date_trunc not immutable

On 12/15/18 3:26 PM, Ravi Krishna wrote:

Version: PG 10.6 on AWS Linux.

I am trying to create an index on function date_trunc('month',timestamp)

PG is complaining that the function must be marked as IMMUTABLE.  So I
assume that date_trunc is not marked as immutable.

Definition of immutable from PG documentation
====================================

All functions and operators used in an index definition must be
"immutable", that is, their results must depend only on their arguments
and never on any outside influence (such as the contents of another
table or the current time). This restriction ensures that the behavior
of the index is well-defined. To use a user-defined function in an index
expression or WHERE clause, remember to mark the function immutable when
you create it.
===================================
What am I missing?  date_trunc will always return the same value for a
given value. Not sure how I can mark a PG function as immutable.

No it won't:

show timezone;
TimeZone
------------
US/Pacific

select date_trunc('hour', now());
date_trunc
------------------------
2018-12-15 15:00:00-08

set timezone='UTC';

select date_trunc('hour', now());

date_trunc
------------------------
2018-12-15 23:00:00+00

--
Adrian Klaver
adrian.klaver@aklaver.com

Ravi Krishna <srkrishna@fastmail.com> writes:

I am trying to create an index on function date_trunc('month',timestamp)
PG is complaining that the function must be marked as IMMUTABLE.

The timestamptz version of it is not immutable, because its effects depend
on the timezone setting:

regression=# set timezone = 'America/New_York';
SET
regression=# select date_trunc('month', now());
date_trunc
------------------------
2018-12-01 00:00:00-05
(1 row)

regression=# set timezone = 'Europe/Paris';
SET
regression=# select date_trunc('month', now());
date_trunc
------------------------
2018-12-01 00:00:00+01
(1 row)

If you want immutability, you need to be working with timestamp-without-tz
or date input, so that timezone isn't a factor.

regards, tom lane

On 2018-12-15, Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 12/15/18 3:26 PM, Ravi Krishna wrote:

Version: PG 10.6 on AWS Linux.

I am trying to create an index on function date_trunc('month',timestamp)

PG is complaining that the function must be marked as IMMUTABLE. So I
assume that date_trunc is not marked as immutable.

Definition of immutable from PG documentation
====================================

All functions and operators used in an index definition must be
"immutable", that is, their results must depend only on their arguments
and never on any outside influence (such as the contents of another
table or the current time). This restriction ensures that the behavior
of the index is well-defined. To use a user-defined function in an index
expression or WHERE clause, remember to mark the function immutable when
you create it.
===================================
What am I missing? date_trunc will always return the same value for a
given value. Not sure how I can mark a PG function as immutable.

No it won't:

show timezone;
TimeZone
------------
US/Pacific

select date_trunc('hour', now());
date_trunc
------------------------
2018-12-15 15:00:00-08

set timezone='UTC';

select date_trunc('hour', now());

date_trunc
------------------------
2018-12-15 23:00:00+00

Ravi, the date_trunc('month',timestamp) is already immutable (at least in PG11):
postgres=# \df+ date_trunc
Schema | Name | Result data type |
Argument data types | Volatility | ...
------------+------------+-----------------------------+-----------------------------------+------------+-...
pg_catalog | date_trunc | timestamp without time zone | text,
timestamp without time zone | immutable | ...

For the "date_trunc(text, timestampTZ) see Adrian's response, why it
does not always return the same values for the same input.

--
Best regards,
Vitaly Burovoy

Thanks all.  I forgot the TZ part.