postgresql - postgres index with date in where clause -

i have large table several million rows in postgresql 9.1. 1 of columns timestamp time zone.

frequently used query looking data using clause 'column > (now()::date - 11)' last ten days.

i want build index work last months data, limit scan. partial index.

so far have not figured out how use actual last month, started hardcoding '2015-12-01' start date index.

create index q on test (i) > '2015-01-01'; 

this worked fine, index created. unfortunately, not used, treats '2015-01-01' ::timestamp, while query ::date. index not used , square one.

next tried modify index compare column date, match. here hit immutable wall.

as to_date or cast date mutable functions, dependent on local timezone, index creation fails.

if have test table this:

create table test (i timestamptz); 

and try create index

create index q on test (i) > to_date('2015-01-01','yyyy-dd-mm'); 

then fails

error:  functions in index predicate must marked immutable 

this understandable. now, when try specific timezone

create index q on test (i) > to_date('2015-01-01','yyyy-dd-mm')  @ time zone 'utc'; 

it still fails

error:  functions in index predicate must marked immutable 

this don't understand anymore. has timezone defined. else immutable?

i tried creating immutable function myself:

create function datacube=# create or replace function immutable_date(timestamptz) returns date $$ select ($1::date @ time zone 'utc')::date; $$ language sql immutable; 

but using function in index:

create index q on test (i) > immutable_date('2015-01-01'); 

fails same error:

error:  functions in index predicate must marked immutable 

i @ loss here. maybe has locales, not timezones? or else makes mutable?

and - maybe there another, simpler way, limit index last month or 2 of data? table partitioning in postgres require rebuilding entire database, , far have not found else.