Calculating the Length of Index Items
For data types other than VARCHAR, the length of an index item is calculated by adding the length of the key value plus 5 bytes for each rowid information associated with the key value.
The key values in an index are typically of fixed length. If an index holds the value of one or more columns of the VARCHAR data type, the length of the key value is at least the sum of the length-plus-one of each VARCHAR value in the key.
In Informix®,
the maximum length of a key value is 390 bytes. The combined size
of VARCHAR columns that make up a key must be less than 390, minus
an additional byte for each VARCHAR column. For example, the key length
of the index that the database server builds for the following statements
equals 390, or
((255+1) + (133+1)): CREATE TABLE T1 (c1 varchar(255, 10), c2 varchar(133, 10));
CREATE INDEX I1 on T1(c1, c2);