- Documentation Library
- Table of Contents
- MySQL 5.0 Manual
- Table of Contents
- 7.4 Optimizar la estructura de una base de datos
- 7.4.1 Elecciones de diseño
- 7.4.2 Haga sus datos lo más pequeños posibles
- 7.4.3 Índices de columna
- 7.4.4 Índices de múltiples columnas
- 7.4.5 Cómo utiliza MySQL los índices
- 7.4.6 La caché de claves de MyISAM
- 7.4.7 Cómo cuenta MySQL las tablas abiertas
- 7.4.8 Cómo abre y cierra tablas MySQL
- 7.4.9 Desventajas de crear muchas tablas en la misma base de datos
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 believe 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 causes a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general-purpose databases.
The more common case is that the index and data are stored together (as in Oracle/Sybase, et al). In this case, you find the row information at the leaf page of the index. The good thing with this layout is that it, in many cases, depending on how well the index is cached, saves a disk read. The bad things with this layout are:
Table scanning is much slower because you have to read through the indexes to get at the data.
You can't use only the index table to retrieve data for a query.
You use more space because you must duplicate indexes from the nodes (you can't store the row in the nodes).
Deletes degenerate the table over time (because indexes in nodes are usually not updated on delete).
It's harder to cache only the index data.
Ésta es una traducción del manual de referencia de MySQL, que puede encontrarse en dev.mysql.com. El manual de referencia original de MySQL está escrito en inglés, y esta traducción no necesariamente está tan actualizada como la versión original. Para cualquier sugerencia sobre la traducción y para señalar errores de cualquier tipo, no dude en dirigirse a mysql-es@vespito.com.
