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Zend_Db Performance - Zend Framework Performance Guide
Zend_Db is a database abstraction layer, and is intended to provide a common API for SQL operations. Zend_Db_Table is a Table Data Gateway, intended to abstract common table-level database operations. Due to their abstract nature and the "magic" they do under the hood to perform their operations, they can sometimes introduce performance overhead.
In order to keep usage as simple as possible, and also to support constantly changing schemas during development, Zend_Db_Table does some magic under the hood: on first use, it fetches the table schema and stores it within object members. This operation is typically expensive, regardless of the database -- which can contribute to bottlenecks in production.
Fortunately, there are techniques for improving the situation.
Zend_Db_Table can optionally utilize Zend_Cache to cache table metadata. This is typically faster to access and less expensive than fetching the metadata from the database itself.
As of 1.7.0, Zend_Db_Table also provides support for hardcoding metadata in the table definition. This is an advanced use case, and should only be used when you know the table schema is unlikely to change, or that you're able to keep the definitions up-to-date.
Zend_Db_Select is relatively good at its job. However, if you are performing complex queries requiring joins or sub-selects, it can often be fairly naive.
The only real answer is to write your own SQL; Zend_Db does not require the usage of Zend_Db_Select, so providing your own, tuned SQL select statements is a perfectly legitimate approach,
Run EXPLAIN on your queries, and test a variety of approaches until you can reliably hit your indices in the most performant way -- and then hardcode the SQL as a class property or constant.
If the SQL requires variable arguments, provide placeholders in the SQL, and utilize a combination of vsprintf() and array_walk() to inject the values into the SQL: