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Global Features

• The Table System resembles a data base system. It consists of tables containing columns with data of any basic AIPS++ data type (integer, floating point, complex, string). Support for keywords (a la FITS) is provided. Furthermore it supports arrays of all data types (thus not only as blobs). It is, for example, possible to get a slice of an array in the table. The shapes of arrays in a column can be fixed or variable.
  A column or keyword data type can also be a record making it possible to store AIPS++ Record objects into the keywords or column cells.
  Finally a keyword can contain a reference to another table which is used for subtables of a table (as extensively used in the MeasurementSet).
• Meta data describe the table layout and help to quickly find data. Table System access is based on row number.
  Persistent indices on table data are not supported, but temporary memory based indices make it possible to quickly find the row numbers containing the required data.
• All I/O is done by means of a storage manager layer, so the logical and physical view of a table is well separated. A table can have multiple storage managers, so each column can be stored in the way most suited for the data in that column. Furthermore the Table System supports so-called virtual columns. The data in those columns are not stored but derived on-the-fly from other data. This feature is mainly used to compress columns containing complex data to short integers.
  The tiled storage manager (TSM) is a unique feature of the Table System. It makes it possible to store array data in a tiled way to achieve that access along all axes is about equally fast.
  All storage managers store the data in a canonical format to make access from multiple platforms possible. Boolean values are stored as bits.
• Concurrent access is fully supported. However, only table locks are available, thus no fine-grained page or row locking is possible. See note 256 for more information about locking in the Table System.
• The Table System uses large files where possible, so there is no 2 Gb file limit. On the rare systems not supporting large files the storage managers can be chosen such that each individual file does not exceed 2 Gb.
• Access to the data is very flexible. Addition, change, retrieval, and removal of columns, rows, and keywords can be done at will.
• A powerful query language (see note 199 on TaQL) makes it possible to select arbitrary subsets of a table. TaQL resembles SQL, but lacks joins. However, it supports subqueries and moreover all its functions work on arrays as well.
  It is important to note that the result of a selection and sort is a table in itself which references the original data (i.e. shallow copy). So changing the data in a selection changes the original data. This is regarded as being very useful. There is a function available to turn a selection into a deep copy.

  Note that TaQL can also be used for doing queries on in-memory objects. This is used in the ACSIS system to make a selection from a set of Record objects.

• The TableMeasures module makes it possible to store and retrieve Measures transparantly in and from a table.