6.1 Optimization Overview

The most important factor in making a system fast is its basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.

The most common system bottlenecks are:

• Disk seeks. It takes time for the disk to find a piece of data. With modern disks, the mean time for this is usually lower than 10ms, so we can in theory do about 100 seeks a second. This time improves slowly with new disks and is very hard to optimize for a single table. The way to optimize seek time is to distribute the data onto more than one disk.

• Disk reading and writing. When the disk is at the correct position, we need to read the data. With modern disks, one disk delivers at least 10-20MB/s throughput. This is easier to optimize than seeks because you can read in parallel from multiple disks.

• CPU cycles. When we have the data in main memory (or if it was already there), we need to process it to get our result. Having small tables compared to the amount of memory is the most common limiting factor. But with small tables, speed is usually not the problem.

• Memory bandwidth. When the CPU needs more data than can fit in the CPU cache, main memory bandwidth becomes a bottleneck. This is an uncommon bottleneck for most systems, but one to be aware of.

6.1.1 MySQL Design Limitations and Tradeoffs

When using the MyISAM storage engine, MySQL uses extremely fast table locking that allows multiple readers or a single writer. The biggest problem with this storage engine occurs when you have a steady stream of mixed updates and slow selects on a single table. If this is a problem for certain tables, you can use another table type for them. See Chapter 8, "MySQL Storage Engines and Table Types."

MySQL can work with both transactional and non-transactional tables. To be able to work smoothly with non-transactional tables (which can't roll back if something goes wrong), MySQL has the following rules:

• All columns have default values.

• If you insert a "wrong" value in a column, such as a too-large numerical value into a numerical column, MySQL sets the column to the "best possible value" instead of giving an error. For numerical values, this is 0, the smallest possible value, or the largest possible value. For strings, this is either the empty string or the longest possible string that can be stored in the column.

• All calculated expressions return a value that can be used instead of signaling an error condition. For example, 1/0 returns NULL .

The implication of these rules is that you should not use MySQL to check column content. Instead, you should check values within your application before storing them in the database.

6.1.2 Designing Applications for Portability

Because all SQL servers implement different parts of standard SQL, it takes work to write portable SQL applications. It is very easy to achieve portability for very simple selects and inserts , but becomes more difficult the more capabilities you require. If you want an application that is fast with many database systems, it becomes even harder!

To make a complex application portable, you need to determine which SQL servers it must work with, then determine what features those servers support.

All database systems have some weak points. That is, they have different design compromises that lead to different behavior.

You can use the MySQL crash-me program to find functions, types, and limits that you can use with a selection of database servers. crash-me does not check for every possible feature, but it is still reasonably comprehensive, performing about 450 tests.

An example of the type of information crash-me can provide is that you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.

The crash-me program and the MySQL benchmarks are all very database independent. By taking a look at how they are written, you can get a feeling for what you have to do to make your own applications database independent. The programs can be found in the sql-bench directory of MySQL source distributions. They are written in Perl and use the DBI database interface. Use of DBI in itself solves part of the portability problem because it provides database-independent access methods .

For crash-me results, visit http://dev.mysql.com/tech-resources/crash-me.php. See http://dev.mysql.com/tech-resources/benchmarks/ for the results from the benchmarks.

If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. For example, MySQL is very fast in retrieving and updating records for MyISAM tables, but will have a problem in mixing slow readers and 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). Transactional databases in general are not very good at generating summary tables from log tables, because in this case row locking is almost useless.

To make your application really database independent, you need to define an easily extendable interface through which you manipulate your data. Because C++ is available on most systems, it makes sense to use a C++ class-based interface to the databases.

If you use some feature that is specific to a given database system (such as the REPLACE statement, which is specific to MySQL), you should implement the same feature for other SQL servers by coding an alternative method. Although the alternative may be slower, it will allow the other servers to perform the same tasks .

With MySQL, you can use the /*! */ syntax to add MySQL-specific keywords to a query. The code inside /**/ will be treated as a comment (and ignored) by most other SQL servers.

If high performance is more important than exactness, as in some Web applications, it is possible to create an application layer that caches all results to give you even higher performance. By letting old results "expire" after a while, you can keep the cache reasonably fresh. This provides a method to handle high load spikes, in which case you can dynamically increase the cache and set the expiration timeout higher until things get back to normal.

In this case, the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.

An alternative to implementing an application cache is to use the MySQL query cache. By enabling the query cache, the server handles the details of determining whether a query result can be reused. This simplifies your application. See Section 4.10, "The MySQL Query Cache."

6.1.3 What We Have Used MySQL For

This section describes an early application for MySQL.

During MySQL initial development, the features of MySQL were made to fit our largest customer, which handled data warehousing for a couple of the largest retailers in Sweden.

From all stores, we got weekly summaries of all bonus card transactions, and were expected to provide useful information for the store owners to help them find how their advertising campaigns were affecting their own customers.

The volume of data was quite huge (about seven million summary transactions per month), and we had data for 4-10 years that we needed to present to the users. We got weekly requests from our customers, who wanted to get "instant" access to new reports from this data.

We solved this problem by storing all information per month in compressed "transaction" tables. We had a set of simple macros that generated summary tables grouped by different criteria (product group , customer id, store, and so on) from the tables in which the transactions were stored. The reports were Web pages that were dynamically generated by a small Perl script. This script parsed a Web page, executed the SQL statements in it, and inserted the results. We would have used PHP or mod_perl instead, but they were not available at the time.

For graphical data, we wrote a simple tool in C that could process SQL query results and produce GIF images based on those results. This tool also was dynamically executed from the Perl script that parsed the Web pages.

In most cases, a new report could be created simply by copying an existing script and modifying the SQL query in it. In some cases, we needed to add more columns to an existing summary table or generate a new one, but this also was quite simple because we kept all transaction-storage tables on disk. (This amounted to about 50GB of transaction tables and 200GB of other customer data.)

We also let our customers access the summary tables directly with ODBC so that the advanced users could experiment with the data themselves .

This system worked well and we had no problems handling the data with quite modest Sun Ultra SPARCstation hardware (2x200MHz). Eventually the system was migrated to Linux.

6.1.4 The MySQL Benchmark Suite

This section should contain a technical description of the MySQL benchmark suite (and crash-me ), but that description has not yet been written. Currently, you can get a good idea of the benchmarks by looking at the code and results in the sql-bench directory in any MySQL source distribution.

This benchmark suite is meant to tell any user what operations a given SQL implementation performs well or poorly.

Note that this benchmark is single-threaded, so it measures the minimum time for the operations performed. We plan to add multi-threaded tests to the benchmark suite in the future.

To use the benchmark suite, the following requirements must be satisfied:

• The benchmark suite is provided with MySQL source distributions. You can either download a released distribution from http://dev.mysql.com/downloads/, or use the current development source tree (see Section 2.3.3, "Installing from the Development Source Tree").

• The benchmark scripts are written in Perl and use the Perl DBI module to access database servers, so DBI must be installed. You will also need the server-specific DBD drivers for each of the servers you want to test. For example, to test MySQL, PostgreSQL, and DB2, you must have the DBD::mysql , DBD::Pg , and DBD::DB2 modules installed. See Section 2.7, "Perl Installation Notes."

After you obtain a MySQL source distribution, you will find the benchmark suite in its sql-bench directory. To run the benchmark tests, build MySQL, then change location into the sql-bench directory and execute the run-all-tests script:

 shell>  cd sql-bench  shell>  perl run-all-tests --server=   server_name  

server_name is one of the supported servers. To get a list of all options and supported servers, invoke this command:

 shell>  perl run-all-tests --help  

The crash-me script also is located in the sql-bench directory. crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

• What column types are supported

• How many indexes are supported

• What functions are supported

• How big a query can be

• How big a VARCHAR column can be

You can find the results from crash-me for many different database servers at http://dev.mysql.com/tech-resources/crash-me.php. For more information about benchmark results, visit http://dev.mysql.com/tech-resources/benchmarks/.

6.1.5 Using Your Own Benchmarks

You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing a bottleneck (or by replacing it with a " dummy module"), you can then easily identify the next bottleneck. Even if the overall performance for your application currently is acceptable, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.

For an example of portable benchmark programs, look at the MySQL benchmark suite. See Section 6.1.4, "The MySQL Benchmark Suite." You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which really is fastest for you.

Another free benchmark suite is the Open Source Database Benchmark , available at http://osdb. sourceforge .net/.

It is very common for a problem to occur only when the system is very heavily loaded. We have had many customers who contact us when they have a ( tested ) system in production and have encountered load problems. In most cases, performance problems turn out to be due to issues of basic database design (for example, table scans are not good at high load) or problems with the operating system or libraries. Most of the time, these problems would be a lot easier to fix if the systems were not already in production.

To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this. It is available at http://jeremy.zawodny.com/mysql/super-smack/. As the name suggests, it can bring a system to its knees if you ask it, so make sure to use it only on your development systems.