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#### 2.3.4 Adapting Sequential to Random Access

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.

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STIL - Starlink Tables Infrastructure Library
Starlink User Note252
STIL web page: http://www.starlink.ac.uk/stil/
Author email: m.b.taylor@bristol.ac.uk