The estimated index size depends on the architecture, so just make
sure the code path is tested, without really caring of what the
estimated size is. Thanks to Christoph Berg for the report.
The estimation (size and cardinality) will probably always be very poor, the
postgres infrastructure get accurate informations by analyzing a real
index. One consequence is that almost only Bitmap Index Scan will be
chosen by the planner.
Some simple and frequent expressions are explicitely handled (for now md5(),
lower() and uppser()), but it only impacts the estimated size, not the
selectivity.
