Re: re-novice coming back to pgsql: porting an SQLite update statement to postgres

> On 14 Sep 2024, at 10:33, Dan Kortschak <dan+pgsql(at)kortschak(dot)io> wrote:

(…)

> I'm still having difficulties with the second part which is to update
> the contents of the amend array in the JSON.
>
> So far I'm able to append the relevant details to the append array, but
> I'm unable to correctly select the corrects elements from the $6
> argument, which is in the form
> [{"start":<RFC3339>,"end":<RFC3339>,"data":<object>}, ...]. The first
> update statement gives me broadly what I want, but includes elements of
> the array that it shouldn't.

(…)

> If I filter on the start and end time, I end up with no element coming
> through at all and the "replace" field ends up null.
>
> update
> events
> set
> datastr = jsonb_set(
> datastr,
> '{amend}',
> datastr->'amend' || jsonb_build_object(
> 'time', $2::TEXT,
> 'msg', $3::TEXT,
> 'replace', (
> select *
> from
> jsonb($6::TEXT) as replacement
> where
> (replacement->>'start')::TIMESTAMP WITH TIME ZONE < endtime and
> (replacement->>'end')::TIMESTAMP WITH TIME ZONE > starttime
> )
> )
> )
> where
> starttime < $5 and
> endtime > $4 and
> bucketrow = (
> select rowid from buckets where id = $1
> );

That’s because the replacement data is an array of objects, not a single object.

You need to iterate through the array elements to build your replacement data, something like what I do here with a select (because that’s way easier to play around with):

with dollar6 as (
select jsonb($$[
{
"data": { "foo": 1, "bar": 2
},
"end": "2023-06-12T19:54:51Z",
"start": "2023-06-12T19:54:39Z"
}
]$$::text) replacement
)
select *
from dollar6
cross join lateral jsonb_array_elements(replacement) r
where (r->>'start')::timestamptz <= current_timestamp;

There are probably other ways to attack this problem, this is the one I came up with.

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.