What do you do if you have a sequential-only table and you
need to perform random access on it?
The Tables.randomTable
utility method
takes any table and returns one which is guaranteed to provide random access.
If the original one is random, it just returns it unchanged,
otherwise it returns a table which contains the same data as the
submitted one, but for which isRandom is guaranteed to
return true.
It effectively does this by taking out a RowSequence and
reading all the data sequentially into some kind of (memory- or disk-based)
data structure which can provide random access, returning a new
StarTable object based on that data structure.
Exactly what kind of data structure is used for caching the data
for later use is determined by the StoragePolicy
currently in effect - this is described in Section 4,
and the
StoragePolicy.randomTable
method may be used explicitly instead to control this.
Clearly, this might be an expensive process. For this reason if you have an application in which random access will be required at various points, it is usually a good idea to ensure you have a random-access table at the application's top level, and for general-purpose utility methods to require random-access tables (throwing an exception if they get a sequential-only one). The alternative practice of utility methods converting argument tables to random-access when they are called might result in this expensive process happening multiple times.