magento database query
Magento's use of data models provide a great way to access and modify data. Using aptly named methods and clever abstraction, Varien hide away the complex SQL needed to perform data operations. While this makes learning models easier, it often impacts the speed of the operation and therefore the responsiveness of your site. This is especially true when saving models that use the EAV architecture. More often that not, this cannot be avoided, however there are some situations where executing direct SQL queries would be simpler and much quicker leading to a more optimised Magento installation. An example of this is updating product prices globally in Magento. It would be easy enough to write some Magento code that looped through all products and modified the price. On a large data set, saving each individual product can take a long time and therefore make the system unusable. To combat this, it is possible to issue a direct SQL query which could update 1000's of products in 1 or 2 seconds.
Database Connections In Magento
By default, Magento will automatically connect to it's database and provide two separate resources which you can use to access data: core_read and core_write. As you can probably guess, core_read is for reading from the database while core_write is for writing to the database. It is important to ensure that you use the correct resource when reading or writing data to the database, especially when writing custom Magento extensions that will be released into the wild.