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Optimize mySql for faster alter table add column

Posted by: admin November 1, 2017 Leave a comment

Questions:

I have a table that has 170,002,225 rows with about 35 columns and two indexes. I want to add a column. The alter table command took about 10 hours. Neither the processor seemed busy during that time nor were there excessive IO waits. This is on a 4 way high performance box with tons of memory.

Is this the best I can do? Is there something I can look at to optimize the add column in tuning of the db?

Answers:

I faced a very similar situation in the past and i improve the performance of the operation in this way :

  1. Create a new table (using the structure of the current table) with the new column(s) included.
  2. execute a INSERT INTO new_table (column1,..columnN) SELECT (column1,..columnN) FROM current_table;
  3. rename the current table
  4. rename the new table using the name of the current table.
Questions:
Answers:

ALTER TABLE in MySQL is actually going to create a new table with new schema, then re-INSERT all the data and delete the old table. You might save some time by creating the new table, loading the data and then renaming the table.

From “High Performance MySQL book” (the percona guys):

The usual trick for loading MyISAM table efficiently is to disable keys, load the data and renalbe the keys:

mysql> ALTER TABLE test.load_data DISABLE KEYS;
-- load data
mysql> ALTER TABLE test.load_data ENABLE KEYS;

Questions:
Answers:

Well, I would recommend using latest Percona MySQL builds plus since there is the following note in MySQL manual

In other cases, MySQL creates a
temporary table, even if the data
wouldn’t strictly need to be copied.
For MyISAM tables, you can speed up
the index re-creation operation (which
is the slowest part of the alteration
process) by setting the
myisam_sort_buffer_size system
variable to a high value.

You can do ALTER TABLE DISABLE KEYS first, then add column and then ALTER TABLE ENABLE KEYS. I don’t see anything can be done here.

BTW, can’t you go MongoDB? It doesn’t rebuild anything when you add column.

Questions:
Answers:

Maybe you can remove the index before alter the table because what is take most of the time to build is the index?