organization of data
Posted: Mon Jan 27, 2025 9:04 am
As for the second question, about the organization of the data, there is only one piece of information available to the CBO: the so-called "clustering factor" of indexes. While the clustering factor is often sufficient information for deciding how to access a single table (access via a pre-sorted index or full table scan across the entire table), depending on the execution plan for operations that link different data sources, there is no information at all for the CBO about how these are organized in relation to one another. Determining this would also be a task that would probably take bosnia and herzegovina telegram screening up too much time and resources given the current state of technology, as there are so many different ways in which data sources can be linked: the order of the data sources can be different and there are potentially many possible access types for each data source. Both can change the order of data access Index Table which means that all possible combinations would have to be evaluated in order to process this information meaningfully and weigh it against each other.
The third question about data caching is currently not taken into account at all by the CBO – the CBO always assumes in its calculations that the data is not in the cache.
However, all of this means that, depending on the data and execution plan, the CBO only has insufficient information to answer the above questions correctly and can therefore easily select an unsuitable strategy. Furthermore, if you have sufficient knowledge of your data and the queries on this data, you may be able to answer these questions much better than the CBO and can therefore help him to select the right strategy or better assess whether the CBO has selected a suitable strategy.
statistics
The CBO essentially applies a mathematical model to the input data available to it (statistics of all kinds, such as object and system statistics) and creates an execution plan as a result.
The third question about data caching is currently not taken into account at all by the CBO – the CBO always assumes in its calculations that the data is not in the cache.
However, all of this means that, depending on the data and execution plan, the CBO only has insufficient information to answer the above questions correctly and can therefore easily select an unsuitable strategy. Furthermore, if you have sufficient knowledge of your data and the queries on this data, you may be able to answer these questions much better than the CBO and can therefore help him to select the right strategy or better assess whether the CBO has selected a suitable strategy.
statistics
The CBO essentially applies a mathematical model to the input data available to it (statistics of all kinds, such as object and system statistics) and creates an execution plan as a result.