Re: Why *exactly* is date_trunc() not immutable ?

followup to self:

On Sun, Feb 18, 2007 at 12:29:17PM +0100, Karsten Hilbert wrote:

> So I figured it would make sense to add a functional index
> on date_trunc('day', dob) to the patients table. Which
> worked (appeared to, at least) with PG 7.4.
>
> One of our users is on PG 8.2
PostgreSQL 8.1 I was to say.

> and gets the warning that
> date_trunc() is not immutable and can thus not be used in a
> functional index.
The code to create the index:

    create index idx_identity_dob_ymd on dem.identity(date_trunc('day', dob))

The exact error it emits:

    functions in index expression must be marked IMMUTABLE

Those were lifted from the error log without further editing.

I know that I could fake immutability by wrapping
date_trunc() in a stored procedure marked IMMUTABLE but
I wonder what pitfalls that might hold.

Thanks,
Karsten
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