What are Derived Columns. What possible problems can a derived column pose?
Câu trả lời What are Derived Columns. What possible problems can a derived column pose?
Derived columns in MySQL, also known as computed or generated columns, are columns that are not stored directly in the table data but are computed from expressions involving other columns. These expressions can include various functions, operators, and references to other columns within the same table. Derived columns can be either stored or virtual. Stored derived columns calculate and store their values in the database when the data is inserted or updated, which can increase the storage requirement but improve read performance. Virtual derived columns, on the other hand, calculate their values on the fly each time they are accessed, which does not require additional storage but can lead to increased CPU usage during data retrieval[2][5][6].
Possible Problems with Derived Columns
Performance Impact: One of the main issues with derived columns, especially virtual ones, is the potential performance impact during query execution. Since virtual derived columns are recalculated every time they are accessed, this can lead to significant CPU overhead if the underlying expression is complex or if the column is accessed frequently[1][5].
Complexity in Query Optimization: Derived columns can make query optimization more challenging. The optimizer must understand and incorporate the logic of the derived column into its execution plans, which can be difficult if the expressions are complex or involve multiple dependencies[1][3].
Storage Overhead: For stored derived columns, there is an additional storage cost because the computed values are stored physically in the database. This can lead to increased storage consumption, which might be a concern in environments where storage is a constraint[5][6].
Maintenance and Debugging: Maintaining queries and schema definitions that involve derived columns can be more complex. Debugging issues related to derived columns can also be challenging, especially if the expressions involve nested or multiple function calls[1][10...
