A_SYB_SKIP_ALTERNATE_KEYS determines whether alternate keys are used to form indexes during table creation. The default value of this variable is 0, which means it’s okay to use alternate keys.
If you set the variable to a nonzero value such as 1, alternate keys are not used to form indexes, which speeds up processing if many writes or rewrites are being performed. A value of 1 may slow processing if the application is reading sequentially using an alternate key.)
While the default value is 0 (OFF, FALSE, NO), this configuration variable can also take the value of ON (TRUE, YES).
You can load a great deal of data into a table rapidly with this variable set to 1, and then create the missing indexes using isql.
See also: The Sybase documentation on isql