Why isn't it possible to cast to a timestamp in an index in PostgreSQL?
-
15-02-2021 - |
Domanda
I am working with a JSON document which I receive from an external source. In this document, there is a timestamp
property which corresponds to the textual format of a timestamp
PostgreSQL data type. So in other words, it can be casted to a timestamp
type: (data->>'timestamp')::timestamp
.
I want to use that timestamp
property in an index. I tried the following:
create table t
(
data jsonb
);
create index on t(((data->>'timestamp')::timestamp));
I am getting:
ERROR: functions in index expression must be marked IMMUTABLE
Why is this? I understood that casting to a timestamptz
is not immutable, because of the localization configuration settings which are not static, but I can't apply that logic to a cast to timestamp
.
Is there still a way to cast my timestamp
property in the index here?
I am working with PostgreSQL 12, as released a few days ago.
Soluzione
Casts from text
to timestamp without time zone
are handled by calling the type input function timestamp_in
, which is STABLE
.
The reason is that timestamp_in
supports some other formats too:
SELECT 'now'::timestamp;
timestamp
----------------------------
2019-10-09 09:53:32.026673
(1 row)
If you know that your input is always an ISO timestamp and this cannot happen, define your own conversion function:
CREATE FUNCTION text2ts(text) RETURNS timestamp without time zone
LANGUAGE sql IMMUTABLE AS
$$SELECT CASE WHEN $1 ~ '^\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}(\.\d+)?$'
THEN CAST($1 AS timestamp without time zone)
END$$;
You can use that in your queries and indexes.
If you lack paranoia and are sure of your data quality, you can omit the test. That will improve performance, but you risk index corruption if somebody sneaks something like now
into the data.
Altri suggerimenti
Try
create table t
(
data jsonb,
data_timestamp TIMESTAMP GENERATED ALWAYS AS ((data->>'timestamp')::timestamp) STORED
);
create index on t(data_timestamp);
From comments:
I also tried that (I forgot to mention that in the question), that gives the same error: ERROR: generation expression is not immutable.
If so the only way which I see is to use static column which' value is generated by INSERT and UPDATE triggers. And index by it.