Set - 6

Question 11 :

MySQL - Drawbacks to Creating Large Numbers of Tables in the Same Database

Answer :

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.


Question 12 :

MySQL - Why So Many Open tables?

Answer :

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.


Question 13 :

How MySQL Uses Memory ?

Answer :

The list below indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the server variable relevant to the memory use is given:

The key buffer (variable key_buffer_size) is shared by all threads; Other buffers used by the server are allocated as needed.

Each connection uses some thread-specific space: A stack (default 64K, variable 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_packet when needed. When a query is running, a copy of the current query string is also allocated.
All threads share the same base memory.
Only the compressed ISAM / MyISAM tables are memory mapped. This is because the 32-bit memory space of 4GB is not large enough for most big tables. When systems with a 64-bit address space become more common we may add general support for memory mapping.
Each request doing a sequential scan over a table allocates a read buffer (variable record_buffer).
All joins are done in one pass, and most joins can be done without even using a temporary table. Most temporary tables are memory-based (HEAP) tables. Temporary tables with a big record length (calculated as the sum of all column lengths) or that contain BLOB columns are stored on disk. One problem in MySQL versions before Version 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 tmp_table_size option to mysqld, or by setting the SQL option SQL_BIG_TABLES in the client program.

In MySQL Version 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 mysqld with the --big-tables option to always store temporary tables on disk. However, this will affect the speed of many complicated queries.
Most requests doing a sort allocates a sort buffer and 0-2 temporary files depending on the result set size.

Almost all parsing and calculating is done in a local memory store. No memory overhead is needed for small items and the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings (this is done with malloc() and free()).
Each index file is opened once and the data file is opened once for each concurrently running thread. For each concurrent thread, a table structure, column structures for each column, and a buffer of size 3 * n is allocated (where n is the maximum row length, not counting BLOB columns). A BLOB uses 5 to 8 bytes plus the length of the BLOB data. The ISAM/MyISAM table handlers will use one extra row buffer for internal usage.
For each table having BLOB columns, a buffer is enlarged dynamically to read in larger BLOB values. If you scan a table, a buffer as large as the largest BLOB value is allocated.
Table handlers for all in-use tables are saved in a cache and managed as a FIFO. Normally the cache has 64 entries. If a table has been used by two running threads at the same time, the cache contains two entries for the table.

A mysqladmin flush-tables command 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 mysqld uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of ps counts the unused memory between stacks as used memory. You can verify this by checking available swap with swap -s. We have tested mysqld with commercial memory-leakage detectors, so there should be no memory leaks.


Question 14 :

How MySQL Locks Tables ?

Answer :

You can find a discussion about different locking methods in the appendix.

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:

If there are no locks on the table, put a write lock on it. Otherwise, put the lock request in the write lock queue. The locking method MySQL uses for READ locks works as follows:

If there are no write locks on the table, put a read lock on it. Otherwise, put the lock request in the read lock queue. 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, SELECT statements will wait until there are no more updates.

To work around this for the case where you want to do many INSERT and 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> TRUNCATE TABLE insert_table;
mysql> UNLOCK TABLES;

You can use the LOW_PRIORITY options with INSERT if you want to prioritize retrieval in some specific cases.

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.


Question 15 :

MySQL - Table Locking Issues

Answer :

The table locking code in MySQL is deadlock free.

MySQL uses table locking (instead of row locking or column locking) on all table types, except BDB tables, to achieve a very high lock speed. For large tables, table locking is MUCH better than row locking for most applications, but there are, of course, some pitfalls.

For BDB tables, MySQL only uses table locking if you explicitely lock the table with LOCK TABLES or execute a command that will modify every row in the table, like ALTER TABLE.

In MySQL Version 3.23.7 and above, you can insert rows into MyISAM tables at the same time other threads are reading from the table. Note that currently this only works if there are no holes after deleted rows in the table at the time the insert is made.

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 other threads that want to access this particular table will wait until the update is ready.

As updates on tables normally are considered to be more important than 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. (You can change this by using LOW_PRIORITY with the statement that does the update or HIGH_PRIORITY with the SELECT statement.)

Starting from MySQL Version 3.23.7 one can use the max_write_lock_count variable to force MySQL to temporary give all SELECT statements, that wait for a table, a higher priority after a specific number of inserts on a table.

Table locking is, however, not very good under the following senario:

A client issues a SELECT that takes a long time to run.
Another client then issues an UPDATE on a used table. This client will wait until the SELECT is finished.
Another client issues another SELECT statement on the same table. As UPDATE has higher priority than SELECT, this SELECT will wait for the UPDATE to finish. It will also wait for the first SELECT to finish!
A thread is waiting for something like full disk, in which case all threads that wants to access the problem table will also be put in a waiting state until more disk space is made available.
Some possible solutions to this problem are:

Try to get the SELECT statements to run faster. You may have to create some summary tables to do this.
Start mysqld with --low-priority-updates. This will give all statements that update (modify) a table lower priority than a SELECT statement. In this case the last SELECT statement in the previous scenario would execute before the INSERT statement. You can give a specific INSERT, UPDATE, or DELETE statement lower priority with the LOW_PRIORITY attribute.
Start mysqld with a low value for max_write_lock_count to give READ locks after a certain number of WRITE locks.
You can specify that all updates from a specific thread should be done with low priority by using the SQL command: SET SQL_LOW_PRIORITY_UPDATES=1.

You can specify that a specific SELECT is very important with the HIGH_PRIORITY attribute.

If you have problems with INSERT combined with SELECT, switch to use the new MyISAM tables as these support concurrent SELECTs and INSERTs.
If you mainly mix INSERT and SELECT statements, the DELAYED attribute to INSERT will probably solve your problems.

If you have problems with SELECT and DELETE, the LIMIT option to DELETE may help.