test=# create table t1(name varchar( 10 ), value integer);
CREATE TABLE
test=# create or replace function f1(integer) returns integer as $$
test$# file = open('/tmp/f1.log', 'a')
test$# file.write('log\n')
test$# file.close
test$# return args[ 0 ]+ 1 
test$# $$ language plpythonu immutable;
CREATE FUNCTION
test=# select name, f1(value) from t1;
 name | f1 
------+----
 r1   |   2 
 r2   |   3 
(записей:  2 )

test=# create index i1 on t1 (f1(value));
CREATE INDEX

test=# select name, f1(value) from t1;
 name | f1 
------+----
 r1   |   2 
 r2   |   3 
(записей:  2 )