Optimization is a complicated task since it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system/application, the more optimal you want your system to become the more you will have to know about it.
So this chapter will try to explain and give some examples of different ways to optimize MySQL. But remember that there are always some (increasingly harder) ways to make the system even faster left to do.
The most important part for getting a system fast is of course the basic design. You also need to know that kinds of things your system will be doing. That is your bottlenecks are.
The most common bottlenecks are.
We start with the system level things sine some of these decisions have to be made very early. In other cases a fast look at this part may suffice since it not that important for the big gains. But it is always nice to have a feeling about how much one gould gain by chancing things at this level.
The default OS to use is really important! To get the most use of multiple CPU machines one should use Solaris (because the threads works really nice) or Linux (because the 2.2 kernel has really good SMP support). Also on 32bit machines Linux has a 2G file size limit by default. Hopefully this will be fixed soon when new filesystems is released (XFS).
Since we have not run production MySQL on that many platforms we advice you to test your intended platform before choosing it if possible.
--skip-lockingMySQL option to avoid external locking. Note that this will not impact MySQL functionality as long that only run one server. Just remember to take down the server (or lock relevant parts) before you run
myisamchk. On some system this switch is mandatory since the external locking does not work in any case. The
--skip-lockingoption is on by default when compiling with MIT-pthreads, because
flock()isn't fully supported by MIT-pthreads on all platforms. The only case when you can't use
--skip-lockingis if you run multiple MySQL SERVERS (not clients) on the same data. Or run
myisamchkon the table without first flushing and locking the
mysqldserver tables first. You can still use
UNLOCK TABLESeven if you are using
Most of the following tests are done on Linux and with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.
You get the fastest executable when you link with
-static. Using Unix
sockets rather than TCP/IP to connect to a database also gives better
On Linux, you will get the fastest code when compiling with
-O6. To compile `sql_yacc.cc' with these options, you
need about 200M memory because
gcc/pgcc needs a lot of memory to
make all functions inline. You should also set
configuring MySQL to avoid inclusion of the
library (it is not needed).
By just using a better compiler and/or better compiler options you can get a 10-30 % speed increase in your application. This is particularly important if you compile the SQL server yourselves!
On Intel you should for example use pgcc or the Cygnus CodeFusion compiler to get maximum speed. We have tested the new Fujitsu compiler but it is not yet bug free enough to compile MySQL with optimizations on.
Here is a list of some mesurements that we have done:
pgccand compile everything with
mysqldserver is 11% faster than with
gccversions older than
-static), the result is 13% slower. Note that you still can use a dynamic linked MySQL library. It is only the server that is critical for performance.
gcc2.7.3 is 13% faster than Sun Pro C++ 4.2.
The MySQL-Linux distribution provided by TcX is compiled with
pgcc and linked statically.
You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space.
If MySQL notices that a table is a symbolically-linked, it will
resolve the symlink and use the table it points to instead. This works
on all systems that support the
realpath() call (at least Linux
and Solaris support
realpath())! On systems that don't support
realpath(), you should not access the table through the real path
and through the symlink at the same time! If you do, the table will be
inconsistent after any update.
MySQL doesn't support linking of databases by default. Things
will work fine as long as you don't make a symbolic link between
databases. Suppose you have a database
db1 under the
MySQL data directory, and then make a symlink
shell> cd /path/to/datadir shell> ln -s db1 db2
Now, for any table
db1, there also appears to be
db2. If one thread updates
and another thread updates
db2.tbl_a, there will be problems.
If you really need this, you must change the following code in `mysys/mf_format.c':
if (!lstat(to,&stat_buff)) /* Check if it's a symbolic link */ if (S_ISLNK(stat_buff.st_mode) && realpath(to,buff))
Change the code to this:
You can get the default buffer sizes used by the
with this command:
shell> mysqld --help
This command produces a list of all
mysqld options and configurable
variables. The output includes the default values and looks something
Possible variables for option --set-variable (-O) are: back_log current value: 5 connect_timeout current value: 5 delayed_insert_timeout current value: 300 delayed_insert_limit current value: 100 delayed_queue_size current value: 1000 flush_time current value: 0 interactive_timeout current value: 28800 join_buffer_size current value: 131072 key_buffer_size current value: 1048540 lower_case_table_names current value: 0 long_query_time current value: 10 max_allowed_packet current value: 1048576 max_connections current value: 100 max_connect_errors current value: 10 max_delayed_threads current value: 20 max_heap_table_size current value: 16777216 max_join_size current value: 4294967295 max_sort_length current value: 1024 max_tmp_tables current value: 32 max_write_lock_count current value: 4294967295 net_buffer_length current value: 16384 query_buffer_size current value: 0 record_buffer current value: 131072 sort_buffer current value: 2097116 table_cache current value: 64 thread_concurrency current value: 10 tmp_table_size current value: 1048576 thread_stack current value: 131072 wait_timeout current value: 28800
If there is a
mysqld server currently running, you can see what
values it actually is using for the variables by executing this command:
shell> mysqladmin variables
Each option is described below. Values for buffer sizes, lengths and stack
sizes are given in bytes. You can specify values with a suffix of `K'
or `M' to indicate kilobytes or megabytes. For example,
indicates 16 megabytes. Case of suffix letters does not matter;
16m are equivalent.
You can also see some statistics from a running server by the command
SHOW STATUS. See section 7.21
SHOW syntax (Get information about tables, columns,...).
mysqldwas started with
--ansi. See section 5.2 Runnning MySQL in ANSI mode.
back_logvalue indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix
listen(2)system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set
back_loghigher than your operating system limit will be ineffective.
MySQLwill allow you to use
MyISAMtables at the same time as you run
SELECTqueries on them. You can turn this option off by starting mysqld with
mysqldserver is waiting for a connect packet before responding with
INSERT DELAYEDthread should wait for
INSERTstatements before terminating.
INSERT DELAYEDhandler will check if there are any
SELECTstatements pending. If so, it allows these to execute before continuing.
CREATE TABLE. This means that the key buffer for tables with this option will not get flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot but you should add automatic checking of all tables with
myisamchk --fast --forceif you use this. Note that if you start
--delay-key-write-for-all-tablesoption this means that all tables will be treated as if they where created with the
delay_key_writeoption. You can clear this flag by starting
INSERT DELAYED. If the queue becomes full, any client that does
INSERT DELAYEDwill wait until there is room in the queue again.
flush_timeseconds all tables will be closed (to free up resources and sync things to disk).
--init-fileoption when you start the server. This is a file of SQL statements you want the server to execute when it starts.
mysql_real_connect(). See also
key_buffer_sizeis the size of the buffer used for index blocks. Increase this get better index handling (for all reads and multiple writes) to as much as you can afford. If you make this too big the system will starte to page and go REAL slow. Remember that since MySQL does not cache data read that you will have to leave some room for the OS filesystem cache. To get even more speed when writing many rows at the same time use
LOCK TABLES. See section 7.24
LOCK TABLES/UNLOCK TABLESsyntax.
Slow_queriescounter will be incremented.
net_buffer_lengthbytes, but can grow up to
max_allowed_packetbytes when needed. This value by default is small to catch big (possibly wrong) packets. You must increase this value if you are using big
BLOBcolumns. It should be as big as the biggest
BLOByou want to use.
mysqldrequires. See below for comments on file descriptor limits. See section 18.2.4
Too many connectionserror.
INSERT DELAYEDstatements. If you try to insert data in a new table after all
INSERT DELAYEDthreads are in use, the row will be inserted as if the
DELAYEDattribute wasn't specified.
max_join_sizerecords return an error. Set this value if your users tend to perform joins without a
WHEREclause that take a long time and return millions of rows.
TEXTvalues (only the first
max_sort_lengthbytes of each value are used; the rest are ignored).
FreeBSDas internal interrupts is sent to all threads.
SHOW DATABASES, if they don't have the
PROCESS_PRIVprivilege. This can improve security if you're concerned about people being able to see what databases and tables other users have.
GROUP BYoperations. See section 18.5 Where MySQL stores temporary files.
mysqldrequires. MySQL needs two file descriptors for each unique open table. See below for comments on file descriptor limits. For information about how the table cache works, see section 10.2.4 How MySQL opens and closes tables.
The table tbl_name is full. Increase the value of
tmp_table_sizeif you do many advanced
thr_setconcurrency()with this value.
thr_setconcurrency()permits the application to give the threads system a hint, for the desired number of threads that should be run at the same time.
crash-metest are dependent on this value. The default is large enough for normal operation. See section 10.8 Using your own benchmarks.
MySQL uses algorithms that are very scalable, so you can usually run with very little memory or give MySQL more memory to get better performance.
If you have much memory and many tables and want maximum performance with a moderate number of clients, you should use something like this:
shell> safe_mysqld -O key_buffer=16M -O table_cache=128 \ -O sort_buffer=4M -O record_buffer=1M &
If you have little memory and lots of connections, use something like this:
shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \ -O record_buffer=100k &
shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \ -O table_cache=32 -O record_buffer=8k -O net_buffer=1K &
If there are very many connections, ``swapping problems'' may occur unless
mysqld has been configured to use very little memory for each
mysqld performs better if you have enough memory for all
connections, of course.
Note that if you change an option to
mysqld, it remains in effect only
for that instance of the server.
To see the effects of a parameter change, do something like this:
shell> mysqld -O key_buffer=32m --help
Make sure that the
--help option is last; otherwise, the effect of any
options listed after it on the command line will not be reflected in the
affect the maximum number of files the server keeps open. If you
increase one or both of these values, you may run up against a limit
imposed by your operating system on the per-process number of open file
descriptors. However, you can increase the limit on many systems.
Consult your OS documentation to find out how to do this, because the
method for changing the limit varies widely from system to system.
table_cache is related to
For example, for 200 open connections, you should have a table cache of
200 * n, where
n is the maximum number of tables in
The cache of open tables can grow to a maximum of
(default 64; this can be changed with with the
mysqld). A table is never closed, except when the
cache is full and another thread tries to open a table or if you use
mysqladmin refresh or
When the table cache fills up, the server uses the following procedure to locate a cache entry to use:
A table is opened for each concurrent access. This means that
if you have two threads accessing the same table or access the table
twice in the same query (with
AS) the table needs to be opened twice.
The first open of any table takes two file descriptors; each additional
use of the table takes only one file descriptor. The extra descriptor
for the first open is used for the index file; this descriptor is shared
among all threads.
If you have many files in a directory, open, close and create operations will
be slow. If you execute
SELECT statements on many different tables,
there will be a little overhead when the table cache is full, because for
every table that has to be opened, another must be closed. You can reduce
this overhead by making the table cache larger.
When you run
mysqladmin status, you'll see something like this:
Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12
This can be somewhat perplexing if you only have 6 tables.
MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.
The list below indicates some of the ways that the
uses memory. Where applicable, the name of the server variable relevant
to the memory use is given.
key_buffer_size) is shared by all threads; Other buffers used by the server are allocated as needed. See section 10.2.3 Tuning server parameters.
thread_stack) a connection buffer (variable
net_buffer_length), and a result buffer (variable
net_buffer_length). The connection buffer and result buffer are dynamically enlarged up to
max_allowed_packetwhen needed. When a query is running a copy of the current query string is also allocated.
BLOBcolumns are stored on disk. One problem in MySQL versions before 3.23.2 is that if a HEAP table exceeds the size of
tmp_table_size, you get the error
The table tbl_name is full. In newer versions this is handled by automatically changing the in-memory (HEAP) table to a disk-based (MyISAM) table as necessary. To work around this problem, you can increase the temporary table size by setting the
mysqld, or by setting the SQL option
SQL_BIG_TABLESin the client program. See section 7.25
SETsyntax. In MySQL 3.20, the maximum size of the temporary table was
record_buffer*16, so if you are using this version, you have to increase the value of
record_buffer. You can also start
--big-tablesoption to always store temporary tables on disk, however, this will affect the speed of many complicated queries.
3 * nis allocated (where
nis the maximum row length, not counting
BLOBuses 5 to 8 bytes plus the length of the
BLOBcolumns, a buffer is enlarged dynamically to read in larger
BLOBvalues. If you scan a table, a buffer as large as the largest
BLOBvalue is allocated.
mysqladmin flush-tablescommand closes all tables that are not in use and marks all in-use tables to be closed when the currently executing thread finishes. This will effectively free most in-use memory.
ps and other system status programs may report that
uses a lot of memory. This may be caused by thread-stacks on different
memory addresses. For example, the Solaris version of
the unused memory
between stacks as used memory. You can verify this by checking available
swap -s. We have tested
mysqld with commercial
memory-leakage detectors, so there should be no memory leaks.
All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.
The locking method MySQL uses for
WRITE locks works as follows:
The locking method MySQL uses for
READ locks works as follows:
When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.
This means that if you have many updates on a table,
statements will wait until there are no more updates.
To work around this for the case where you want to do many
SELECT operations on a table, you can insert rows in a temporary
table and update the real table with the records from the temporary table
once in a while.
This can be done with the following code:
mysql> LOCK TABLES real_table WRITE, insert_table WRITE; mysql> insert into real_table select * from insert_table; mysql> delete from insert_table; mysql> UNLOCK TABLES;
You can use the
LOW_PRIORITY options with
INSERT if you
want to prioritize retrieval in some specific cases. See section 7.14
You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.
The table locking code in MySQL is deadlock free.
MySQL uses table locking (instead of row locking or column locking) to achieve a very high lock speed. For large tables, table locking is for most applications MUCH better than row locking, but there are of course some pitfalls.
In MySQL 3.23.7 and above, you can insert rows into
MyISAM tables at the same time as other threads are reading from
the table. Note that currently this only works if there are no deleted
rows in the table.
Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update all others threads that want to access this particular table will wait until the update is ready.
As updates of databases normally are considered to be more important
SELECT, all statements that update a table have higher
priority than statements that retrieve information from a table. This
should ensure that updates are not 'starved' because one issues a lot of
heavy queries against a specific table.
Starting from MySQL 3.23.7 one can use the
max_write_lock_count variable to force MySQL to issue
SELECT after a specific number of inserts on a table.
One main problem with this is the following:
SELECTthat takes a long time to run.
UPDATEon a used table; This client will wait until the
SELECTstatement on the same table; As
UPDATEhas higher priority than
SELECTwill wait for the
UPDATEto finish. It will also wait for the first
Some possible solutions to this problem are:
SELECTstatements to run faster; You may have to create some summary tables to do this.
--low-priority-updates. This will give all statements that update (modify) a table lower priority than a
SELECTstatement. In this case the last
SELECTstatement in the previous scenario would execute before the
DELETEstatement lower priority with the
mysqldwith a low value for max_write_lock_count to give
READlocks after a certain number of
SET SQL_LOW_PRIORITY_UPDATES=1. See section 7.25
SELECTis very important with the
HIGH_PRIORITYattribute. See section 7.12
SELECT, switch to use the new
MyISAMtables as these supports concurrent
INSERTwill probably solve your problems. See section 7.14
DELETEmay help. See section 7.11
One of the most basic optimization is to get your data (and indexes) to take as little space on the disk (and in memory) as possible. This can give huge improvements since disk reads are faster and normally less main memory will also be used. Indexing also takes less resources if done on smaller columns.
You can get better performance on a table and minimize storage space using the techniques listed below:
MEDIUMINTis often better than
NOT NULLif possible. It makes everything faster and you save one bit per column. Note that if you really need
NULLin your application you should definitely use it. Just avoid haveing it on all columns by default.
BLOBcolumns), a fixed-size record format is used. This is faster but unfortunately may waste some space. See section 10.6 Choosing a table type.
Indexes are used to find find a row with a specific calue on one column fast. Without a index MySQL has to start with the first record and then read through the whole table until it find the relevent rows. The bigger the table the more this costs. If the table has a index for the colums in question MySQL can get fast a possition to seek to in the middle of the data file without having to look at all data. If a table have 1000 rows this is at least 100 times faster than reading sequentially. Note that is you need to access almost all 1000 rows it is faster to read sequentially since we when avoid disk seeks.
All MySQL indexes (
stored in B-trees. Strings are automatically prefix- and end-space
compressed. See section 7.27
CREATE INDEX syntax.
Indexes are used to:
MIN()value for a specific indexed column.
ORDER BY key_part_1,key_part_2). The key is read in reverse order if all key parts are followed by
Suppose you issue the following
mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;
If a multiple-column index exists on
appropriate rows can be fetched directly. If separate single-column
indexes exist on
col2, the optimizer tries to
find the most restrictive index by deciding which index will find fewer
rows and using that index to fetch the rows.
If the table has a multiple-column index, any leftmost prefix of the
index can be used by the optimizer to find rows. For example, if you
have a three-column index on
(col1,col2,col3), you have indexed
search capabilities on
MySQL can't use a partial index if the columns don't form a
leftmost prefix of the index. Suppose you have the
statements shown below:
mysql> SELECT * FROM tbl_name WHERE col1=val1; mysql> SELECT * FROM tbl_name WHERE col2=val2; mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;
If an index exists on
(col1,col2,col3), only the first query
shown above uses the index. The second and third queries do involve
indexed columns, but
(col2,col3) are not
leftmost prefixes of
MySQL also uses indexes for
LIKE comparisons if the argument
LIKE is a constant string that doesn't start with a wildcard
character. For example, the following
SELECT statements use indexes:
mysql> select * from tbl_name where key_col LIKE "Patrick%"; mysql> select * from tbl_name where key_col LIKE "Pat%_ck%";
In the first statement, only rows with
"Patrick" <= key_col <
"Patricl" are considered. In the second statement, only rows with
"Pat" <= key_col < "Pau" are considered.
SELECT statements will not use indexes:
mysql> select * from tbl_name where key_col LIKE "%Patrick%"; mysql> select * from tbl_name where key_col LIKE other_col;
In the first statement, the
LIKE value begins with a wildcard character.
In the second statement, the
LIKE value is not a constant.
column_name IS NULL will use indexes if column_name
is a index.
MySQL normally uses the index that finds least number of rows. An
index is used for columns that you compare with the following operators:
BETWEEN and a
LIKE with a non-wildcard prefix like
Any index that doesn't span all
AND levels in the
is not used to optimize the query. In other words: To be able to use an
index, a prefix of the index must be used in every
WHERE clauses use indexes:
... WHERE index_part1=1 AND index_part2=2 AND other_column=3 ... WHERE index=1 OR A=10 AND index=2 /* index = 1 OR index = 2 */ ... WHERE index_part1='hello' AND index_part_3=5 /* optimized like "index_part1='hello'" */ ... WHERE index1=1 and index2=2 or index1=3 and index3=3; /* Can use index on index1 but not on index2 or index 3 */
WHERE clauses do NOT use indexes:
... WHERE index_part2=1 AND index_part3=2 /* index_part_1 is not used */ ... WHERE index=1 OR A=10 /* Index is not used in both AND parts */ ... WHERE index_part1=1 OR index_part2=10 /* No index spans all rows */
First, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get.
If you do not have any
GRANT statements done MySQL will optimize
the permission checking somewhat. So if you have a very high volume it
may be worth the time to avoid grants. Otherwise more permission check
results in a larger overhead.
If your problem is with some explicit MySQL function, you can always time this in the MySQL client:
mysql> select benchmark(1000000,1+1); +------------------------+ | benchmark(1000000,1+1) | +------------------------+ | 0 | +------------------------+ 1 row in set (0.32 sec)
The above shows that MySQL can execute 1,000,000
expressions in 0.32 seconds on a
All MySQL functions should be very optimized, but there may be
some exceptions and the
benchmark(loop_count,expression) is a
great tool to find if this is a problem with your query.
In most cases you can estimate the performance by counting disk seeks.
For small tables you can usually find the row in 1 disk seek (as the
index is probably cached). For bigger tables, you can estimate that,
(using B++ tree indexes), you will need:
log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) +
1 seeks to find a row.
In MySQL an index block is usually 1024 bytes and the data
pointer is usually 4 bytes, which gives for a 500,000 row table with a
index length of 3 (medium integer) gives you:
log(500,000)/log(1024/3*2/(3+4)) + 1 = 4 seeks.
As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3 (which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.
For writes you will however need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.
Note that the above doesn't mean that your application will slowly degenerate by N log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by N log N). To avoid this increase the index cache as the data grows. See section 10.2.3 Tuning server parameters.
In general, when you want to make a slow
SELECT ... WHERE faster,
the first thing to check is whether or not you can add an
index. See section 10.4 MySQL index use. All references between different tables
should usually be done with indexes. You can use the
command to determine which indexes are used for a
See section 7.22
EXPLAIN syntax (Get information about a
Some general tips:
myisamchk --analyzeon a table after it has been loaded with relevant data. This updates a value for each index that indicates the average number of rows that have the same value. (For unique indexes, this is always 1, of course.)
myisamchk --sort-index --sort-records=1(if you want to sort on index 1). If you have a unique index from which you want to read all records in order according to that index, this is a good way to make that faster. Note however that this sorting isn't written optimally and will take a long time for a large table!
The where optimizes are put in the
SELECT part here since they
are mostly used there. But the same optimizations are used for there in
Also note that this section is incomplete. MySQL does many optimizations and we have not had time to document them all.
Some of the optimizations performed by MySQL are listed below:
((a AND b) AND c OR (((a AND b) AND (c AND d)))) -> (a AND b AND c) OR (a AND b AND c AND d)
(a<b AND b=c) AND a=5 -> b>5 AND b=c AND a=5
(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6) -> B=5 OR B=6
COUNT(*)on a single table without a
WHEREis retrieved directly from the table information. This is also done for any
NOT NULLexpression when used with only one table.
SELECTstatements are impossible and returns no rows.
HAVINGis merged with
WHEREif you don't use
GROUP BYor group functions (
WHEREis constructed to get a fast
WHEREevaluation for each sub join and also to skip records as soon as possible.
WHEREclause on a
UNIQUEindex, or a
PRIMARY KEY, where all index parts are used with constant expressions and the index parts are defined as
mysql> SELECT * FROM t WHERE primary_key=1; mysql> SELECT * FROM t1,t2 WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
ORDER BYand in
GROUP BYcome from the same table, then this table is preferred first when joining.
ORDER BYclause and a different
GROUP BYclause, or if the
GROUP BYcontains columns from tables other than the first table in the join queue, a temporary table is created.
SQL_SMALL_RESULT, MySQL will use an in-memory temporary table.
DISTINCTis converted to a
GROUP BYon all columns,
ORDER BYwill in many cases also need a temporary table.
HAVINGclause are skipped.
Some examples of queries that are very fast:
mysql> SELECT COUNT(*) FROM tbl_name; mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name; mysql> SELECT MAX(key_part2) FROM tbl_name WHERE key_part_1=constant; mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... LIMIT 10; mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;
The following queries are resolved using only the index tree (assuming the indexed columns are numeric):
mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val; mysql> SELECT COUNT(*) FROM tbl_name WHERE key_part1=val1 AND key_part2=val2; mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;
The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:
mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,...
A LEFT JOIN B is in MySQL implemented as follows
Bis set to be dependent on table
Aand all tables that
Ais dependent on.
Ais set to be dependent on all tables (except
B) that are used in the
LEFT JOINconditions are moved to the
WHEREoptimzations are done.
Athat matches the
WHEREclause, but there wasn't any row in
Bthat matched the
LEFT JOINcondition, then an extra
Brow is generated with all columns set to
LEFT JOINto find rows that doesn't exist in some table and you have the following test:
column_name IS NULLin the
WHEREpart, where column_name is a column that is declared as
NOT NULL, then
MySQLwill stop searching after more rows (for a particular key combination) after it has found one row that matches the
The table read order forced by
LEFT JOIN and
STRAIGHT JOIN will help
the join optimiser (which calculates in which order tables should be joined) to do
its work much quickly as there is fewer table permutations to check.
Note that the above means that if you do a query of type:
SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key
Then MySQL will do a full scan on
b as the
LEFT JOIN will
force it to be read before
The fix in this case is to change the query to:
SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key
In some cases MySQL will handle the query differently when you are
LIMIT # and not using
LIMIT, MySQL will use indexes in some cases when it normally would prefer to do a full table scan.
ORDER BY, MySQL will end the sorting as soon as it has found the first
#lines instead of sorting the whole table.
DISTINCT, MySQL will stop as soon as it finds
GROUP BYcan be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case
LIMIT #will not calculate any unnecessary
#rows to the client, it will abort the query.
LIMIT 0will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns.
LIMIT #to calculate how much space is needed to resolve the query.
The time to insert a record consists approximately of:
Where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently-running query).
The size of the table slows down the insertion of indexes by N log N (B-trees).
Some ways to speed up inserts:
INSERTstatements. This is much faster (many times in some cases) than using separate
INSERT DELAYEDstatement. See section 7.14
MyISAMyou can insert rows at the same time
SELECTs are running if there are no deleted rows in the tables.
LOAD DATA INFILE. This is usually 20 times faster than using a lot of
INSERTstatements. See section 7.16
LOAD DATA INFILEsyntax.
LOAD DATA INFILErun even faster when the table has many indexes. Use the following procedure:
CREATE TABLE. For example using
FLUSH TABLESstatement or the shell command
myisamchk --keys-used=0 -rq /path/to/db/tbl_name. This will remove all usage of all indexes from the table.
LOAD DATA INFILE. This will not update any indexes and will therefore be very fast.
myisampackand want to compress the table, run
myisampackon it. See section 10.6.3 Compressed table characteristics.
myisamchk -r -q /path/to/db/tbl_name. This will create the index tree in memory before writing it to disk, which is much faster since it avoid lots of disk seeks. The resulting index tree is also perfectly balanced.
FLUSH TABLESstatement or the shell command
LOAD DATA INFILEin some future version of MySQL.
mysql> LOCK TABLES a WRITE; mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33); mysql> INSERT INTO a VALUES (8,26),(6,29); mysql> UNLOCK TABLES;The main speed difference is that the index buffer is flushed to disk only once, after all
INSERTstatements have completed. Normally there would be as many index buffer flushes as there are different
INSERTstatements. Locking is not needed if you can insert all rows with a single statement. Locking will also lower the total time of multi-connection tests, but the maximum wait time for some threads will go up (because they wait for locks). For example:
thread 1 does 1000 inserts thread 2, 3, and 4 does 1 insert thread 5 does 1000 insertsIf you don't use locking, 2, 3 and 4 will finish before 1 and 5. If you use locking, 2, 3 and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster. As
DELETEoperations are very fast in MySQL, you will obtain better overall performance by adding locks around everything that does more than about 5 inserts or updates in a row. If you do very many inserts in a row, you could do a
LOCK TABLESfollowed by a
UNLOCK TABLESonce in a while (about each 1000 rows) to allow other threads access to the table. This would still result in a nice performance gain. Of course,
LOAD DATA INFILEis much faster still for loading data.
To get some more speed for both
LOAD DATA INFILE and
INSERT, enlarge the key buffer. See section 10.2.3 Tuning server parameters.
Update queries are optimized as a
SELECT query with the additional
overhead of a write. The speed of the write is dependent on the size of
the data that are being updated and the number of indexes that are
updated. Indexes that are not changed will not be updated.
Also another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.
Not that with dynamic record format updating a record with to a longer
total length may split the record. So if you do this often it is very
OPTIMIZE TABLE sometimes. See section 7.9
OPTIMIZE TABLE syntax.
The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 10.2.3 Tuning server parameters.
Its also much faster to remove all rows than to remove a big part of the rows from a table.
With MySQL you can currently (version 3.23.5) choose between four usable table formats from a speed point of view.
myisamchkcan easily figure out where each row starts and ends. So it can usually reclaim all records except the partially written one. Not that in MySQL all indexes can always be reconstructed.
myisamchkto defragment a table. If you have static data that you acess/change a lot in the same table as some
BLOBcolumns, it might be a good idea to move the dynamic columns to other tables just to avoid fragmentation.
SELECT tab1.a, tab3.a FROM tab1, tab2, tab3 WHERE tab1.a = tab2.a and tab2.a = tab3.a and tab2.c != 0;To speed this up we could create a temporary table with the join of tab2 and tab3 since that are looked up using the same column (tab1.a). Here is the command to create that table and the resulting select.
CREATE TEMPORARY TABLE test TYPE=HEAP SELECT tab2.a as a2, tab3.a as a3 FROM tab2, tab3 WHERE tab2.a = tab3.a and c = 0; SELECT tab1.a, test.a3 from tab1, test where tab1.a = test.a2; SELECT tab1.b, test.a3 from tab1, test where tab1.a = test.a2 and something;
DECIMALcolumns are space-padded to the column width.
myisamchk) unless a huge number of records are deleted and you want to return free disk space to the operating system.
'') for string columns, or zero for numeric columns (this isn't the same as columns containing
NULLvalues). If a string column has a length of zero after removal of trailing spaces, or a numeric column has a value of zero, it is marked in the bit map and not saved to disk. Non-empty strings are saved as a length byte plus the string contents.
myisamchk -rfrom time to time to get better performance. Use
myisamchk -ei tbl_namefor some statistics.
3 + (number of columns + 7) / 8 + (number of char columns) + packed size of numeric columns + length of strings + (number of NULL columns + 7) / 8There is a penalty of 6 bytes for each link. A dynamic record is linked whenever an update causes an enlargement of the record. Each new link will be at least 20 bytes, so the next enlargement will probably go in the same link. If not, there will be another link. You may check how many links there are with
myisamchk -ed. All links may be removed with
myisampackutility. All customers with extended MySQL email support are entitled to a copy of
myisampackfor their internal usage.
myisampackcan read tables that were compressed with
0are stored using 1 bit.
BIGINTcolumn (8 bytes) may be stored as a
TINYINTcolumn (1 byte) if all values are in the range
MySQL can support different index types, but the normal type is
ISAM. This is a B-tree index and you can roughly calculate the size for the
index file as
(key_length+4)*0.67, summed over all keys. (This is for
the worst case when all keys are inserted in sorted order.)
String indexes are space compressed. If the first index part is a string, it
will also be prefix compressed. Space compression makes the index file
smaller if the string column has a lot of trailing space or is a
column that is not always used to the full length. Prefix compression helps
if there are many strings with an identical prefix.
HEAP tables only exists in memory so they are lost if
taken down or crashes. But since they are very fast they are
usefull as anyway.
The MySQL internal HEAP tables uses 100% dynamic hashing without overflow areas and don't have problems with delete.
You can only access things by equality using a index (usually by the
= operator) whith a heap table.
The downside with HEAPS are:
Unsorted tips for faster systems:
EXPLAINcommand. See section 7.22
EXPLAINsyntax (Get information about a
SELECTqueries on tables that are updated a lot. This is to avoid problems with table locking.
MyISAMtables can insert rows in a table without deleted rows at the same time another table is reading from it. If this is important for you, you should consider methods where you don't have to delete rows or run
OPTIMIZE TABLEafter you have deleted a lot of rows.
SELECT * from table where hash='calculated hash on col1 and col2' and col_1='constant' and col_2='constant' and ..
BLOBcolumns. You will get dynamic row length as soon as you are using a single
BLOBcolumns. See section 9.4 MySQL table types.
UPDATE table set count=count+1 where index_column=constantis very fast! This is really important when you use databases like MySQL that only has table locking (multiple readers / single writers). This will also give better performance with most databases as the row locking manager in this case will have less to do.
INSERT /*! DELAYED */when you do not need to now when your data is written. This speeds things up since many records can be written with a single disk write.
INSERT /*! LOW_PRIORITY */when you want your selects are more important.
SELECT /*! HIGH_PRIORITY */to get selects that jumps the queue. That is the select is done even if there is somebody waiting to do a write.
INSERTstatement to store many rows with one SQL command (many SQL servers supports this)
LOAD DATA INFILEto load bigger amounts of data. This if faster than normal inserts and will be even faster when
myisamchkis integrated in
AUTO_INCREMENTcolumns to make unique values.
OPTIMIZE TABLEonce in a while to avoid fragmentation when using dynamic table format.See section 7.9
HEAPtables to get more speed when possibe. See section 9.4 MySQL table types.
customer_namein the customer table). To make your names portable to other SQL servers you should keep them shorter than 18 characters.
MyISAMdirectly you could get a speed increase of 2-5 times compared to using the SQL interface. The be able to do this the data must however be on the same server as the application and usually it should only be accessed by on processes (because external file locking is real slow). One could eliminate the above problems by introducing low level
MyISAMcommands in the MySQL server (this could be one easy way to get more performance if needed). By carefully designing the database interface it should be quite easy to support this types of optimisations.
DELAY_KEY_WRITE=1will make the updating of indexes faster as these are not logged to disk until the file is closed. The downside is that you should run
myisamchkon these tables before you start
mysqldto ensure that they are ok if something killed
mysqldin the middle. As the key information can always be generated from the data you should not lose anything by using
You should definitely benchmark your application and database to find out where is the bottlenecks. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is 'good enough' you should at least make a 'plan', for each bottleneck, how to solve it if you someday 'really need it fix it'.
For some example portable becnchmark programs look at the MySQL benchmark suite. See section 11 The MySQL benchmark suite. You can take any program this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.
It is very common that some problems only occur then the system is very heavily loaded. And we have had many customer who contacts us then they have a (tested) system in production and have have got load problems. In every on these cases so far it has been problems with basic design (table scans are NOT good at high load) or OS/Library issues. Most of this would be a LOT easier to fix if the system where not already in production.
To avoid probles like this you should put some effort into benchmarking your whole appliction under the worst possible load!
MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We belive that the MySQL choice is better for a very wide range of modern systems.
Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will get a performance hit for every query that access more than one column. Since this degenerates so quickly when more that when one columns are accessed we believe that this model is not good for general purpose databases.
The more common case is there the index and data are stored together (like in Oracle/Sybase at all). In this case you will find the row information at the leaf page of the index. The good thing with this layout is that it in many cases (depends on how well the index is cached) saves a disk read. The bad things with this layout is:
Since MySQL uses extremely fast table locking (multiple readers / single writers) the biggest remaining problem is a mix of a steady stream of inserts and slow selects on the same table.
We belive that for a huge number of systems the extremely fast performance in other cases make this choice a win. This case is usually also possible to solve by having multiple copies of the table. But it takes more effort and hardware.
We are also working on some extension to solve this problem for some common application niches.
Since all SQL servers implement different parts of SQL it takes work to write portable SQL applications. For very simple selects/inserts it is very easy but the more you need the harder it gets. And if you want a application that is fast with many databases it becomes even harder!
To make a complex application portable you need to choose a number of SQL server that it should work with.
When you can use the MySQL crash-me program/web-page http://www.mysql.com/crash-me-choose.htmy to find functions, types and limits you can use with a selection of database servers. Crash-me now test a long way from everything possible but it still is vīcomprehensive with about 450 things tested.
For example, you shouldn't have longer column names than 18 characters if you want to be able to use Informix or DB2.
Both the MySQL benchmarks and Crash-me programs are very database independent. By taking a look of how we have handled this, you can get a feeling of what you have to do to write your application database independent. The benchmark themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem.
See http://www.mysql.com/benchmark.html the results from this benchmark.
As you can see in these results all databases has some weak points. That is they have different design compromises that lead to different behavior.
If you strive for database independence you need to get a good feeling of each SQL servers bottlenecks. MySQL is VERY fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle on the other hand has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good in generating summary tables from log tables as in this case row locking is almost useless.
To get your application 'really database independent' you need to define a easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.
If you use some specific feature for some database (like the
REPLACE command in MySQL), you should code a method for
the other SQL servers to implement the same feature (but slower). With
MySQL you can use the
/*! */ syntax to add
MySQL specific keywords to a query. The code inside
/**/ will be treated as a comment (ignored) by most other SQL
If REAL high performance is more important than exactness, like in some web applications. A possibility is to create a application layer that caches all results to give you even higher performance. By just letting old results 'expire' after a while you can keep the cache reasonable fresh. This is quite nice in case of extremely high load, in which case you can dynamicly increase the cache to be bigger and set the expire timeout higher until things gets back to normal.
In this case the table creating information should contain information of the initial size of the cache and how often the table should normally be refreshed.
During MySQL initial development the features of MySQL where made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.
We get from all stores weekly summaries of all bonus card transactions and we are expected to provide useful information for the store owners to help them find how their advertisements campaigns are affecting their customers.
The data is quite huge (about 7 million summary transactions per month) and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.
We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros/script that generate summary tables grouped by different criterias (product group, customer id, store ...) from the transaction tables. The reports are web pages that are dynamicly generated by a small perl script that parses a web pages, executes the SQL statements in it and inserts the results. Now we would have used PHP or mod_perl instead but they where not available at that time.
For graphical data we wrote a simple tool in
C that can produce
gifs based on the result of a SQL query (with some processing of the
result). This is also dynamicly executed from the perl script that
In most cases a new report can simple by done by copying a existing script and modifying the SQL query in it. In some cases we will need to add more fields to an existing summary table or generate a new one, but this is also quite simply as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data).
We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.
We haven't had any problems handling this with quite modest Sun Ultra sparcstation (2x200 Mz). We recently upgrade one of our servers to a 2 CPU 400 Mz Ultra sparc and we are now planing to start handling transactions on the product level, which would mean a 10 fold increase of data. We think we can keep up with this by just adding more disk to our systems.
We are also experimenting with Intel-Linux to be able to get more cpu power cheaper. Now that we have the binary portable database format (new in 3.32) we will start to use this for some parts of the application.
Our initial feelings are that Linux will perform much better on low to medium load but Solaris will perform better when you start to get a a high load because of extrema disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This make the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.
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