Re: create index on a jsonb timestamp field?

Larry Rosenman <ler@lerctr.org> writes:
> when I try to create an index on the query_time field of the json
> structure I get:
> ler=# create index dns_query_time_idx on dns_query(((data -> 'message'
> ->> 'query_time')::text::timestamptz));
> ERROR:  functions in index expression must be marked IMMUTABLE

Yeah, because the timestamptz input function has dependencies on
both the datestyle and timezone GUCs.  Given that your input is
ISO-format with explicit time zone, you don't really care about
either of those things, but the mutability check doesn't know that.

> Is there any easy way to do this?  Or, what would the experts recommend
> here?

The sanest way to deal with this IMO is to make a column containing
the extracted timestamp, which you could maintain with a trigger,
and then index that.  You could alternatively make a custom function
that you (mis?)label as immutable, but your queries would have to
use that same function in order to get matched to the index, so
I dunno about that being a user-friendly approach.

BTW, I'd had the idea that the GENERATED option in PG v13 would allow
setting up this sort of case without bothering with a handwritten trigger,
but it seems not:

regression=# create table foo(data jsonb, ts timestamptz GENERATED ALWAYS AS ((data->>'ts')::timestamptz) stored);
psql: ERROR:  generation expression is not immutable

I wonder if that's really necessary to insist on?

            regards, tom lane