Re: create index on a jsonb timestamp field?

On 05/18/2019 8:17 pm, Tom Lane wrote:
> Larry Rosenman <ler(at)lerctr(dot)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.

This is what I wound up figuring out between my original post and yours,
and it works great.

>
> 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?

Good question. Is that something the project is going to look into?

>
> regards, tom lane

--
Larry Rosenman http://www.lerctr.org/~ler
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