variable unlimited length column has slower search performance. Since our entry.xsd has this:
```
    <complexType name="UsageCategory">
        <attribute name="term" type="atom:CategoryName" use="required"/>
        <attribute name="scheme" type="xsd:anyURI" use="optional"/>
        <attribute name="label" type="xsd:string" use="optional"/>
    </complexType>
    <simpleType name="CategoryName">
        <restriction base="xsd:string">
            <maxLength value="256"/>
        </restriction>
    </simpleType>
```
we should pick char(256) for the column type, instead of text.