As section Searching for NCHAR or NVARCHAR Columns in an Index describes, indexing certain data sets cause significant performance degradation. Sorting certain data sets can also degrade performance.
If you do not need a non-ASCII collation sequence, it is recommended that you use the CHAR and VARCHAR data types for character columns whenever possible. Because CHAR and VARCHAR data require simple value-based comparison, sorting and indexing these columns is less expensive than for non-ASCII data types (NCHAR or NVARCHAR, for example). For more information on other character data types, see the IBM Informix: GLS User's Guide.
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