The number of keys stored on each page affects the size of a Btree and how it is maintained. Therefore, it also affects the retrieval and search performance of the tree. For each Btree, Berkeley DB computes a maximum key and data size. This size is a function of the page size and the fact that at least two key/data pairs must fit on any Btree page. Whenever key or data items exceed the calculated size, they are stored on overflow pages instead of in the standard Btree leaf pages.
Applications may use the DB->set_bt_minkey function to change the minimum number of keys that must fit on a Btree page from two to another value. Altering this value in turn alters the on-page maximum size, and can be used to force key and data items which would normally be stored in the Btree leaf pages onto overflow pages.
Some data sets can benefit from this tuning. For example, consider an application using large page sizes, with a data set almost entirely consisting of small key and data items, but with a few large items. By setting the minimum number of keys that must fit on a page, the application can force the outsized items to be stored on overflow pages. That in turn can potentially keep the tree more compact, that is, with fewer internal levels to traverse during searches.
The following calculation is similar to the one performed by the Btree implementation. (The minimum_keys value is multiplied by 2 because each key/data pair requires 2 slots on a Btree page.)
maximum_size = page_size / (minimum_keys * 2)
Using this calculation, if the page size is 8KB and the default minimum_keys value of 2 is used, then any key or data items larger than 2KB will be forced to an overflow page. If an application were to specify a minimum_key value of 100, then any key or data items larger than roughly 40 bytes would be forced to overflow pages.
It is important to remember that accesses to overflow pages do not perform as well as accesses to the standard Btree leaf pages, and so setting the value incorrectly can result in overusing overflow pages and decreasing the application's overall performance.
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