[GENERAL] Creating index on concatenated char columns fails is Postgres 9 (regression)

[Andrus]

Steps to reproduce:

Run commands

    create temp table test (kuupaev date, kellaaeg char(5)  ) on commit drop;
    create index test on test ((kuupaev||kellaaeg));     

in 

    "PostgreSQL 9.3.4, compiled by Visual C++ build 1600, 32-bit"

Observed result:

    ERROR:  functions in index expression must be marked IMMUTABLE

In 

    "PostgreSQL 8.4.4, compiled by Visual C++ build 1400, 32-bit"

those commands work OK.

I need this index to speed up query

SELECT 
    max( kuupaev||kellaaeg ) 
  from ALGSA 
  where laonr=?nlaonr and kuupaev <=?prmLOPP and kuupaev||kellaaeg <= ?someparam

How to fix or other way to speed this query?

Posted also in  
http://stackoverflow.com/questions/26161561/how-to-create-composite-index-in-postgres-9

Andrus.