If you have existing tables, and applications that use them, that you want to convert to
for better reliability and scalability, use the following guidelines and tips. This section assumes most such
tables were originally
MyISAM, which was formerly the default.
As you transition away from
MyISAM tables, lower the value of the
key_buffer_size configuration option to free memory no longer needed for caching
results. Increase the value of the
innodb_buffer_pool_size configuration option, which performs a similar role
of allocating cache memory for
InnoDB tables. The
pool caches both table data and index data, so it does double duty in speeding up lookups for queries
and keeping query results in memory for reuse.
Allocate as much memory to this option as you can afford, often up to 80% of physical memory on the server.
If the operating system runs short of memory for other processes and begins to
swap, reduce the
innodb_buffer_pool_size value. Swapping is such an expensive
operation that it drastically reduces the benefit of the cache memory.
innodb_buffer_pool_size value is several gigabytes or higher, consider
increasing the values of
innodb_buffer_pool_instances. Doing so helps on busy servers where
many connections are reading data into the cache at the same time.
On a busy server, run benchmarks with the Query Cache turned off. The
InnoDB buffer pool provides similar benefits, so the Query Cache might be
tying up memory unnecessarily.
MyISAM tables do not support transactions, you
might not have paid much attention to the
autocommit configuration option and the
ROLLBACK statements. These keywords are important to allow multiple sessions
to read and write
InnoDB tables concurrently, providing substantial scalability
benefits in write-heavy workloads.
While a transaction is open, the system keeps a snapshot of the data as seen at the beginning of the transaction, which can cause substantial overhead if the system inserts, updates, and deletes millions of rows while a stray transaction keeps running. Thus, take care to avoid transactions that run for too long:
If you are using a mysql session for interactive experiments, always
COMMIT (to finalize the changes) or
ROLLBACK (to undo the changes) when finished. Close down interactive
sessions rather than leaving them open for long periods, to avoid keeping transactions open for long
periods by accident.
ROLLBACK is a relatively expensive operation, because
DELETE operations are written to
tables prior to the
COMMIT, with the expectation that most changes will be committed
successfully and rollbacks will be rare. When experimenting with large volumes of data, avoid making
changes to large numbers of rows and then rolling back those changes.
When loading large volumes of data with a sequence of
INSERT statements, periodically
COMMIT the results to avoid having transactions that last for hours. In
typical load operations for data warehousing, if something goes wrong, you
TRUNCATE TABLE and start over from the beginning rather than doing a
The preceding tips save memory and disk space that can be wasted during too-long transactions. When transactions
are shorter than they should be, the problem is excessive I/O. With each
COMMIT, MySQL makes sure each change is safely recorded to disk, which
involves some I/O.
For most operations on
InnoDB tables, you should use
From an efficiency perspective, this avoids unnecessary I/O when you issue large numbers of consecutive
statements. From a safety perspective, this allows you to issue a
ROLLBACK statement to recover lost or garbled data if you make a
mistake on the mysql command line, or in an exception handler in
The time when
autocommit=1 is suitable for
is when running a sequence of queries for generating reports or analyzing statistics. In this situation,
there is no I/O penalty related to
InnoDB can automatically
optimize the read-only workload.
If you make a series of related changes, finalize all those changes at once with a
COMMIT at the end. For example, if you insert related pieces of
information into several tables, do a single
COMMIT after making all the changes. Or if you run many consecutive
INSERT statements, do a single
COMMIT after all the data is loaded; if you are doing millions of
statements, perhaps split up the huge transaction by issuing a
COMMIT every ten thousand or hundred thousand records, so the
transaction does not grow too large.
You might see warning messages referring to "deadlocks" in
the MySQL error log, or the output of
INNODB STATUS. Despite the scary-sounding name, a deadlock is not a
serious issue for
InnoDB tables, and often does not require any corrective action.
When two transactions start modifying multiple tables, accessing the tables in a different order, they can reach
a state where each transaction is waiting for the other and neither can proceed. MySQL immediately detects this
condition and cancels (rolls back) the
"smaller" transaction, allowing the other to proceed.
Your applications do need error-handling logic to restart a transaction that is forcibly cancelled like this. When you re-issue the same SQL statements as before, the original timing issue no longer applies: either the other transaction has already finished and yours can proceed, or the other transaction is still in progress and your transaction waits until it finishes.
If deadlock warnings occur constantly, you might review the application code to reorder the SQL operations in a
consistent way, or to shorten the transactions. You can test with the
innodb_print_all_deadlocks option enabled to see all deadlock warnings in the
MySQL error log, rather than only the last warning in the
SHOW ENGINE INNODB STATUS output.
To get the best performance from
InnoDB tables, you can adjust a number of
parameters related to storage layout.
When you convert
MyISAM tables that are large, frequently accessed, and hold vital
data, investigate and consider the
innodb_page_size configuration options, and the
KEY_BLOCK_SIZE clauses of the
CREATE TABLE statement.
During your initial experiments, the most important setting is
innodb_file_per_table. Enabling this option before creating new
InnoDB tables ensures that the
tablespace files do not allocate disk space permanently for all the
DROP TABLE and
TRUNCATE TABLE free disk space as you would expect.
To convert a non-
InnoDB table to use
Do not convert MySQL system tables in the
mysql database (such as
host) to the
InnoDB type. This is an unsupported operation. The system tables must always
be of the
You might make an InnoDB table that is a clone of a MyISAM table, rather than doing the
ALTER TABLE conversion, to test the old and new table side-by-side before
Create an empty
InnoDB table with identical column and index definitions. Use
show create table to see the
CREATE TABLE statement to use. Change the
To transfer a large volume of data into an empty
InnoDB table created as shown in
the previous section, insert the rows with
INSERT INTO .
SELECT * FROM
myisam_table ORDER BY
You can also create the indexes for the
InnoDB table after inserting the data.
Historically, creating new secondary indexes was a slow operation for InnoDB, but now you can create the indexes
after the data is loaded with relatively little overhead from the index creation step.
If you have
UNIQUE constraints on secondary keys, you can speed up a table import
by turning off the uniqueness checks temporarily during the import operation:
... import operation ...SET unique_checks=1;
For big tables, this saves disk I/O because
InnoDB can use its insert
buffer to write secondary index records as a batch. Be certain that the data contains no duplicate keys.
unique_checks permits but does not require storage engines to ignore duplicate
To get better control over the insertion process, you might insert big tables in pieces:
INSERT INTO newtable SELECT * FROM oldtable WHERE yourkey >
somethingAND yourkey <=
After all records have been inserted, you can rename the tables.
During the conversion of big tables, increase the size of the
InnoDB buffer pool to
reduce disk I/O, to a maximum of 80% of physical memory. You can also increase the sizes of the
InnoDB log files.
By this point, as already mentioned, you should already have the
innodb_file_per_table option enabled, so that if you temporarily make several
copies of your data in
InnoDB tables, you can recover all that disk space by
dropping unneeded tables afterward.
Whether you convert the
MyISAM table directly or create a cloned
InnoDB table, make sure that you have sufficient disk space to hold both the old and
new tables during the process.
InnoDB tables require more disk space than
MyISAM tables. If an
TABLE operation runs out of space, it starts a rollback, and that can take hours if it is
disk-bound. For inserts,
InnoDB uses the insert buffer to merge secondary index
records to indexes in batches. That saves a lot of disk I/O. For rollback, no such mechanism is used, and the
rollback can take 30 times longer than the insertion.
In the case of a runaway rollback, if you do not have valuable data in your database, it may be advisable to
kill the database process rather than wait for millions of disk I/O operations to complete. For the complete
procedure, see Section 22.214.171.124, "Starting
InnoDB on a Corrupted Database".
PRIMARY KEY clause is a critical factor affecting the performance of MySQL
queries and the space usage for tables and indexes. Perhaps you have phoned a financial institution where you
are asked for an account number. If you do not have the number, you are asked for a dozen different pieces of
information to "uniquely identify" yourself. The primary
key is like that unique account number that lets you get straight down to business when querying or modifying
the information in a table. Every row in the table must have a primary key value, and no two rows can have the
same primary key value.
Here are some guidelines for the primary key, followed by more detailed explanations.
PRIMARY KEY for each table. Typically, it is
the most important column that you refer to in
WHERE clauses when looking
up a single row.
Choose the column and its data type carefully. Prefer numeric columns over character or string ones.
Consider using an auto-increment column if there is not another stable, unique, non-null, numeric column to use.
An auto-increment column is also a good choice if there is any doubt whether the value of the primary key column could ever change. Changing the value of a primary key column is an expensive operation, possibly involving rearranging data within the table and within each secondary index.
Consider adding a primary key to any table that does not already have one. Use the smallest practical numeric type based on the maximum projected size of the table. This can make each row slightly more compact, which can yield substantial space savings for large tables. The space savings are multiplied if the table has any secondary indexes, because the primary key value is repeated in each secondary index entry. In addition to reducing data size on disk, a small primary key also lets more data fit into the buffer pool, speeding up all kinds of operations and improving concurrency.
If the table already has a primary key on some longer column, such as a
consider adding a new unsigned
AUTO_INCREMENT column and switching the primary key
to that, even if that column is not referenced in queries. This design change can produce substantial space
savings in the secondary indexes. You can designate the former primary key columns as
NOT NULL to enforce the same constraints as the
PRIMARY KEY clause, that
is, to prevent duplicate or null values across all those columns.
If you spread related information across multiple tables, typically each table uses the same column for its primary key. For example, a personnel database might have several tables, each with a primary key of employee number. A sales database might have some tables with a primary key of customer number, and other tables with a primary key of order number. Because lookups using the primary key are very fast, you can construct efficient join queries for such tables.
If you leave the
PRIMARY KEY clause out entirely, MySQL creates an invisible one
for you. It is a 6-byte value that might be longer than you need, thus wasting space. Because it is hidden, you
cannot refer to it in queries.
The extra reliability and scalability features of
InnoDB do require more disk
storage than equivalent
MyISAM tables. You might change the column and index
definitions slightly, for better space utilization, reduced I/O and memory consumption when processing result
sets, and better query optimization plans making efficient use of index lookups.
If you do set up a numeric ID column for the primary key, use that value to cross-reference with related values
in any other tables, particularly for join
queries. For example, rather than accepting a country name as input and doing queries searching for the same
name, do one lookup to determine the country ID, then do other queries (or a single join query) to look up
relevant information across several tables. Rather than storing a customer or catalog item number as a string of
digits, potentially using up several bytes, convert it to a numeric ID for storing and querying. A 4-byte
INT column can index over 4 billion items (with the US meaning of billion:
1000 million). For the ranges of the different integer types, see Section
11.2.1, "Integer Types (Exact Value) -
InnoDB files require more care and planning than