Each Teradata Database table has an associated subtable called a table header. One copy of each table header is stored on each AMP in the system, having one row per table per AMP. Table headers are composed of a row header, one fixed-length field, and six variable‑length fields (see “Table Header Components” on page 787).
Teradata Database uses table headers internally to maintain various information about each table. Table headers are described here primarily because they are used to store value‑compressed column values (see “Compression Types Supported by Teradata Database” on page 695), and it is difficult to explain how compression works without having at least a superficial understanding of this file structure.
The size limit for a table header is 1 MB, so it is not possible to create a table defined with all possible features (2,048 columns, 32 indexes with 64 columns each (a composite NUSI that specifies an ORDER BY clause counts as 2 indexes in this calculation), 32 BLOB, CLOB, or XML columns, 255 distinct compressed values in each compressible column, and so on) without exceeding the defined byte limit. The maxima stated in Appendix B: “Teradata System Limits,” are for the individual table parameters only.