Our server gets slow, "To get data from mysql database". So I search for it on google. They told me, "Use INDEX for the select query to get data from the database it becomes more fastest execution".
Index is a small copy of a database table sorted by key values.
U need to create index first.
CREATE INDEX index_name ON table_name(column_name)
Then:
SELECT * FROM table_name
USE INDEX (index_name)
WHERE condition;
Related
There is my SQL query. Table system_mailer is for logging sent e-mails. When i want to search some data, query is 10 seconds long. It is possible on any way to optimize this query?
SELECT `ID`
FROM `system_mailer`
WHERE `group` = 'selling_center'
AND `group_parameter_1` = '1'
AND `group_parameter_2` = '2138'
Timins is around couple of seconds, how could it be optimised?
You might find the following index on 4 columns would help performance:
CREATE INDEX idx ON system_mailer (`group`, group_parameter_1, group_parameter_2, ID);
MySQL should be able to use this index on your current query. By the way, if you are using InnoDB, and ID is the primary key, then you might be able to drop it from the explicit index definition, and just use this:
CREATE INDEX idx ON system_mailer (`group`, group_parameter_1, group_parameter_2);
Please avoid naming your columns and tables with reserved MySQL keywords like group. Because you made this design decision, you will now be forced to forever escape that column name with backticks (ugly).
just be sure you have a composite index on table system_mailer for the columns
(`group`, `group_parameter_1`, `group_parameter_2`)
and you can use redudancy adding the id to index for avoid data table access in query
(`group`, `group_parameter_1`, `group_parameter_2`, ID)
I'm having trouble using a where clause on CREATE UNIQUE INDEX on mysql. I know you cannot just add where clause at the end of CREATE UNIQUE INDEX. Example below.
CREATE UNIQUE INDEX FAKE_TABLE_INDEX ON TABLE_NAME (COLUMN_NAME) WHERE INACTIVE = 0;
and this query above gives me an error. So is there alternative I can do to fix this query.
MySQL doesn't have filtered indexes. If I understand what they do (from reading the Microsoft docs) I think the closest analogous feature is multi-column indexes:
CREATE INDEX fake_table_index ON table_name (inactive, column_name);
This is more expensive than the filtered index because it indexes all the values of inactive, not just where inactive = 0.
This also doesn't have the unique constraint that the filtered index does. It's only useful for optimizing queries, not enforcing uniqueness. You'll have to do that with a trigger if you need it.
I'm using SugarCRM and a few weeks ago I executed a a query on MySQL which created an index to prevent duplicate rows. Where can I see that or find it and edit or delete this ? I'm not able to remember the exact query but it's needed to add more columns. Using MySQL only just a few weeks.
MySQL error 1062: Duplicate entry 'example-dyplicate' for key
'idx_name'
To see the structure of a table, including all the indexes, use:
SHOW CREATE TABLE tablename;
You can delete an index with:
DROP INDEX indexname ON tablename;
There's no way to edit an index. If you want to change an index, you drop it and then add a new index with the new columns you want. However, you can do both in a single query using ALTER TABLE:
ALTER TABLE tablename DROP INDEX indexname ADD INDEX indexname (col1, col2, ...);
I am using MySQL v5.1.
I would like to create index on a table by executing the following SQL statement:
CREATE INDEX index_name
ON table_name (column_name)
But, I wan to firstly check if the index on that column has already been created or not, if not, create it (otherwise do not create). What is the SQL syntax for this?
Mysql doesn't have IF NOT EXISTS for CREATE INDEX. You can work it out by querying information_schema.statistics table. Take a look here, there is an example of stored procedure that does what you are looking for (search for "CREATE INDEX IF NOT EXISTS" on the page)
You want SHOW INDEX.
To get all the indexes on a table:
SHOW INDEX FROM table_name
MySQL allows you to add a WHERE clause to limit the results as well.
Lock the table while you're checking to see if the index exists (and if it doesn't exist, creating the index) so that another process doesn't create the index right after you've checked for it but before you've created it yourself.
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?
I faced a very similar situation in the past and i improve the performance of the operation in this way :
Create a new table (using the structure of the current table) with the new column(s) included.
execute a INSERT INTO new_table (column1,..columnN) SELECT (column1,..columnN) FROM current_table;
rename the current table
rename the new table using the name of the current table.
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;
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.
Maybe you can remove the index before alter the table because what is take most of the time to build is the index?
Combining some of the comments on the other answers, this was the solution that worked for me (MySQL 5.6):
create table mytablenew like mytable;
alter table mytablenew add column col4a varchar(12) not null after col4;
alter table mytablenew drop index index1, drop index index2,...drop index indexN;
insert into mytablenew (col1,col2,...colN) select col1,col2,...colN from mytable;
alter table mytablenew add index index1 (col1), add index index2 (col2),...add index indexN (colN);
rename table mytable to mytableold, mytablenew to mytable
On a 75M row table, dropping the indexes before the insert caused the query to complete in 24 minutes rather than 43 minutes.
Other answers/comments have insert into mytablenew (col1) select (col1) from mytable, but this results in ERROR 1241 (21000): Operand should contain 1 column(s) if you have the parenthesis in the select query.
Other answers/comments have insert into mytablenew select * from mytable;, but this results in ERROR 1136 (21S01): Column count doesn't match value count at row 1 if you've already added a column.