This function compares string_a with string_baccording to the collation rules specified by the indexing collation configuration file indicated by the environment variable APTIXLANG.

The function returns -1 if string_a collates before string_b, 0 if string_a and string_b sort the same, and 1 if string_a collates after string_b.

This function is useful in the ixsorthook function or if you wish to write your own index processing.