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My SQL Interview Questions and Answers


MySQL - Using Symbolic Links for Databases and Tables
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 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 that you link one directory to multiple databases. Replacing a database directory with a symbolic link 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 db2 that points to db1:

shell> cd /path/to/datadir
shell> ln -s db1 db2

Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one thread updates db1.tbl_a 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 (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode)))

to

if (1)

On Windows you can use internal symbolic links to directories by compiling MySQL with -DUSE_SYMDIR. This allows you to put different databases on different disks.

MySQL - Tuning Server Parameters
You can get the default buffer sizes used by the mysqld server 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 like this:

Possible variables for option --set-variable (-O) are:
back_log current value: 5
bdb_cache_size current value: 1048540
binlog_cache_size current_value: 32768
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_binlog_cache_size current_value: 4294967295
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
myisam_sort_buffer_size current value: 8388608
net_buffer_length current value: 16384
net_retry_count current value: 10
net_read_timeout current value: 30
net_write_timeout current value: 60
query_buffer_size current value: 0
record_buffer current value: 131072
slow_launch_time current value: 2
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

You can find a full description for all variables in the SHOW VARIABLES section in this manual.

You can also see some statistics from a running server by issuing the command SHOW STATUS.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. If you, however, give MySQL more memory, you will normally also get better performance.

When tuning a MySQL server, the two most important variables to use are key_buffer_size and table_cache. You should first feel confident that you have these right before trying to change any of the other variables.

If you have much memory (>=256M) 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=64M -O table_cache=256 \
-O sort_buffer=4M -O record_buffer=1M &

If you have only 128M and only a few tables, but you still do a lot of sorting, you can use something like:

shell> safe_mysqld -O key_buffer=16M -O sort_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 &

or even:

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \
-O table_cache=32 -O record_buffer=8k -O net_buffer=1K &

When you have installed MySQL, the `support-files' directory will contain some different my.cnf example files, `my-huge.cnf', `my-large.cnf', `my-medium.cnf', and `my-small.cnf', you can use as a base to optimize your system.
If there are very many connections, ``swapping problems'' may occur unless mysqld has been configured to use very little memory for each connection. 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 output.

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