The problem is that the function arguments are not considered as possible key
candidates for index scan and so only a sequential scan is possible inside
the body of a function.  I have therefore made some patches to the optimizer
so that indices are now used also by functions.  I have also moved the plan
debug message from pg_eval to pg_plan so that it is printed also for plans
genereated for function execution.  I had also to add an index rescan to the
executor because it ignored the parameters set in the execution state, they
were flagged as runtime variables in ExecInitIndexScan but then never used
by the executor so that the scan were always done with any key=1. Very odd.
This means that an index rescan is now done twice for each function execution
which uses an index, the first time when the index scan is initialized and
the second when the actual function arguments are finally available for the
execution.  I don't know what is the cost of an double index scan but I
suppose it is anyway less than the cost of a full sequential scan, at leat
for large tables. This is my patch, you must also add -DINDEXSCAN_PATCH in
Makefile.global to enable the changes.

Submitted by: Massimo Dal Zotto <dz@cs.unitn.it>