implements this, will perform all operations in a single scan:
The main reason for providing the option to specify multiple changes in a single ALTER TABLE is that multiple table scans or rewrites can thereby be combined into a single pass over the table.
Although for DROP COLUMN specifically it will often not even need do that:
The DROP COLUMN form does not physically remove the column, but simply makes it invisible to SQL operations...
Questions:
Would the multi-column statement result in traversing all the rows just once and performing all changes needed?
How does MySQL actually perform DROP COLUMN? Does it also "hide" the columns first, or does it delete the data straight away?
Assumptions:
Using InnoDB
No indexes/complex defaults are involved in any of the columns we want to change/drop (so basically changes that would not require a temporary table when run as individual alter statements)
References:
[1] MySQL ALTER TABLE docs
[2] PostgreSQL ALTER TABLE docs
MySQL's InnoDB:
(This does not really answer the Questions, but provides a little more insight in the the bigger question of ALTER.)
If any of the alters needs to copy the table over, you are probably better off putting all alters into the same statement. Changing the PRIMARY KEY, for example, requires rebuilding the data that is clustered with the PK.
Some alters can be achieved by simply altering the schema; these are virtually instantaneous, and could be done via separate alter statements. Adding an option to ENUM was implemented long ago.
Some alters need some form of scan, but can do it "in the background". DROP INDEX can be done by quickly "hiding" it, then freeing up the BTree in the background.
I have left out a grey area in which you batch 'simple' alters. One would hope that ALTER is smart enough to simply go through them quickly, rather than deciding to copy the table over.
I got some useful feedback but decided to respond to my own question to provide a more concrete set of answers.
Would the multi-column statement result in traversing all the rows just once and performing all changes needed?
Yes, if the alter statement results in rebuilding the table then it only needs to do it once.*
* This answer comes from my own testing and other mostly anecdotal evidence (including #Uueerdo 's in this post). It would be useful to have some official docs for this...
How does MySQL actually perform DROP COLUMN? Does it also "hide" the columns first, or does it delete the data straight away?
MySQL will rebuild the table in place (rather than create a copy or just change metadata) for most column operations. Each specific case can be found in the Online DDL docs for InnoDB.
A few operations like renaming a column or setting a default value will just alter metadata, so they don't require a table rebuild.
However, dropping a column DOES require a full table rebuild.
I am under the impression that the MySQL command DROP TABLE User will just remove the data, columns, and relevant constraints. However, the table shell still exists. Is this correct?
Using DROP TABLE will remove the entire table and all records contained inside it. If you want to retain the table structure but remove all data, then consider using TRUNCATE TABLE. Truncating a table is implemented by dropping the entire table and then recreating it. This is faster than doing DELETE FROM yourTable, which removes records one-by-one.
After Drop Table, the table will not exist anymore, so no data and no table definition(which you called 'table shell'); TRUNCATE TABLE keep your table definition and delete all the data ,and reset table auto-increment as well, but be careful about TRUNCATE if the table size is huge, it will expand your tablespace and not easy to shrink.
As mysql manual on drop table says:
Be careful with this statement! It removes the table definition and all table data.
So, no shell (whatever that should mean) remains after dropping a table.
What do you mean by table shell?
From MqSQL dev site:
DROP TABLE removes one or more tables. You must have the DROP privilege for each table.
Be careful with this statement! It removes the table definition and all table data. For a partitioned table, it permanently removes the table definition, all its partitions, and all data stored in those partitions. It also removes partition definitions associated with the dropped table.
I have two MySQL database k_db1 and k_db2 on a single server.
In k_db1, I have k_db1.table1 and k_db1.table2.
In k_db2, I have k_db2.table3 and k_db2.table4.
I want to create a third database k_db3 where I copy/paste tables of others databases.
It will result in k_db3.db1-table1, k_db3.db1-table2, k_db3.db2-table3, k_db3.db2-table4. I want to transfer data, indexes etc... and I don't want to delete k_db1 and k_db2 tables in the process. It must duplicate datas.
Do you know a way to do this just with SQL command?
Thanks in advance for your help.
You can try something like this:
DROP TABLE IF EXISTS k_db3.db1_table_1;
CREATE TABLE k_db3.db1_table_1 AS
SELECT * FROM db1.table_1;
Then you can recreate the indexes on the new table via ALTER TABLE statements.
Also I would avoid using - in table names.
Is there a way to do this?
In case the DBMS command history got cleaned or, in my case, when many ALTER TABLE were used in the course of time.
I'm using MySQL.
Yes, it is as simple as
SHOW CREATE TABLE yourtable;
This will include all the subsequent ALTER TABLE statements. You cannot retrieve the table's original state.
Here is the relevant documentation
Is there a way to rename a column on an InnoDB table without a major alter?
The table is pretty big and I want to avoid major downtime.
Renaming a column (with ALTER TABLE ... CHANGE COLUMN) unfortunately requires MySQL to run a full table copy.
Check out pt-online-schema-change. This helps you to make many types of ALTER changes to a table without locking the whole table for the duration of the ALTER. You can continue to read and write the original table while it's copying the data into the new table. Changes are captured and applied to the new table through triggers.
Example:
pt-online-schema-change h=localhost,D=databasename,t=tablename \
--alter 'CHANGE COLUMN oldname newname NUMERIC(9,2) NOT NULL'
Update: MySQL 5.6 can do some types of ALTER operations without rebuilding the table, and changing the name of a column is one of those supported as an online change. See http://dev.mysql.com/doc/refman/5.6/en/innodb-create-index-overview.html for an overview of which types of alterations do or don't support this.
If there aren't any constraints on it, you can alter it without a hassle as far as I know. If there are you'll have to remove the constraints first, alter and add the constraints back.
Altering a table with many rows can take a long time (though if the columns involved are not indexed, it may be trivial).
If you specifically want to avoid using the ALTER TABLE syntax created specifically for that purpose, you can always create a table with almost the exact same structure (but different name) and copy all the data into it, like so:
CREATE TABLE `your_table2` ...;
-- (using the query from SHOW CREATE TABLE `your_table`,
-- but modified with your new column changes)
LOCK TABLES `your_table` WRITE;
INSERT INTO `your_table2` SELECT * FROM `your_table`;
RENAME TABLE `your_table` TO `your_table_old`, `your_table2` TO `your_table`;
For some ALTER TABLE queries, the above can be quite a bit faster. However, for a simple column name change, it could be trivial. I might try creating an identical table and performing the change on it in order to see how much time you're actually looking at.