title: ERROR: functions in index expression must be marked IMMUTABLE in Postgres
body: <p>I want to create a Multicolumn expression index, but when I create the index, the following message is output:</p>
  
  <pre><code>--detail message 
  wapgrowth=&gt; create index CONCURRENTLY idx_test on  tmp_table using btree (skyid, to_char(create_time, 'YYYY-MM-DD'), actiontype );
  ERROR:  functions in index expression must be marked IMMUTABLE
  
  
  --table ddl
  wapgrowth=&gt; \d tmp_table
                 Table "wapgrowth.tmp_table"
     Column    |            Type             |   Modifiers   
  -------------+-----------------------------+---------------
   id          | integer                     | not null
   actiontype  | character varying(20)       | 
   apptype     | character varying(20)       | 
   score       | integer                     | 
   create_time | timestamp without time zone | default now()
   skyid       | integer                     | 
  Indexes:
  </code></pre>