Dyntex.net Support Centre: MySQL Reference Manual p.498

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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

498 MySQL Technical Reference for Version 4.0.3 7.1.2.1 Static (Fixed-length) Table Characteristics This is the default format. It's used when the table contains no VARCHAR, BLOB, or TEXT columns. This format is the simplest and most secure format. It is also the fastest of the on-disk formats. The speed comes from the easy way data can be found on disk. When looking up something with an index and static format it is very simple. Just multiply the row number by the row length. Also, when scanning a table it is very easy to read a constant number of records with each disk read. The security is evidenced if your computer crashes when writing to a xed-size MyISAM le, in which case myisamchk can easily gure out where each row starts and ends. So it can usually reclaim all records except the partially written one. Note that in MySQL all indexes can always be reconstructed: All CHAR, NUMERIC, and DECIMAL columns are space-padded to the column width. Very quick. Easy to cache. Easy to reconstruct after a crash, because records are located in xed positions. Doesn't have to be reorganised (with myisamchk) unless a huge number of records are deleted and you want to return free disk space to the operating system. Usually requires more disk space than dynamic tables. 7.1.2.2 Dynamic Table Characteristics This format is used if the table contains any VARCHAR, BLOB, or TEXT columns or if the table was created with ROW_FORMAT=dynamic. This format is a little more complex because each row has to have a header that says how long it is. One record can also end up at more than one location when it is made longer at an update. You can use OPTIMIZE table or myisamchk to defragment a table. If you have static data that you access/change a lot in the same table as some VARCHAR or BLOB columns, it might be a good idea to move the dynamic columns to other tables just to avoid fragmentation: All string columns are dynamic (except those with a length less than 4). Each record is preceded by a bitmap indicating which columns are empty ('') for string columns, or zero for numeric columns. (This isn't the same as columns containing NULL values.) 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. Usually takes much less disk space than xed-length tables. Each record uses only as much space as is required. If a record becomes larger, it is split into as many pieces as are required. This results in record fragmentation.

MySQL Reference Manual

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